NTT Technical Review
August 2016 Vol. 14 No. 8
Front-line Researchers
Akira Fujiwara, Senior Distinguished Researcher, NTT Basic ResearchLaboratories

Feature Articles: Forefront Research on Bio-soft Materials
Overview of Bio-soft Material Research at NTT
Fabrication of Nanobiodevices that Utilize the Function of Membrane Proteins
Pattern Formation of Supported Lipid Bilayer for Molecular Manipulation
Neuronal Growth on Artificial Structures with Different Materials
Time-lapse Imaging of Neural Morphological Changes Relating to Cellular
FunctionsOn-chip Graphene Biosensor
Conductive Composite Material for Vital Data Measurement

Regular Articles
Substrate-transfer Technique Using h-BN for GaN-based High-power Transistors
Path Accommodation Design Engine for Simply and Reliably DesigningMulti-layer Transport Networks

Global Standardization Activities
Trajectory of ITU-T Standardization and Key Issues at the ITU-T CJK CTO
Meeting

Practical Field Information about Telecommunication
Technologies
Recent Case Study of Fault in IP Phone User System

External Awards/Papers Published in Technical Journals
and Conference Proceedings
External Awards/Papers Published in Technical Journals and Conference
Proceedings

Front-line Researchers

Research and Development that Will
Be Valued 100 Years from Now—
Making Dreams a Reality with
Single-electron Devices Manipulating
Individual Electrons
Akira Fujiwara
Senior Distinguished Researcher,
NTT Basic Research Laboratories
Overview
Semiconductor technology—the backbone of information andcommunication technology—has made amazing progress over
the years, and more recently, attention has come to focus on theultimate-level control of single electrons and single atoms using
nanofabrication and crystal-growth techniques. What kind ofbreakthroughs can we expect from NTT’s research on ultimate
electronics? We asked Dr. Akira Fujiwara, Senior Distinguished
Researcher at NTT Basic Research Laboratories, to update us on
recent research achievements and to tell us about his life as a
researcher.
 eywords: semiconductor, single-electron device, current stanK
dard“Ultimate electronics” based on new principles
and concepts—Dr. Fujiwara, please tell us about your current
research activities.
The computers, smartphones, and other information terminals and devices that surround us today allembed a countless number of current switches to process large amounts of information at high speed.
Semiconductor technology provides the backbone forthis processing. “Semiconductor” is the name given
1

to a material that exhibits properties halfway between
a metal (good conductor) and an insulator. The transistors used in integrated circuits (ICs) consist of
semiconductor material, and controlling the current
flowing through the ICs enables various types of
information processing to be done.
Up to now, the research in this area has focused ona means of downsizing IC chips and increasing the
integration density as much as possible. It has so far
become possible to integrate more than 100 million
transistors interconnected by massive amounts ofwiring on a 1-cm-square chip.
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Front-line Researchers

Nanometer-scale silicon structures fabricated by ultra-fine-processing technology

New approach to system construction based on ultimate electron manipulation
Transfers one electron ata timeSingle-electron transfer

Transfers and reads one electron at a time

Detects (reads) one electron at a timeSingle-electron detection

Evaluation of single-electron
manipulation error

Transfers, reads, and controls feedback
Single-electron error
correction

Achieving high-accuracy current standards
Establishing basic technology for high-sensitivity sensing and ultra-low-energy
information processing

Fig. 1. Ultimate electronics.

However, the present mechanism for charging suchwiring requires the flow of more than 10,000 electrons per one bit of information. At this rate, simply
increasing the integration density will drasticallyincrease power consumption. This is a serious issue
in terms of the environment and energy-related costs.
With this in mind, we are researching single-electron
devices that can manipulate individual electrons withthe aim of achieving devices and circuits that operateon new principles and concepts. If this can beachieved, we foresee the creation of a form of electronics having ultimate performance (in accuracy andsensitivity) and low-power characteristics that are different from the functions and performance of today’s
simple electron switches (Fig. 1).
—This is certainly the seed of new technology arisingfrom unconventional ideas. In what kind of researchand in what fields would this technology be useful?
The basic functions of transistors that make up an
IC are often compared to the holding back and releasing of the flow of water. This analogy, however, only
holds for bulk semiconductors of a sufficiently largesize.
In contrast, the field of research that we are currently pursuing is low-dimensional semiconductorsthat trap electrons in a small space. This concerns
technology that confines electrons in nanometerscale spaces by forming a laminated structure ofsemiconductors and insulators and contracting its
Vol. 14 No. 8 Aug. 2016

dimensions using ultra-fine-processing technology.
In such low-dimensional semiconductors, quantummechanical wave properties intrinsic to electrons andrepulsive forces between electrons are particularly
prominent. Here, the structure that we are dealing
with is called a quantum dot, which is considered to
be zero-dimensional. Within this structure, we canconfine an electron in a small space and use the repulsive forces that arise between electrons to individually control electrons one by one. Such an electronconfining space is often called a single-electron
island (Fig. 2). Electrically controlling the movementof electrons into and out of this single-electron island
enables high-speed and high-accuracy transfer ofelectrons. In addition, preparing a high-sensitivity
charge detection device near the island enables thedetection of transferred electrons one electron at atime.
A hot topic of late is a “current standard” for accurately measuring electrical current using silicon single-electron devices that can precisely manipulate
individual electrons (Fig. 3). A resistance standard
using semiconductor devices and a voltage standard
using superconducting devices are already in practical use, but if a current standard using a single-electron device can be achieved, it should be possible toconstruct a more solid electrical standards and units
system.
Research on low-dimensional semiconductors has
been going on for some time, and various devices
have been studied. This is because it’s an attractive
2

Front-line Researchers

Transfer clocke−

100 nmCross-section ofsilicon wiree−e−Single-electron island

Single-electron island(a) Single-electron transfer device
Charge detection device(b) Electron potential diagram
Charge node
LG2

S-S
S-G

Single-electron transfer device

LG1
e− S

e−

S-D

D

LG3

200 nm(c) Single-electron transfer/detection device

Fig. 2. Single-electron transfer and detection using silicon transistors.

Single-electrontransfer device
e−

e

−

e− e

Single electron
e

−

e

−

−

e−

e

−

e−

Output current
ef

Clock signal
Producesunidirectional flow

Frequency fCompletion of quantum metrology triangle
FrequencyfJosephson voltage
standard

Single-electron
current standard

V

I
Quantum Hall
resistance standard

Fig. 3. Application of single-electron current standard.

research theme that has much potential not only in therealm of fundamental physics but also in the field ofengineering applications.
3

We began our research on single-electron manipulationand transfer devices around the year 2000, and in
March 2001, we published our research results on the
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Front-line Researchers

first silicon-based device for manipulating elementary charge in Nature, the distinguished British scientific journal [1]. Later, in research focusing on highspeed operation of this device, we succeeded inachieving single-electron transfer by a gigahertz
clock in 2008 [2]. Next, through assistance received
from the Funding Program for Next Generation
World-Leading Researchers (NEXT Program) of the
Cabinet Office of the Japanese government, we conducted research on silicon low-power nanodevices
based on the control of single or a few electrons overa three-year period starting in 2011. In this research,
we developed a different approach to using an artificially fabricated single-electron island. That is, we
proposed and successfully demonstrated high-speed
single-electron transfer using a localized state withina semiconductor crystal to trap an electron in an evensmaller space. We published the results of thisresearch in Nature Communications in 2014 [3]. In
more recent research, we have been quantitatively
evaluating the accuracy of single-electron transfersand making sound progress while setting new world
records in this field [4, 5].
From a “Why not?” attitude at first, to a totallycommitted love of research—Why did you choose physics as your research
theme? Have you made steady progress as a
researcher?
It all started during my studies in high school andcollege; physics struck me as something “very beautiful.” But I also enjoyed studies related to nature andthe environment, so there was a period in which I wasin a quandary over which to choose as a major: physics or the environment. In the end, knowing that I was
someone who liked to probe deeply into matters,
physics seemed like a good fit, so that’s what I chose.
However, I can’t say that this decision was a firm
reflection of my career objectives. At the time, it wasmore like “Why not?”I think that people who can say “This is my life’s
calling!” from the start are extremely rare. For thisreason, I believe that it’s best to first get involved insomething that you are somewhat skilled in or thatyou feel is worthwhile or rewarding from a social
perspective. Then, by moving forward even without aclear understanding of your objectives, your callingin life should one day come into view. Looking backto the time when I entered NTT, I remember how
motivated I felt as a corporate researcher on a clear
Vol. 14 No. 8 Aug. 2016career path. Today, I still have fond memories of theemotion I felt in my second year at the company,
when I worked on the fabrication of transistors andactually got them to work under the instruction of mysuperiors and senior colleagues. In Japanese, the Chinese characters for the word “device” include thecharacter for “child,” and the emotion that I felt on
creating a device was much like the emotion one
might feel on the birth of one’s child. This feelingmotivated me to advance to the next step in myresearch career.
Later on, I shifted to what is now my current line ofresearch—the manipulation of single electrons. My
research activities have not necessarily been smoothand steady, but one reason why I have been able topersevere in my research is that I have been blessedwith great superiors and colleagues, and a great family too; there has always been someone to consult
with whenever I hit an impasse in my work.
Starting in 2004, I also spent two years in charge ofrecruiting staff at NTT laboratories, and because I
could not do any research during that time, I became
quite frustrated. However, this period proved to be agreat learning experience for me on what drives people and an organization. What may appear to be anegative environment at first glance can be turned
into a positive environment with the right mindset.
—Where do good ideas come from in research?
If there is anything that I feel to be true in myresearch activities up to now is that discoveries lie inlaboratory experiments. I really don’t feel muchinsight from desktop thinking and contemplation. In
the laboratory, however, I start by acquiring data,
which may generate a response from me such as“Say, something is strange here!” Then, by probing
deeper here and there, I can obtain many useful
results. In fact, results that are different from those
expected are food for an inquiring mind. This, inshort, is my research style.
It’s exactly at these key moments that I am particularly focused rather than in regular, everyday
research. At such times, I find myself deep in thoughteven on the train or in the middle of a meal. When I
was younger, staying up all night to work on a problem was a matter of course. I remember sleeping only
six hours over a three-day period when writing my
doctoral thesis.
In regards to the paper we published in Nature in
2001, what supported our achievement in manipulating single electrons was our detection method for4Front-line Researchers

verifying whether an individual electron was actually
being manipulated. This detection method was
unprecedented and highly creative, and it enabled us
to obtain and analyze experimental results that were
not expected on the basis of previous experiments. It
took about three or four years from the discovery ofthis detection method to the manipulation of single
electrons, but I was totally absorbed in everyday
experiments, and I enjoyed the entire process.
Digging deeply into the cause of observed phenomena leads to the discovery of new research themes.
And the criterion for assessing such a discovery is
insight based on hard physics, so logical thinking isvery important here.
Brilliant researchers look for brilliant partners—What is important for producing good results as anactive researcher?
Starting in 2003, I spent a year as a researcher at the
National Institute of Standards and Technology in the
United States. At that time, I was already researching
high-speed, high-accuracy single-electron transfer,and I thought that this institution would be a goodplace to move my research forward. I faced a varietyof challenges there, such as assembling experimental
equipment, devising measurement techniques, andcoming up with good ideas. These challenges tested
my competence as a researcher in an American stylethat evaluates not only one’s position but one’s actual
abilities as well. It took several months for my abilities to be recognized, but once that happened, myresearch activities began to change in a big way. As
my ideas came to be accepted, I began to win theconfidence of those around me and to engage inlively discussions.
An internationally active researcher places great
importance on whom to hold discussions with. In
other words, such a researcher wants to be stimulated
by a brilliant partner and is always on the lookout forsuch a person to team up with. A researcher will not
grow by withdrawing into a shell. It is imperative toseek out situations enabling contact and stimulating
discussions with excellent researchers. Researchers
need to challenge themselves with an open frame ofmind.
It is also important for a researcher to have a personality that is conducive to direct interaction in aface-to-face manner. At present, we are involved in ajoint research project with the National PhysicalLaboratory of the United Kingdom, and I believe that5

making visits is one important way of earning thetrust of other team members. This is my researchstyle; when it comes to gaining the confidence of others, personality matters more than research results.
Deciding whom I would like to establish a rapport
with is very important. Of course, scientific credentials are necessary in a partner, but what counts in theend is whether I think that I would like to team up
with that person over the long term.
Moreover, to achieve breakthroughs as a researcher,
it is important to maintain an interest in otherresearchers and research studies and to keep the channels of communication open. It is also important to
ask insightful questions at international conferences
and other gatherings to keep others interested in you
and your activities. Even if you are unable to ask
questions at a presentation for whatever reason, you
can always make your voice known later in one way
or another. In addition, simply chatting about common interests or hobbies is a great way of promoting
communication. Such an approach can open yourheart to that country’s culture and to human-interest
conversations outside of research.
—Can you say a few words to our young researchers?
Please value intellectual curiosity and teamwork.
Research results cannot be achieved by one person. In
short, amassing wisdom and working hard together
are essential components. You cannot achieve effective teamwork with a reserved attitude. It goes without saying that the technologies of monozukuri(manufacturing) are important, so I would like to see
all of you make an active effort in accumulating andsharing know-how within your teams. There are threecriteria for doing significant work in the world ofbasic research. The first is “Do work that can be recognized by the world.” The second is “Do work thatcan be evaluated and cited by other researchers in thesame field.” And the third is “Do work that includes
content that has the potential of being praised in the
future even if it appears at first glance to be out-offashion research.” In other words, are you doing workthat includes useful data or information that later
researchers will have the urge to reference?
To fulfill these criteria, keep asking yourself, “Am I
falling victim to the Galapagos syndrome?” “Am I
becoming a frog in the well?” Take a broad look at
your research field and its history and do some objective self-analysis. Find out exactly where yourresearch lies in the lineage or historical flow of that
research. Contributing to the progress of science and
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Front-line Researchers

technology means leaving one’s footprint on a long
list of results achieved to date by other researchers.
Pay homage to past research and continue to ask
yourself “Am I really doing something meaningful?”[2]—Dr. Fujiwara, what challenges will you take up
going forward?

[4]

We expect an electric current standard using singleelectron transfer to be used as a means of directly
establishing the ampere, the unit of current, which isscheduled to be redefined as part of a new International System of Units after 2018. The aim is to complete the quantum metrology triangle as a milestone
toward achieving applications of this current standard. This is an experiment that aims to check therelationships among the electric current standard
using single-electron transfer, the Josephson voltage
standard, and the quantum Hall resistance standard. I
mention here that discoverers of the Josephson effectand the quantum Hall effect each received a Nobel
Prize in Physics. Our work, meanwhile, is an engineering challenge that seeks to determine the extent
to which an electron can be accurately controlled. We
are undertaking this work as an exit path for ultimate
electronics. At present, our joint research with a teamat the National Physical Laboratory of the United
Kingdom is moving forward, and research projects
centered particularly about Europe are progressing.
Over the long term, I hope to connect this researchwith the further pursuit of ultimate electronics, theapplication of sensors, and the achievement of lowpower information processing systems.
In addition, while pursuing my research goals, I
would like to convey my thoughts and opinions to myyounger colleagues. This is not to mean that I want topush my ideas on others. Rather, I would hope tostimulate their intellectual curiosity and assist themin any way that I can.
References[1]

A. Fujiwara and Y. Takahashi, “Manipulation of Elementary Charge
in a Silicon Charge-coupled Device,” Nature, Vol. 462, pp. 560–562,
2001.

Vol. 14 No. 8 Aug. 2016

[3]

[5]

A. Fujiwara, K. Nishiguchi, and Y. Ono, “Nanoampere Charge Pumpby Single-electron Ratchet Using Silicon Nanowire Metal-oxidesemiconductor Field-effect transistor,” Appl. Phys. Lett., Vol. 92,
042102, 2008.
G. Yamahata, K. Nishiguchi, and A. Fujiwara, “Gigahertz Single-trap
Electron Pumps in Silicon,” Nat. Commun., Vol. 5, Article no. 5038,
2014.
G. Yamahata, T. Karasawa, S. P. Giblin, M. Kataoka, and A. Fujiwara,“Gigahertz Single-electron Pumping in Silicon with an Accuracy Better than 9.2 Parts in 107,” Appl. Phys. Lett., Vol. 109, 013101, 2016.
Press release issued by NTT on July 5, 2016.
http://www.ntt.co.jp/news2016/1607e/160705a.html

Interviewee profile
Akira Fujiwara
Senior Distinguished Researcher, Senior
Manager of Physical Science Laboratory andGroup Leader of Nanodevices Research Group,
NTT Basic Research Laboratories.
He received his B.S., M.S., and Ph.D. in applied
physics from the University of Tokyo in 1989,
1991, and 1994. He joined NTT in 1994 and has
been engaged in research on silicon nanostructures and their application to nanodevices andsingle-electron devices. He was a guest researcher at the National Institute of Standards andTechnology, Gaithersburg, MD, USA, in 2003–
2004. He was a director of the Japanese Societyof Applied Physics (JSAP) in 2010–2011 and avisiting professor at Hokkaido University in2013. He received the International Conference
on Solid State Devices and Materials (SSDM)
Young Researcher Award in 1998, the SSDM
Paper Award in 1999, and JJAP (Japanese Journalof Applied Physics) Paper Awards in 2003, 2006,
and 2013. He was awarded the Young Scientist
Award from the Ministry of Education, Culture,
Sports, Science and Technology in 2006. He was
supported by the Funding Program for Next
Generation World-Leading Researchers (NEXT
Program), Japan Society for the Promotion of
Science, during 2011–2014. He is a member of
JSAP and a senior member of IEEE (Institute ofElectrical and Electronics Engineers).

6

Feature Articles: Forefront Research on Bio-soft
Materials

Overview of Bio-soft Material
Research at NTT
Hiroshi Nakashima
Abstract

At NTT Basic Research Laboratories, we are aiming to create a novel nanobiointerface that allows
direct access to the human body and brain by utilizing the functions and structures of biomolecules andsoft materials. We are also attempting to understand the fundamental principles of bioinformation processing by managing the combination of nanotechnology and biotechnology. The Feature Articles in thisissue introduce our bio-soft material research in relation to the fabrication of nanobiodevices, whichconstitute one of the components of a future artificial synapse. We also present a highly sensitive andlong-term stable biosensing system.
Keywords: bio-soft material, nanobiointerface, bioelectrode

1. Introduction
Soft and flexible materials now play an importantrole in supporting modern society in the same way as
hard materials such as metals, semiconductors, andceramics. Soft materials include rubber, plastic, gel,colloid, and liquid crystal, and biological tissue also
consists of large and complex soft materials. In the20th century, hard materials played major roles in theevolution of various industries.
In the 21st century, the manufacture of soft materials including their biological applications has been
greatly expanded. The size, shape, and function ofsoft materials are easily controllable over a wide
range. These materials provide us with the potential
to discover unknown physical and chemical properties through the synthesis of novel soft and flexible
materials or by precisely controlling their composition and structure on a nanometer-scale. In addition,these materials are not simply an interesting target forbasic research; in fact, a wide range of industrial
applications have also been developed.
In the field of information technology and electronics, for example, plastic optical fiber and organic
electro-luminescent (EL) material have been put topractical use. A tough and flexible polymer material
is used as a surface coating material for mobile
1

phones, and its surface is both highly waterproof andscratch-resistant.
In recent years, it has become increasingly important to ensure that the production, use, and recyclingof materials have as little effect on the environment as
possible. Therefore, organic and soft materials thatare people- and environment-friendly have been
widely used.
Another issue in the current aging society in termsof the development of medical implant materials andmedical and healthcare devices is that it is importantthat we construct a biointerface that enables directcontact with biological systems. A sophisticated biointerface design is thus required that employs soft
material with a high affinity to soft biological tissue,
without causing it any damage.
2. Biointerface application
If we are to produce a functional biointerface that iscompatible with biological systems, it is essential that
we understand fundamental biostructures and design
the biointerface by mimicking these biostructures.
Proteins, nucleic acids, polysaccharides, and the lipid
molecules of a cell membrane are often used to produce biointerfaces. These molecules have a specificfeature, namely that the shape and characteristics
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Feature Articles

vary greatly depending on the external environment
and external stimulus. Deoxyribonucleic acid (DNA)or protein chips, various biosensors, and a drug delivery system have been developed using these molecules.
In recent years, high-performance biointerfaces
have been reported that can be applied to advanced
medical treatments. For example, soft and highlybiocompatible silicone enables us to produce an
implant material that can be flexibly bent and that
does not result in inflammation or rejection by the
body. Damage to the spinal cord caused by an injury
or an accident can result in paralysis, and to restore
bodily functions, it is necessary to repair the damagesite using artificial biomaterials. Thus far, it has beendifficult to repair such damage. However, a researchgroup has reported exciting results in which a paralyzed laboratory animal began to move after siliconmaterial was embedded in the damaged part and electrical stimulation provided.
Another research group has reported a functionalfilm material that can adhere to the surface of a beating heart for a long time without falling off and without the need for sutures or adhesive. A metal or semiconductor device is printed on the film, and it features
various sensor functions for detecting distortion, pH,
and temperature; it also includes an actuator functionthat can provide electricity, heat, and a light stimulus.
There is the potential to add further utilities to thefunctional film, and thus, it is expected that we will
be able to enhance clinical applications in relation to
the study and medical treatment of heart disease.
Biointerface research has been conducted at NTT
Basic Research Laboratories since the 1980s, and theresearch target is to understand neural information
processing in the brain and realize a similar bioprocess using artificial devices. As an example, we havecultured a nerve cell in an artificial environment andhave tried using various approaches to elucidate thenerve cell function and control neural growth. Thereis a synapse at the terminal of a nerve cell that transmits electrical signals to the corresponding neuron. It
is well known that synapses control information processing in the brain with respect to memory andlearning.
The fundamental mechanism of bioelectrical signaltransmission via a synapse is as follows: 1) the nerve
action potential is transmitted to the synapse of aneuron; 2) the electrical signal is converted into achemical signal (neurotransmitter), and the neurotransmitter is then released from the synapse to theoutside; 3) the neurotransmitter is received via aVol. 14 No. 8 Aug. 2016

receptor protein of the recipient cell; and finally, 4)
the chemical signal is converted back to an electricalsignal in the synapse of the recipient cell, and theelectrical signal is then propagated in the cell. Here,the synapse that releases the neurotransmitter iscalled a pre-synapse, and the other part that includes
a receptor protein is called a post-synapse. Information is coded by changing either the release amount or
the release frequency of the neurotransmitter andmodified by changing the receptor sensitivity at thesynapse.
Certain phenomena and functions regarding theexchange of information at the synapse are as yet
unexplained. Therefore, one of our research targets
involves using nanotechnology and biotechnology toartificially develop a post-synapse structure thatworks as an interface with a neural signal receiver. In
the future, we will attempt to fabricate an artificial
synapse with a simple and basic structure through thefusion of our post-synapse device and an actual nerve
cell extracted from a biological pre-synapse (Fig. 1).
Furthermore, we will explore the principles of bioinformation processing and clarify the information
signaling mechanism in the brain. We also hope tofabricate a distinctive and valuable nanobiodevice
that can work as a substitute for damaged neurotransmission elements.
3. Bio-soft material research at NTT
In our research group, we are attempting to accessfundamental bioinformation processing by utilizing ananobiointerface or nanobiodevices, which we havefabricated by combining nanotechnology and biotechnology. We are also undertaking the research anddevelopment of novel and highly sensitive biosensors
and long-term biomonitoring systems (Fig. 2). Here,
we outline the research topics of each group.
The article “Fabrication of Nanobiodevices that
Utilize the Function of Membrane Proteins” [1]explains our effort to focus on a cell membrane and amembrane protein as an interface for configuring
nanobiodevices. The artificially created cell membranes (artificial lipid membranes) are deposited on asilicon substrate containing a nano- or microscalehole array, and the composed structure constitutes thebasic and pseudo-skeletal structure of an artificial
cell. In addition, the membrane proteins, which work
as a biosignal receiver, are reconstituted in the artificial lipid membrane, and this provides a platform formeasuring a single protein function. The platform
structure works as a post-synapse in the designed
2

Feature Articles

Nanofabrication
Nanopillar
Nanohole array

Artificial synapse
Nanochannel
Pre-synapse (transmit signals)
Nerve cell
Fusion of
nano- andbiotechnology

Bio-soft material
Membraneprotein
Giant vesicle

Post-synapse (receive signals)

NeuronArtificial nanobiodevice
Open
Close

Fig. 1. Nano-bio fusion technology: artificial synapse.

Membrane protein approach

Nerve cell approach

Membrane protein based
nanobiodevice

Control of nerve growth andsynapse formation

Bio-soft material research

Biomolecular sensing approach
Graphene aptasensor• Biomimetic nanodevice• Biosensing• Study of deep bioinformation• Healthcare/life assistance• Repair damaged biofunctions• Personalized medicine

Biocompatible material
approach
Biosignal measurement with
conductive polymer based
bioelectrodeFig. 2. Bio-soft material research at NTT.

artificial synapse.
The artificial lipid membrane is a soft molecular
film with high fluidity, and its dynamic characteristics are an interesting research subject. In the article“Pattern Formation of Supported Lipid Bilayer for
Molecular Manipulation” [2], we present an originaltechnique for forming an artificial lipid membrane ona nanostructured surface in a precisely controlled3manner. Furthermore, we describe how the lipid
membrane is used as a dynamic molecule carrier.
We use nerve cells to investigate the structural control of a pre-synapse that is responsible for the signaltransmission of the artificial synapse. The article
entitled “Neuronal Growth on Artificial Structureswith Different Materials” [3] describes our study ofthe interfacial structure of nerve cells cultured on
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Feature Articles

various substrates for which we use a technique thatcombines a focused ion beam with a scanning electron microscope. The topic also includes direction
control in the process of nerve cell growth on a substrate with a nanopillar structure, which leads to thepatterning of the nerve cell network. Moreover, in“Time-lapse Imaging of Neural Morphological
Changes Relating to Cellular Functions” [4], we
describe how we have used a scanning ion conductance microscope for the time-lapse imaging of thechange in nerve cell structure in a physiological solution. Here, we have focused on an apoptotic process,which plays an important role in forming the mostappropriate and effective neural network structure.
We have clarified the correlation between morphological cell changes and the biological function by
employing live imaging of the apoptotic process.
In “On-chip Graphene Biosensor” [5], we describe
a novel biosensor based on a graphene platform
where an aptamer molecule (single-stranded DNA),
which binds with a specific protein, is bound to thegraphene surface. We have succeeded in selectively
detecting very small amounts (below 1 μL) of proteins such as cancer or blood coagulation markers by
combining a microchannel device with this biosensor.
We are currently undertaking proof-of-concept
experiments on the sensor, but our aim is to fabricate
an on-chip biosensor that can be universally used forthe qualitative and quantitative analysis of very small
amounts of biomolecules.
The article “Conductive Composite Material for
Vital Data Measurement” [6] introduces the fabrication of a flexible and non-cytotoxic bioelectrodemade of a composite consisting of silk protein and abiocompatible conductive polymer. We have used
this bioelectrode to measure the electrical propertiesof single cells, and we have also demonstrated thelong-term and stable detection of biosignals acquired
from the surface of the body.
4. Evolution to wearable bioelectrodeThe bioelectrode, which is composed of two soft

Vol. 14 No. 8 Aug. 2016

materials, was originally produced as a soft andfibrous microelectrode for recording very weak signals emitted by neurons in the brain. When a conventional rigid metal electrode is used, necrosis of thecells around the electrode occurs immediately. However, this soft bioelectrode has the great advantage ofbeing hydrophilic and biologically friendly. Therefore, we can measure the nerve action potential of anexperimental animal with little cell damage.
By expanding this knowledge and collaboratingwith Toray Industries Inc., we developed a conductive fabric material called “hitoe” based on the coupling of an advanced fiber material nanofiber and a
conductive polymer. “hitoe” has excellent flexibility,
stretchability, breathability, and biocompatibility, andit can accurately detect biological signals such asheart rate, and provide electrocardiograms (ECG) andelectromyograms (EMG) [7]. A single nanofiber isapproximately 700 nm in diameter. Thanks to the finefiber structure, adhesion with the skin is greatly
improved. In addition, the high conductivity anddurability of “hitoe” both help to achieve stable andlong-term biological signal monitoring. Moreover, by
combining “hitoe” with an undershirt, we have successfully developed a bioelectrode system that canmeasure the heart rate and ECG simply by having the
subject wear the undershirt and by monitoring theresults on a smartphone. Thanks to our collaborative
work with GOLDWIN INC. and NTT DOCOMO,
the undershirt has been marketed for sports applications (Fig. 3).
“hitoe” is expected to be used with a medical quality electrode for help in the early detection and treatment of heart disease. This is because “hitoe” canmonitor biological information comfortably and conveniently during normal daily life without placing aburden on the wearer. Our goal is to couple “hitoe”
with various wearable devices and information andcommunication technology, and we are therefore
conducting research that will allow us to utilize bioelectrodes in a wide range of fields including medicine, healthcare, worker safety, sports, and entertainment.

4

Feature Articles

R

Waveform

Measured nerve action potential
with implantable electrode74 bpmT

P

Heart rate

Q
S

Lead-off alarm(Turns red when bad connection occurs.)

Developed functional
wear combinedwith “hitoe”

C3fit IN-pulse series (GOLDWIN)“hitoe” transmitter 01 (NTT DOCOMO)
Sports use

Development andcommercialization of functional
material “hitoe” (Toray / NTT)
Bioelectrodes composed of
conductive polymer and textile fiber

Developed conductive polymer
and fiber composite material

Stable ECG measurement exceeding 24 hours
with T-shirt type wearable electrodeFig. 3. Evolution of wearable-type bioelectrode.

References
[1]

[2]

[3]

[4]

Y. Kashimura, A. Oshima, and K. Sumitomo, “Fabrication of Nanobiodevices that Utilize the Function of Membrane Proteins,” NTT
Technical Review, Vol. 14, No. 8, 2016.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201608fa2.html
K. Furukawa, “Pattern Formation of Supported Lipid Bilayer for
Molecular Manipulation,” NTT Technical Review, Vol. 14, No. 8,
2016.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201608fa3.html
N. Kasai and T. Goto, “Neuronal Growth on Artificial Structures withDifferent Materials,” NTT Technical Review, Vol. 14, No. 8, 2016.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201608fa4.html
A. Tanaka, “Time-lapse Imaging of Neural Morphological Changes

[5]

[6]

[7]

Relating to Cellular Functions,” NTT Technical Review, Vol. 14, No.
8, 2016.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201608fa5.html
Y. Ueno, “On-chip Graphene Biosensor,” NTT Technical Review, Vol.
14, No. 8, 2016.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201608fa6.html
T. Teshima, S. Tsukada, and H. Nakashima, “Conductive Composite
Material for Vital Data Measurement,” NTT Technical Review, Vol.
14, No. 8, 2016.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201608fa7.html
K. Takagahara, K. Ono, N. Oda, and T. Teshigawara, “‘hitoe’—A
Wearable Sensor Developed through Cross-industrial Collaboration,”
NTT Technical Review, Vol. 12, No. 9, 2014.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201409ra1.html

Hiroshi Nakashima

Senior Research Scientist, Supervisor, NTT
Basic Research Laboratories.
He received his B.E., M.E., and Ph.D. inapplied chemistry from Waseda University,
Tokyo, in 1995, 1997, and 2002. He joined NTT
Basic Research Laboratories in 1997. Since then,he has been engaged in research on the synthesis
and control of the optoelectrical properties of(semi) conductive polymers and nanobiomaterials. His current work includes research on biosignal monitoring using a hybrid material composedof conductive polymer and textile fiber. He is amember of the Chemical Society of Japan and
the Japan Society of Applied Physics.

5

NTT Technical Review

Feature Articles: Forefront Research on Bio-soft
Materials

Fabrication of Nanobiodevices that
Utilize the Function of Membrane
Proteins
Yoshiaki Kashimura, Azusa Oshima, and Koji Sumitomo
Abstract

Membrane proteins play vital roles in a wide variety of functions in living organisms. The development of nanobiodevices that fuse semiconductor nanotechnology and membrane proteins makes it possible to understand various biological processes as well as engineering applications. In this article, we
introduce the fabrication of nanobiodevices that are functionalized by membrane proteins and designed
to mimic the synaptic signal transmission mechanism in the nervous system.
Keywords: lipid bilayer, membrane protein, nanobiodevice

1. Introduction
Membrane proteins have attracted interest as a
post-genome target since the mapping of the humangenome was completed in 2003. Membrane proteinsthat are located in cell membranes perform a variety
of functions vital to the cell membrane. There areseveral types of membrane proteins. Most of them aretransmembrane proteins that span the membrane andin most cases work as gateways that permit the transport of specific chemicals across the membrane.
Transmembrane proteins can be classified into three
types based on their transport mechanisms: channels
that form transmembrane passive pores and transportmolecules and ions; transporters that require conformational changes when transporting substances; andpumps that move ions against a concentration gradient by consuming energy sources such as ATP (adenosine triphosphate).
Among channel proteins, ion channels that allow
ions to flow into cells play an important role in thesignal transduction in living organisms. The nervous
system responsible for informational signal transmission consists of a network of neurons. A synapse is aconnection that is formed at a narrow gap between
neurons. At this connection, an electrical signal isconverted into a chemical signal such as the release of1

a chemical called a neurotransmitter. The neurotransmitter is released from a presynaptic neuron andbinds to ion channels called receptors located in a
postsynaptic neuron. This causes the ion channels toform passive pores leading to an influx of ions into
the postsynaptic neuron, and consequently induces asynaptic potential that spans other neurons. Thus, ion
channels play pivotal roles in the signal transduction
in living organisms. Therefore, a loss or a gain of achannel function can cause brain diseases such as
depression, addiction, dementia, and anxiety disorders.
As described above, membrane proteins including
ion channels are biomolecules associated with many
physiological and behavioral functions such as drug
response, immune response, and the outbreak of adisease. Membrane protein research will become
increasingly important since it is known that morethan 60% of commercially available pharmaceuticals
are targeted at membrane proteins. The transport rate
through an ion channel is typically 106 ions per second or greater. This is interesting from the viewpointof technological applications because the channel
gating is controlled solely by the attachment of a molecule to a channel. The fusion of semiconductor
nanotechnology and membrane proteins should leadto further progress on diagnostic tools, the medical
NTT Technical Review

Feature Articles

SynapseArtificial synapse
Presynapse
Neurotransmitter

Si

Electrode

Channelprotein

Artificial lipid
membrane

Artificial
postsynapse

Channel protein

Ionic current passing
through channel proteins

Open
Close

Postsynapse
Signals are received on a microwell sealed with an artificial
lipid membrane that works as a postsynapse.

Fig. 1. Nanobiodevice that mimics the neuronal transmission mechanism.

treatment of diseases, and environmental applicationsin addition to basic research in nanobioscience.
Despite its importance, research in this fieldremains a challenge. One reason is that membraneproteins are very difficult to handle compared with
DNA (deoxyribonucleic acids), which has alreadybeen put to practical use. For example, membraneproteins denature on a semiconductor substrate orunder atmospheric conditions. In this article, we
introduce our efforts to overcome these hurdles and tocreate nanobiodevices that are functionalized bymembrane proteins.
2. Construction of artificial cells
A schematic diagram of a nanobiodevice that utilizes membrane protein functions is shown in Fig. 1.
We have designed a new type of nanobiodevice called
an artificial synapse that mimics the structure of apostsynaptic neuron, which is the signal-receiving
part of the synapse. One difficulty when using membrane proteins is that they can only function withinlipid membranes. If the proteins are simply disposedon a substrate, they are deformed as a result of an
unfavorable protein-substrate interaction, whichleads to their denaturation. Membrane proteinsaccount for half the weight of a cell membrane. The
other main membrane component is a lipid molecule,which is an amphiphilic molecule containing both ahydrophilic head group and a hydrophobic tail group.
In an aqueous solution, lipid molecules arrange themVol. 14 No. 8 Aug. 2016

selves into topologically closed structures that protect
the hydrophobic part from being exposed to the surrounding aqueous solution. This results in the formation of a lipid bilayer membrane, which is a fundamental component of a cell membrane.
Membrane proteins can diffuse laterally within alipid membrane in vivo. Our first goal was to reproduce a biological cell-like environment on a semiconductor substrate. A promising way to achieve this is
to use microwells on a Si (silicon) substrate sealedwith a lipid membrane. A sealed microwell is comparable in size to a biological cell and has a suspended
lipid membrane area that works as a cell-mimetic
field on the substrate. When membrane proteins arereconstituted within a suspended lipid membrane,
they can diffuse within the membrane and function in
the same way as those in vivo. Thus, individual
microwells can be considered the simplest model of abiological cell.
An example method to fabricate the above-mentioned artificial cell is illustrated in Fig. 2. First, lipid
vesicles (or liposomes), which are one stable form ofa lipid membrane, were prepared by electroformation[1]. We used giant unilamellar vesicles (GUVs) withdiameters in the 10–100 µm range, which should belarger than those of the microwells. We used a lithographic technique to fabricate microwells (2–4 µm in
diameter and 1 µm deep) on a silicon dioxide (SiO2)
substrate [2]. The microwells were sealed with lipid
membranes by rupturing the GUVs in an electrolyte
solution such as calcium chloride (CaCl2 ). We
2

Feature Articles

20 µm

20 µm

(a) Giant unilamellar vesicle (GUV)GUV(b) Confinement of probes in microwells

Chemical (probe)(c) GUV rupture on microwells

Fig. 2. Fabrication of microwells sealed with a lipid membrane.

confirmed that the microwells were sealed by confining fluorescent probes in them. A fluorescent imageof a lipid membrane patch labeled with red fluorescent rhodamine B, where green fluorescent calcein
was confined in the microwells, is shown in Fig. 2(b).
The fluorescence was clearly observed from the calcein confined in the microwells. This result showsthat the lipid membrane successfully seals the
microwells. Depending on our target functions, otherchemicals can be confined in the microwells using thesame protocol instead of fluorescent probes.
3. Biodevice functionalized bymembrane proteins
A functional analysis of ion channel activity
requires first reconstituting the ion channels within alipid membrane. To demonstrate this, we began byusing a-hemolysin, which is widely studied and is
known as a bacterial exotoxin with a membranedamaging function [3]. We inserted a-hemolysin
monomers into the lipid membrane and then oligomerized it to form a heptameric transmembrane porethrough which ions or small molecules pass. The
function of a-hemolysins is easily identified by theirability to pass ions through unregulated pores, thus
providing a good prototype for confirming the operating principle of the device.
For this purpose, a calcium ion (Ca2+) indicator(fluo-4) that emits green fluorescence in the presenceof Ca2+ was confined in the microwells instead ofcalcein. Then, to analyze the Ca2+ transport throughthe ion channels, a Ca 2+ concentration gradient3

between the outside and inside of the microwells wascreated by adding a CaCl2 solution to the outer solution. When a-hemolysin was added to the outer solution, it diffused into the suspended lipid bilayer andformed ion channels. Time-lapse images of the fluorescent intensity of microwells containing fluo-4
after adding the CaCl2 solution (t = 0) are shown in
Fig. 3. The obtained images reveal that most of the
microwells became bright after several minutes andtheir fluorescence intensity gradually increased. This
result indicates that Ca2+ were transported from theouter solution to inside the microwells through theion channels formed by a-hemolysin.
A fluorescence intensity analysis revealed that thedetection limit for Ca2+ transport was estimated to be
several tens of ions/s/µm2, which is much smaller
than the ion current in a standard electrophysiologicalmeasurement. This is because the small microwellvolume, which is a minimum of several hundred attoliters, induces a large change in the ionic concentration even when the ion permeation is small. This
demonstration enables us to mimic a local ion concentration change in the biological cells when we use
more sophisticated membrane proteins.
4. Toward electrophysiological detection of ionchannel activity on a device
The most widely used methods for studying ion
channel functions are related to electrophysiological
techniques that directly measure the ionic current
through channel proteins. Among them, the patch
clamp technique is the gold standard, and it can be
NTT Technical Review

Feature Articles

Membrane protein
Ion
Lipid bilayer

20 µm

20 µm

(a) 0 min20 µm(b) 5 min20 µm(d) 7 min

20 µm

Fluorescent probe(c) 6 min20 µm(e) 10 min

20 µm(f) 15 min(g) 15 min(including the fluorescenceof lipid bilayer)

Fig. 3. Functional analysis of a-hemolysin with fluorescent microscopy.

applied to cells and tissues using a glass micropipette
as a recording electrode. However, the use of an electrophysiological technique to detect an ion channel
current on a semiconductor device still remains challenging. One difficulty is that the ion channel current
is very low and comparable to the background noise(several pA). Although we succeeded in observing
Ca2+ transport through ion channels with fluorescent
microscopy using our proposed microwell device,when we try to achieve electrophysiological detection
we are faced with the problem of ion leakage throughthe water layer between the lipid bilayer and the substrate, as shown in Fig. 4 [4]. This phenomenon did
not cause any serious changes in the optical measurements; however, it is not negligible in an electrophysiological measurement, which detects a verysmall signal.
To overcome this problem, we proposed a newly
designed microwell structure that uses a self-assembled monolayer (SAM) on a gold (Au) surface toprevent ion leakage from/into the microwells, as
shown in Fig. 5 [5]. Microwells with a slightly offset
Au ring were fabricated using a lithographic technique. A SAM of octadecanethiol, which has a similar structure to a lipid, was created on the Au surface.
By rupturing GUVs on the structure, a lipid monolayer was formed with the hydrophobic tails of both
molecules facing each other to minimize unfavorable
interactions with the aqueous regions. A supported
Vol. 14 No. 8 Aug. 2016

Lipid membrane
Ca2+

SiO2

Water layer(1–2 nm)Si

Fig. 4. S chematic illustration of microwell structure sealed
by a lipid bilayer showing the passage of Ca 2+
through the water layer.

lipid bilayer was formed in the SiO2 regions of thesurface, and a suspended lipid bilayer was formed at
the microwell aperture.
The fluorescence from the rhodamine B-labeled
patch of a lipid membrane suspended over the structure is shown in Fig. 5(c). The green fluorescencefrom the microwell indicates that fluorescent probes
are successfully sealed with the lipid membrane.
There is no fluorescence on the Au ring, probably
owing to the energy transfer between dyes and gold.
The fluorescence recovery after the photobleachingobservation revealed that lateral fluidity is maintained across the Au ring. This result supports the
structural geometry predicted in Fig. 5(a).
The time course of the fluorescence intensity of theCa2+ indicators confined in the microwells when4Feature Articles

Lipid membrane
Unmodified microwell

SiO2
SiN
Au

(a)
Au ring

SiO2

Au ring

SiO2

40 µm

5 µm

SiO2
(b)

Fluorescence intensity (arb. unit)

Au

Au

(c)

Modified microwell

SEM: scanning electron microscopy
SiN: silicon nitride

Fig. 5. ( a) Schematic illustration of microwell structure.
(b) SEM image of the Au ring substrate. (c)Fluorescence from the rhodamine-B labeled
lipid membrane. The green fluorescent probe
is confined in the microwell.

0

500

1000
Time (s)

1500

2000

Fig. 6. C hange in fluorescence intensity from fluo-4
confined microwells as a function of time.

their efforts to achieve nanobiodevices.
CaCl2 solution was added to the outer solution isshown in Fig. 6. Unmodified substrates (without a Auring) caused a significant increase in the fluorescenceintensity within 20 min. However, with a modified
substrate (with a Au ring), there was no increase influorescence intensity during the observation time.
Separating the microwells and the outer regions
effectively reduced the ion diffusion through thewater layer. Toward the aim of achieving the electrophysiological detection of ion channel activity, we
estimated the membrane resistance by using a conventional patch-clamp amplifier. The membrane
resistances for the modified microwells were oneorder higher than those of the unmodified microwells,
leading to a reduction in background noise. The
improved membrane resistance and backgroundnoise for the modified microwells resulted from thereduced ion leakage. The device structure used in thisstudy therefore has the potential to be used as a platform for the very low signal-to-noise ratio measurement of ion channels.
In addition to the ion leakage problem, there arecertain other hurdles to overcome such as the purification and alignment of membrane proteins. In ourgroup, researchers from a variety of backgrounds
including physics, chemistry, and medical engineering are working hard to overcome such hurdles in
5

5. Conclusion
Despite recent developments in biological information transmission, many important problems remain
unsolved such as the synaptic plasticity associatedwith learning and the understanding of mechanisms
at the neuronal circuit level. In addition, the mechanisms of neurological disorders such as Alzheimer’s
disease, which are caused by synapse abnormalities,
have not yet been described in detail. One of our aims
for our proposed nanobiodevice is to artificiallyreplace synapses, which will allow us to access our
information transmission system. By constructing amodel synapse system in a controlled manner in an
artificial environment, it will be possible to elucidate
the mechanisms and functions in as simple a way as
possible. The acquisition of information at a single
protein level allows us to reveal the biological information transmission mechanism at the molecularlevel and, furthermore, to find ways of treating incurable diseases.
References[1]

D. S. Dimitrov and M. I. Angelova, “Lipid Swelling and Liposome
Formation Mediated by Electric Fields,” Bioelectrochem. Bioenerg.,
Vol. 19, No. 2, pp. 323–336, 1988.

NTT Technical Review

Feature Articles

[2]

[3]

K. Sumitomo, Y. Tamba, Y. Shinozaki, and K. Torimitsu, “Confinement of Fluorescent Probes in Microwells on Si Substrates by Sealing
with Lipid Bilayers,” Appl. Phys. Express, Vol. 3, No. 10, pp. 1070011–3, 2010.
K. Sumitomo, A. McAllister, Y. Tamba, Y. Kashimura, A. Tanaka, Y.
Shinozaki, and K. Torimitsu, “Ca2+ Ion Transport through Channels
Formed by a-hemolysin Analyzed Using a Microwell Array on a Si
Substrate,” Biosens. Bioelectron., Vol. 31, No. 1, pp. 445–450, 2012.

Yoshiaki Kashimura

Research Scientist, NTT Basic Research Laboratories.
He received his B.S., M.S., and Ph.D. in chemistry from the University of Tokyo in 1993, 1995,
and 1999. He joined NTT Basic Research Laboratories in 1999. From 2005 to 2006, he was avisiting professor at the Chinese Academy of
Sciences, Beijing, China. He has been researching transport properties at the single molecule
level ranging from small molecules to macrobiomolecules. His current work includes conducting
research on nanobiodevices. He is a member of
the Japan Society of Applied Physics (JSAP), theSurface Science Society of Japan (SSSJ), theInstitute of Electronics, Information and Communication Engineers, and the Material ResearchSociety.

[4][5]

S. J. Johnson, T. M. Bayerl, D. C. McDermott, G. W. Adam, A. R.
Rennie, R. K. Thomas, and E. Sackmann, “Structure of an Adsorbed
Dimyristoylphosphatidylcholine Bilayer Measured with Specular
Reflection of Neutrons,” Biophys. J., Vol. 59, No. 2, pp. 289–294,
1991.
R. Forbes, Y. Kashimura, and K. Sumitomo, “Hermetically Sealed
Microwell with a Lipid Bilayer Created Using a Self-assembled
Monolayer,” Appl. Phys. Express, Vol. 8, No. 11, pp. 117201-1–4,
2015.

Koji Sumitomo

Professor, Department of Materials and Synchrotron Radiation Engineering, University of
Hyogo.
He received his B.E., M.E., and Ph.D. in electronics from Osaka University in 1986, 1988, and1991. He joined NTT Basic Research Laboratories in 1991, where until his retirement in 2016,
he conducted research on surface science, nanobioscience, and fabrication of nanobiodevices.
He is a member of JSAP, the Physical Society ofJapan, SSSJ, the Biophysical Society of Japan,
and the Biophysical Society.

Azusa Oshima

Research Scientist, NTT Basic Research Laboratories.
She received a B.S. in physics from Nihon
University, Tokyo, in 2008, and an M.E. and
Ph.D. in biomedical engineering from Tohoku
University, Miyagi, in 2010 and 2013. She joined
NTT Basic Research Laboratories in 2013. She
has been engaged in research on nanobioscience.
She is a member of JSAP and the Japan Society
for Analytical Chemistry.

Vol. 14 No. 8 Aug. 2016

6

Feature Articles: Forefront Research on Bio-soft
Materials

Pattern Formation of Supported
Lipid Bilayer for Molecular
Manipulation
Kazuaki Furukawa
Abstract

A supported lipid bilayer (SLB) is an artificial membrane formed on a solid support. We have developed an original technique for controlling the position of an SLB using a pattern fabricated on a solid
surface. In this article, I report on two new results obtained using our technique; one involves the use of
an SLB microarray for protein detection and the other a molecular gate device that can control a few
individual molecules within an SLB.
Keywords: lipid bilayer, self-spreading, molecule manipulation

1. Introduction
Ten years have passed since we published our first
paper describing the positional control of a supported
lipid bilayer (SLB) and its application to microfluidic
devices [1]. Our achievements in the early stages
were detailed in a previous review [2]. I will focus on
new developments in this article, especially on
research using an SLB for transporting and manipulating biological molecules.
Cell membranes are formed by various biologicalmolecules. The main components of a membrane arelipids, which are molecules with both hydrophilic andhydrophobic parts. Therefore, a monolayer formedby lipid molecules has a hydrophilic surface and ahydrophobic surface. In aqueous conditions, thehydrophobic surfaces of two monolayers face eachother to form a lipid bilayer, which is the basic structure of a cell membrane. Here, the lipid molecules canbe obtained by extraction from natural products suchas egg yolk and/or by chemical synthesis.
A schematic drawing of self-spreading is shown in
Fig. 1. First, the lipid molecules are placed on ahydrophilic solid surface. The solid surface isimmersed gently in a buffer solution such as physiological saline. The lipid molecules form an SLB at theboundary between the lipid source and the solid sur1

face, and it spreads across the solid surface over time.
This phenomenon is driven by the self-organization
process exhibited by the lipid molecules. Self-spreading occurs selectively on hydrophilic surfaces. Thus,
when a solid has a hydrophilic/hydrophobic patternsurface, we can grow an SLB only on the hydrophilic
surface. This is a very important technique for NTT
Basic Research Laboratories in terms of accelerating
our research in this field [1, 3].
We observe SLBs using a fluorescence microscope.
For the observations, we add a small quantity (~1%)
of lipid molecules containing a fluorescent dye to thelipid source. Because the dye-conjugated lipid

In buffer solution
Source lipid
SLB

Lipid
Hydrophilic part
Hydrophobic part

Direction of formation

Hydrophilic surface

Fig. 1. Fabrication of SLB by self-spreading technique.

NTT Technical Review

Feature Articles

Microarray area
SiO2

Au

500 µm200 µm(a) Designed pattern(b) With lipid sources

100 µm(c) After self-spreading

Fig. 2. Fabrication of dense SLB microarray.

distributes homogeneously in the SLB, fluorescence
images correspond to the shape of the SLB.
2. Fabrication of SLB microarray
A microarray is a two-dimensional array of biological molecules on a solid surface, which is usefulfor high-throughput medical screening. The bestknown microarray is a deoxyribonucleic acid (DNA)chip. It can detect complementary DNA molecules ina sample solution on the basis of the hybridization of
DNA fixed on a solid surface. Recent research has
started to extend the technique to protein targets. Thismeans proteins instead of DNA must be fixed on asolid surface. DNA is a stable molecule, and it maintains its activity when fixed on a solid surface. However, proteins are usually delicate molecules, and theyoften lose their activity as the result of fixation. One
way to overcome this problem is to use an SLB as acushion for proteins. If we are to develop the protein
microarray on the basis of this idea, we must fabricate
an SLB microarray with a high integration density.
Our technique is useful for this purpose [4].
Our original proposed technique for fabricating a
dense SLB microarray is shown in Fig. 2. Our strategy is to use pattern-guided self-spreading for the
integration. A gold (Au) pattern fabricated on silicon
dioxide (SiO2) is shown in Fig. 2(a). Different kindsof lipid sources, which show blue, green, and red
fluorescence, are placed on the 500 × 250-μm areas(Fig. 2(b)) of the surface pattern on the solid. The
sample is then immersed in buffer solution to initiate
the self-spreading. As shown in Fig. 2(c), SLBs growalong the surface guide. At the end of the guide, we
can obtain a linear pattern formed of 10-μm-wide
SLBs with 5-μm intervals. The different colored dyes
do not mix with each other because the lipid molecules are not soluble in aqueous media.
Vol. 14 No. 8 Aug. 2016

A technique for fabricating a dense SLB microarray
was reported before our work. The technique is basedon dropping vesicle solutions from different sources.
One problem with the technique is that the droplets
must not mix with each other during fabrication.
Another problem is that of drying when the droplets
are too small. The integration limit is thus determinedby the droplet size. By contrast, SLBs are grown in asingle buffer in our process. The integration limit isthus determined by the pattern size. This is the advantage of our technique, which yields an SLB microarray with a much higher integration density than theprevious approach.
3. Protein detection by SLB microarray
We applied our microarray to protein detection
using the specific binding of streptavidin and biotin[4]. The results are shown in Fig. 3. The microarrayhas three different sets of SLBs, each formed of threelines. The first set contains 1% of nitro-benzoxadiazole (NBD)-conjugated lipid, the second set contains
1% of biotin-conjugated lipid, and the third set contains them both.
A fluorescence image of the microarray is shown in
Fig. 3(a). In the initial stage, only NBD has green
fluorescence. Thus, in Fig. 3(a) we observe green
fluorescence in the first and third sets. Here we add a
streptavidin solution. The streptavidin we used waslabeled with red fluorescent dye, and thus, the red
fluorescence must be observed from an area where
the streptavidin is present. The fluorescence image ofthe microarray after the reaction is shown in Fig. 3(b).
It can be separated into the green and red fluorescence images shown in Fig. 3(c). The red fluorescence, which corresponds to the position of streptavidin, is observed only in the second and third sets of
SLBs, which include biotin-conjugated lipid. This
2

Feature Articles

Set 1
NBD 1%

Texas redconjugatedstreptavidin

Set 2
biotin 1%
Set 3
Both

10 min(a) Before reaction(b) After reaction

Green: NBD
Red: streptavidin
Dark: biotin

u

Ti/A

u

Ti/A

SiO2
Si

(c) Color separation images of (b): streptavidin: biotin

Fig. 3. Demonstration of protein detection with SLB microarray.

confirms that our microarray works for protein detection.

SLB

Nanogap structure

SEM image
Au

4. SLB in nanostructures

Au

We can grow an SLB in the desired area by using asurface pattern as described above. Our next question
concerns how narrow the area is in which an SLB cangrow. To find the answer, we performed the following
experiments [5, 6].
We designed the new structure shown in Fig. 4. It
has a pair of gold electrodes with a 10–100 nm gap.
The SLB is about 4 nm thick, and the gap is slightly
larger than the thickness. The nanogap structure ismuch smaller than the resolution of an optical microscope. We therefore use a scanning electron microscope to observe the nanogap structure.
A self-spreading SLB passing through a nanogap isshown in Fig. 5(a). The SLB grows inside a
10-μm-wide line (1) and reaches the nanogap position (2). It continues to grow through the nanogap,
which is located in the center of the line (3). The SLB
keeps growing and reaches the other side of the
nanogap (4–6).
The most common way to fabricate SLBs is thevesicle fusion technique. However, it is difficult todetermine if the SLB exists or not when we intend to

Au

3

SiO2
V

SiO2

Au

Photoresist

SEM: scanning electron microscope

Fig. 4. Design of new pattern with nanogap structure.

fabricate the SLB in a nanostructure area such as thespace in a nanogap structure. With our self-spreading
method, it is clear that the SLB certainly grows on the
nanogap area.
5. Manipulation of molecules inside SLB
We can use our nanogap structure as a pair of electrodes because they are made of Au metal. Next, we
examined self-spreading when voltage was applied tothe electrode.
The self-spreading when –50 mV was applied
NTT Technical Review

Feature Articles(1) t = t0 − 810
Microchannel

Microchannel
10 µm

Photoresist(2) t = t0 − 390(3) t = t0

(4) t = t0 + 600(4) t = t0 + 360(5) t = t0 + 1140

(5) t = t0 + 720(6) t = t0 + 1680

(6) t = t0 + 750

10 µm

t0

(3) t = t0 + 60(a) Without voltage applied

V/mV(1) t = t0 − 540
Photoresist(2) t = t0

−50

Nanogap position0

Nanogap position(7) t = t0 + 1110

Time/s(8) t = t0 + 1470(b) With on/off voltage applied
Red fluorescence is SLB. White dotted holizontal line indicates 10-µm-wide line pattern,
and yellow dotted vertical line indicates nanogap position. The time at which SLB reachedthe nanogap position is set as t0.

Fig. 5. Self-spreading through nanogap structure.

between the electrodes is shown in Fig. 5(b) [5, 6].
The self-spreading behavior is no different from theexample shown in Fig. 5(a) until the SLB reaches the
nanogap (1–2). However, when the SLB reaches the
nanogap (3), the SLB growth stops (4). After a certain
period, the voltage is turned off (5), and the selfspreading restarts through the nanogap (6). The selfspreading after that is no different from that of theprevious experiments shown in Fig. 5(a).
We explain the results with the following model.
When an effective electric field is generated in the
nanogap by the applied voltage, the field traps themolecule inside the SLB. This blocks the flow of thelipid molecules for SLB growth. In an electrolyte
solution such as a physiological buffer, the electricfield is only effective at nanometer-scale distances(called Debye length) from the electrode surface. In
our experiments, we show that an effective electricfield can be generated by using a nanometer-scalegap.
If we reapply the voltage in Fig. 5(b)(8), we canagain stop the growth. It stops the supply of the lipid
molecules required for the growth by trapping themin the nanogap. Because the nanogap is very narrow
Vol. 14 No. 8 Aug. 2016

and the effective electric field is applied to a limitedarea, we think that few molecules are trapped. Thismeans we can control a macroscopic event such as
self-spreading simply with a small number of molecules, which is analogous to a gate in a field-effecttransistor [5]. The conditions for trapping molecules
are determined by the gap distance and the Debye
length, which varies depending on the electrolyte
concentration in the solution. We checked the effectof these parameters by performing thorough experiments and revealed the validity of our model.
6. Future perspectives
I have described some of our recent results—especially those related to biosensing applications—that
we have obtained as a result of our basic research
activities. One of the goals is to develop a protein chipusing our SLB microarray.
A technique for manipulating molecules with
nanogap electrodes can be further extended to singlemolecule manipulation. It will constitute an original
tool that will help NTT Basic Research Laboratories
to undertake pioneering work in the new nano-bio
4Feature Articles

research field.

[3]

References[1][2]

K. Furukawa, H. Nakashima, Y. Kashimura, and K. Torimitsu,“Microchannel Device Using Self-spreading Lipid Bilayer as Molecule Carrier,” Lab Chip, Vol. 6, No. 8, pp. 1001–1006, 2006.
K. Furukawa, “Artificial Cell Membrane on Patterned Surface—
Growth Control and Microchannel Device Application,” NTT Technical Review, Vol. 7, No. 8, 2009.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr200908sf4.html

[4]
[5]
[6]

K. Furukawa and H. Hibino, “Self-spreading of Supported Lipid
Bilayer on SiO2 Surface Bearing Graphene Oxide,” Chem. Lett., Vol.
41, No. 10, pp. 1259–1261, 2012.
K. Furukawa and T. Aiba, “Supported Lipid Bilayer Composition
Microarrays Fabricated by Pattern-guided Self-spreading,” Langmuir,Vol. 27, No. 12, pp. 7341–7344, 2011.
Y. Kashimura, K. Furukawa, and K. Torimitsu, “Electrostatic Control
of Lipid Bilayer Self-spreading Using a Nanogap Gate on a Solid
Support,” J. Am. Chem. Soc., Vol. 133, No. 16, pp. 6118–6121, 2011.
Y. Kashimura, K. Sumitomo, and K. Furukawa, “Electrostatic Controlof the Dynamics of Lipid Bilayer Self-spreading Using a Nanogap
Gate,” Mater. Res. Express, Vol. 1, No. 3, 035404, 2014.

Kazuaki Furukawa

Professor, School of Science and Engineering,
Meisei University.
He received a B.E. and M.E. in physics fromWaseda University, Tokyo, in 1986 and 1988, anda Dr. Sci. from the same university in 2000. He
joined NTT Basic Research Laboratories in1988. He was engaged in materials science
research with a particular focus on soft materials
including polymers and biomolecules until heretired from NTT in 2016. He is a member of theJapan Society of Applied Physics, the PhysicalSociety of Japan, the Chemical Society of Japan,
the Society of Polymer Science, Japan, and theJapanese Research Association for Organic Electronics Materials. He is currently with Meisei
University, Tokyo.

5

NTT Technical Review

Feature Articles: Forefront Research on Bio-soft
Materials

Neuronal Growth on Artificial
Structures with Different Materials
Nahoko Kasai and Toichiro Goto
Abstract

Neuronal guidance on a device is a key issue to achieve artificial synapses towards the goal of developing artificial post-synaptic structures. NTT Basic Research Laboratories has established a method for
examining the in vitro affinity of neurons to a substrate by observing in detail the interface between aneuron and a substrate surface using a focused ion beam/scanning electron microscope at the single
neuron level. With this knowledge in hand, we fabricated nanostructures using materials with a highaffinity to neurons and examined the ability to control neuronal growth on the structure. We demonstrated the possibility of achieving neuronal guidance on a nanopillar substrate of amorphous silicon,
which solves one of several difficulties related to the realization of artificial synapses.
Keywords: neuron, FIB/SEM, nanostructure

1. Introduction
Recent advances in nanotechnology and measurement technology have enabled us to obtain highly
accurate biological information at high speed. Nanotechnology has been useful not only for diagnostics
and treatments but also for developing nanobiodevices, which are nanometer-scale devices with biological functions. Nanobiodevices that utilize thesuperior functions of biological molecules such as
high selectivity and high efficiency have limitlesspotential for medical care, innovative drug development, and science applications.
There are more than 100 billion neurons in a humanbrain, and they develop their function from a complicated network by delivering signals through synaptic
connections. The goal of NTT Basic Research Laboratories is to realize an artificial synapse as a nanobiodevice by mimicking this synapse. A schematic
illustration of our artificial synapse is shown in Fig. 1.
The artificial synapse can be achieved with neurons
forming a synaptic connection with an artificial postsynapse, which can be fabricated by combining nanotechnology and biomolecules. Thus, the mechanismof the brain can be investigated from the signal ofeach synapse at the molecule and synapse level. For
1example, we can identify the membrane proteinsneeded to induce a synaptic connection, and elucidate
the mechanism of synapse signal transmission.
We require various elemental techniques if we areto achieve an artificial synapse. One such techniqueinvolves moving neurons towards the artificial postsynapse to achieve a synaptic connection. To do this,
it is necessary to prepare an environment in which
neurons can grow in vitro to adhere to the substrate,
and to elongate the neurites properly. However, toprovide an environment for a post-synapse membrane, proteins should be capable of being reconstituted into a lipid bilayer because they can function
only when reconstituted into a lipid bilayer in vivo.
This means that the device should be covered with alipid bilayer as shown in Fig. 1(a). However, neurons
cannot grow on or adhere to a substrate covered witha lipid bilayer. Therefore, at NTT Basic ResearchLaboratories, we have proposed nanopillars, which arenanostructures, as a scaffold on which the neurons cangrow without adhering to the lipid bilayer (Fig. 1(b)).
If neurons grow on nanopillars used as scaffolds, thenanopillars may control the neuronal growth direction, namely providing guidance.
We succeeded in evaluating the neuronal affinity
with the substrate material at the single cell level by
NTT Technical Review

Feature Articles

Electrode

Solution
SiO2
Lipid bilayer
Si Electrode Membrane proteins

Artificial post-synapse(a)
NeuronPre-synapse

Pillar(b)
Si: silicon
SiO2: silicon dioxideFig. 1. A rtificial synapse to communicate with neuron
using artificial post-synapse via synaptic
connection.

observing their interface by combining a scanningelectron microscope (SEM) and a focused ion beam
(FIB) to examine the substrate material for neuronal
growth control so that the material could be utilized
as a nanostructure for a scaffold.
Then, with the knowledge we obtained, we showedthat neurons grew on the nanostructures by patterningthem using the materials for which neurons showhigh affinity. In this article, we introduce our examination of the interface between neurons and a substrate, and neuronal growth control, namely guidance, using a nanostructure.
2. Evaluation of neuronal affinity and growthwith different substrates
Neuronal in vitro cultivation has been widely usedwith the aim of elucidating neuronal signaling mechanisms and for applications in the neural engineering
field. Our group has cultivated neurons on various
substrates to create interfacial devices for neuronalguidance and thus realize artificial synapses.
The interfacial state between neurons and substrates is important information in regard to controlling neuronal growth when we fabricate neuron-integrated devices. Detailed information about the interface between single neurons and substrates, espeVol. 14 No. 8 Aug. 2016

cially nanostructures, can be important in terms ofcontrolling neuronal growth on nanostructured substrates.
Several techniques can be used to observe biological samples, ranging from the use of optical microscopes through scanning probe microscopes such as
atomic force microscopes. These optical and scanning probe microscopes enable us to observe cellsurfaces in detail. However, they cannot be used todirectly observe the internal structure of the cells or
the interface between the cells and the substrate.
NTT Basic Research Laboratories has succeeded in
evaluating the interfaces between single neurons andsubstrates by using FIB milling and subsequent SEM.
FIB can process samples by accelerating a galliumion beam by high voltage [1]. This FIB with an SEM
enables us to observe both the cross-sectional andthree-dimensional structure of the neurons at a single
cell level. In this study, a cell sample was sliced withan FIB/SEM dual system, and its cross-section wasobserved very precisely [2]. This article introduces
the result of observing neurons cultivated on either agold (Au) or titanium (Ti) substrate obtained with afluorescent microscope or an FIB/SEM dual system.
Neurons were obtained from a rat cerebral cortex.
Au has been widely used in the nanotechnology field
because its surface chemical characteristics can be
modified by functionalizing the surface through selfassembled monolayers of thiols. Ti has superior biocompatibility, and its oxide is used for medical applications such as implants. Thus, the evaluation of theneuronal affinity to different materials can lead to the
optimization of neuronal growth scaffolds using
nanostructures.
Fluorescent images of neurons cultivated on Au andTi are respectively shown in Figs. 2(a) and 2(b).
Green indicates neurons stained with fluorescent dye.
A relatively large green grainy lump was observed on
the Au, suggesting the aggregation of neurons. In
contrast, the neurons on the Ti were observed to beelongated, which means they had dispersed without
aggregating. This suggests Ti is more cytophilic thanAu. To investigate the neuronal affinity expectedfrom the fluorescent results in more detail, neurons
cultivated on Au or Ti were examined using crosssectional images of the interface between the neurons
and the substrate obtained with an FIB/SEM dual
system (Figs. 2(c) and 2(d)). The neuron-Au interface exhibited partial adhesion. In contrast, almost all
the neurons adhered to the Ti interface. These results
are consistent with an assessment made using fluorescence microscopy, and the difference reflects the2

Feature Articles(b) Ti(a) AuAu surface

2 µm

2 µm

Neuronal
cross-section
Substrate cross-section(c) AuNeuronal
cross-section
Ti surface
Substrate cross-section(d) TiFig. 2. Microscopic images of neurons cultivated on Au and Ti surfaces. Red lines show the substrate surface [2].

neuronal affinity of the materials.
A fluorescent microscope can be used to evaluate
the affinity of the multiple cells, but a combination of
FIB and SEM can examine the difference in the affinity of a single neuron, and a precise evaluation can bemade of the substrate surface. An SEM can provide us
with more detailed information because of its muchhigher spatial resolution, and a detailed interfacial
examination will be possible even for nanostructures.
In the future, we will aim for an evaluation that
includes two-dimensional interfacial mapping andthat uses low temperature (cryo) FIB/SEM, which
requires fewer samples in the preparation process offixation and dehydration and thus avoids changing
the cellular topologies.
3. Neuronal guidance using nanostructures
Neuronal guidance using nanometer-scale structures including grooves, fibers, and pillars has been
attracting attention since the late 2000s, and the structural influences on neuronal adhesion and growth
have been examined [3]. However, little attention has
been paid to nanostructure materials.
As stated above, we examined neuronal affinity
with different substrates using the cross-sectional
observation of a single neuron. In this section, we
examine neuronal growth and guidance capability
using nanopillar arrays made of amorphous silicon(a-Si) and Au fabricated as scaffolds for neuronal
growth.
We fabricated nanopillars 100 and 500 nm in diameter on quartz substrates using electron-beam lithography; then we cultivated rat cortical neurons on
3them. The samples were then fixed and observed byusing either an SEM or confocal laser scanningmicroscope (CLSM) after the treatment. The firstprocess in the treatment was fixation to stabilize theprotein, which is the main structural molecule of thecell. Thus, the observed cell structures resembled
their intact structures after fixation. The sample for
SEM observation was further dehydrated and freezedried, and the sample for CLSM was also immunostained to identify the specific protein localization.
SEM images of neurites (axons or dendrites) elongated from neurons cultivated for 7 days on a-Si
nanopillars are shown in Fig. 3. The neurites wereobserved to adhere to the nanopillars, and they grewon pillars with diameters of 500 and 100 nm. Some of
the narrower pillars were bent by the neurite growth(magnified in the inset in Fig. 3(b)).
By comparing the widths of neurites cultivated on
different nanopillar patterns, we found that the neurites were significantly wider on wider-diameter pillars (Fig. 3(c)). Neurites on the wider pillars were
almost as wide as neurites on a plane substrate. Thissuggests that the size of the neuronal adhesion area
affects neurite width. The neuronal width can be
determined by the adhesion area; a neurite adhering
to a substrate promotes the expression of cytoskeletal
proteins. This result shows that 500-nm-diameter pillars allow neurites to grow in the same manner as ona plane substrate.
Then, we examined neurons on different pillar
materials, as shown in Fig. 4. Neurons grew longer
along with the patterns on a-Si pillars (Fig. 4(a), thickarrow) while they elongated randomly on both Au
pillars and a quartz substrate (Fig. 4(b)). A quantitative
NTT Technical Review

Feature Articles

180

2 µm
(a) Pillar diameter: 500 nm /distance: 1000 nm

Neurite width (nm)

160
140
120

**

100

**

80
60
40
20

N
o

pa
tte
50 rn
0/
10
50 00
0/
20
0
10 0
0/
20
10 0
0/
40
0

0

** Statistically significant difference

2 µm(b) 100/200 nm(c) Neurite width on pillars

Fig. 3. SEM images of neurites on pillars, and neurite width [4].

QuartzNeurons
Pillars
50 µm(a) a-Si pillars

Ratio in neurite tip position (%)

100
90
On
* substrate

80
70
60
50
40
30

On pillar

20
10
0
a-Si
(n=218)Au(n=168)* Statistically significant difference(b) Au pillars(c) Neurite tip positions

Fig. 4. (a)(b) Fluorescent images of neurons cultivated on pillars of different materials. (c) Neurite tip positions [4].

analysis revealed that there was a higher ratio of
neurite tips on the a-Si pillars than on the Au pillars(Fig. 4(c)) [4]. This low affinity of neurons for Au
corresponds to neuronal cross-section results obtainedusing FIB/SEM. These results demonstrate the possibility of achieving neuronal guidance using nanoVol. 14 No. 8 Aug. 2016

pillars made of appropriate materials.
4. Realizing an artificial synapse
We established a method of evaluating the in vitro
neuronal affinity to a substrate with different materials
4

Feature Articles

using a single neuron by observing neuronal crosssections. Furthermore, we demonstrated the possibility of controlling the neuronal growth using nanopillars made of high affinity materials. The neuronal
growth direction can be controlled by controlling thenanopillar array. Then, neurons can be expected to
grow towards the device, namely the artificial postsynapse, which would be fabricated in advance. Thus,in the future, we will investigate the mechanism ofsynaptogenesis by realizing an artificial synapse by
the bottom-up examination of molecules; for example, the identification of proteins (synaptogenesis
factors) for synapse formation.

[1]

Acknowledgments[4]This work was supported by Grants-in-Aid for Sci-

Nahoko Kasai

Senior Research Scientist, Materials Science
Laboratory, NTT Basic Research Laboratories.
She received a Ph.D. in applied chemistry fromTohoku University, Miyagi, in 1996. She joined
NTT Interdisciplinary Laboratories in 1996 andhas been based at NTT Basic Research Laboratories since 1998. She was a guest researcher at the
University of Oxford, UK, in 2006. She is currently working on neuronal growth control using
nanostructured substrates for artificial synapse
applications, and is also working on a wearable
electrode project. She is a member of the Japanese Society of Applied Physics, the Electrochemical Society of Japan, the Japan Neuroscience Society, and the Chemical Society of Japan.

5

entific Research (KAKENHI; Grant Number
15H03541) from the Japan Society for the Promotionof Science.
References

[2]

[3]

V. Lešer, M. Milani, F. Tatti, Ž. P. Tkalec, J. Štrus, and D. Drobne,“Focused Ion Beam (FIB)/Scanning Electron Microscopy (SEM) in
Tissue Structural Research,” Protoplasma, Vol. 246, No. 1, pp. 41–48,
2010.
T. Goto, N. Kasai, R. Lu, R. Filip, and K. Sumitomo, “Scanning Electron Microscopy Observation of Interface between Single Neurons
and Conductive Surfaces,” J. Nanosci. Nanotechnol., Vol. 16, No. 4,
pp. 3383–3387, 2016.
D. Hoffman-Kim, J.A. Mitchel, and R.V. Bellamkonda, “Topography,
Cell Response, and Nerve Regeneration,” Annu. Rev. Biomed. Eng.,
Vol. 12, pp. 203–231, 2010.
N. Kasai, R. Lu, R. Filip, T. Goto, A. Tanaka, and K. Sumitomo,“Neuronal Growth on a-Si and Au Nanopillars,” Electrochemistry,
Vol. 84, No. 5, pp. 296–298, 2016.

Toichiro Goto

Research Scientist, Materials Science Laboratory, NTT Basic Research Laboratories.
He received a Ph.D. in molecular devices from
Shizuoka University in 2011. He was originallywith NTT Micro System Integration Laboratories, where he studied micro-optical devices for
optical communications. In 2003, he joined NTT
Basic Research Laboratories and studied nanoelectrodes for use in molecular electronics. His
current interest is the fabrication of nanostructures for neural growth control. He is a memberof the Japanese Society of Applied Physics andthe Surface Science Society of Japan.

NTT Technical Review

Feature Articles: Forefront Research on Bio-soft
Materials

Time-lapse Imaging of NeuralMorphological Changes Relating to
Cellular Functions
Aya Tanaka
Abstract

The human body is composed of trillions of cells. The association between the cells with specialized
functions is important in the formation of the skeleton and organs, whose integration is vital for humanactivity. The nervous system plays a central role in effectively integrating the functions of the skeleton
and organs. Here, we introduce research aimed at understanding the relationship between neural function
and morphology in the early stages of apoptosis, which plays an important role in the process of neuralnetwork formation.
Keywords: bioimaging, SICM, neuron

1. Introduction
The human body is composed of several trillion
cells. The association between the cells with specificfunctions plays a vital role in the formation of theskeleton and organs, whose integration is crucial forhuman activity. The nervous system plays a central
role in integrating their functions so that they operate
effectively. The nervous system is classified into twoparts: the central nervous system, which is composedof the brain and spinal cord, and the peripheral nervous system, which is composed of the nerves
throughout the body. In the nervous system, external
stimuli detected by the peripheral nervous system aretransmitted to the central nervous system. The central
nervous system processes the information and sends
the responses to the peripheral nervous system. For
the nervous system to operate efficiently, a neurotransmission pathway network must be formed
between neurons or between neurons and other cells.
Therefore, it is important to understand the processes
that form a neural network.
There are two main processes involved in the formation of a neural network inside the brain. One is
the neural maturation of individual neurons; thismaturation provides functions for neurotransmission
1and the formation of a platform for neurotransmission, namely a synapse, between cells during neuralnetwork formation. The neural maturation processes
can be classified into five stages based on establishedmorphological criteria (Fig. 1). These consist of
immature neurons lacking neurites (stage 1), neurons
with multiple short neurites without establishedpolarity (stage 2), and neurons that have achieved
polarity and that possess neurites considerably longer
than the rest, namely axons, which act as transmission paths (stage 3). The remaining neurites mature as
dendrites with functions for receiving neural information from axons (stage 4). With further maturation,
the axon and the dendrites elongate to form synapses
among target cells (stage 5).
The other process is the formation of a neural network. One of the key biological events in this process
is apoptosis. Apoptosis is defined as programmed cell
death. In a developing nervous system, apoptosis
occurs as a homeostatic mechanism to maintain cell
populations in a neural network. It is a highly regulated and controlled process, and apoptotic cells arequickly eliminated without causing any damage tosurrounding cells. In the neural network formation
process, target cells secrete signals such as neural
growth factors, which are essential for neural survival.
NTT Technical Review

Feature Articles

NeuriteNeuriteAxon

Dendrite

Axon

Dendrite

Axon

Growthcone

Fig. 1. Morphological changes in neural network formation process.

Glass pipette

Electrolyte solution

Electrode

Cell

Fig. 2. Experimental system overview of SICM.

Neurons that successfully receive the signals continue growing in the direction of the target cells. In
contrast, neurons that fail to receive the signals are
eliminated from the network formation process via
several biological processes. If we are to understandthis apoptotic process, we must investigate cellularfunctions at molecular and morphological levels.
2. Observation of biological samples by
microscopy
Cellular morphologies are observed using someform of microscopy. Fluorescence microscopy isused to visualize biological samples by attachingfluorescence probes to them. This technique helps us
to visualize the interaction between biological samples during biological events, although it is limited interms of imaging morphological detail. Electron
microscopy has been the conventional method for
obtaining structural information about cells withhigher resolution than optical microscopy, but it
requires a fixed sample [1]. Therefore, it is impossible to obtain dynamic morphological images of living
cells. One effective dynamic imaging technique,Vol. 14 No. 8 Aug. 2016

scanning probe microscopy [2, 3], and specifically,scanning ion conductance microscopy (SICM) [4], is
expected to be a powerful tool for obtaining imagesof living cells (Fig. 2). This technique uses a glass
pipette filled with an electrolyte that senses an ion
current and feeds back its position relative to samples
completely immersed in a liquid buffer containing
electrolytes. Since the tip-sample distance is maintained at the radius of the pipette during the scan bykeeping the ionic current constant, SICM enables
stable and non-contact imaging of soft and sticky
biological samples at a resolution of better than 100 nm.
Because the application of external stimuli to living
cells may induce cell death, SICM will allow us toimage the morphological dynamics of living cells.
3. Neurons during apoptosis
As described above, apoptosis plays an importantrole in the neural network formation process. It
exhibits a distinct set of the biological changes
including apoptotic volume decrease, the formationof cell buds or membrane blebs, and phosphatidylserine (PS) translocation. The cell membrane is2

Feature Articles

0 min

60 min

120 min180 min(a) Time-lapse images of neural morphological changes observed by SICM
1.4

8 0 min
6

180 min

1.2
v/v0

Height (µm)

10120 min

4
2
0

1
0.8
0.6

0

5
10
15
Distance (µm)(b) Cross-sectional graph of (a)

0.4

20

0

60
120
Time (min)180(c) Time-course volume change of single neurons

Fig. 3. Morphological changes in neurons in early stages of apoptosis.

composed of the inner and outer leaflets of a lipid
membrane. PS normally resides in the inner leaflet ofthe cell membrane and is translocated from the inner
to the outer leaflet in the early stages of apoptosis.
This PS translocation is considered to be a key biological event in relation to phagocytosis. As regards
morphological changes during the early stages of
apoptosis, the cell volume decreases, and membrane
blebs are subsequently formed on the surface of thecell membrane. Despite the importance of these
apoptotic morphological changes, no one has yet
revealed the morphological dynamics at a resolutionof better than a micrometer, or revealed the relationship between biochemical changes such as PS translocation and the cellular morphology. SICM is asuitable technique for investigating a series of thesechanges because it allows us to observe the morphology of living cells without any mechanical interactions between the probe and the sample surface witha high resolution. In this article, we discuss the morphological changes that occur in neurons in the earlystages of apoptosis including apoptotic volume
decrease and membrane blebbing. In addition, we
explain our investigation of the relationship between
membrane blebbing and PS translocation. Additionally, we use the results of these experiments to discuss the order of morphological dynamics during
apoptosis and the relationship between biochemical
and morphological changes [5].
3

4. Bioimaging of neurons
We investigated the morphological changes in
apoptotic neurons by exposing cultivated neurons tostaurosporine (STS), which is an apoptosis inducer.
The morphological changes in a single neuron are
shown in Fig. 3(a), and line scans for the SICM
image indicated by the dashed line in Fig. 3(a) are
shown in Fig. 3(b). In the first 120 min after exposure
to STS, bulge-like structures known as membrane
blebs were observed at the neuron (Fig. 3(a), whitearrows). The number and size of the blebs increased
with time. A graph of the volume change over time asa ratio of volume to initial volume, v/v0, shown in
Fig. 3(c), was obtained from five neurons, and individual time-course images were obtained in an independent experiment. The v/v0 ranged from 0.6 to 0.8,
meaning that the volume decreased from 20 to 40%
in the first 120 min after exposure to STS. Once thevolume reached its minimum value, it increased
again, and membrane bleb formation was observed.
The SICM results indicate that the time course of themorphological changes begins with an apoptotic volume decrease, followed by membrane blebbing.
We used fluorescence microscopy to investigate therelationship between membrane blebbing and PS
translocation by detecting the PS translocation with
Annexin V, which binds to the PS residing in the outerleaflet of the cell membrane in the presence of calcium
NTT Technical Review

Apoptosis

Ca2+

Ca2+ Ca2+

Annexin V

Annexin V

Ca2+

Annexin V

Ca2+

Annexin V

Annexin V

Ca2+

Annexin V

Feature Articles

Cell membrane

PS

Fig. 4. Detection of PS translocation using Annexin V.

25
Cell number ratios (%)

ions. Normally, PS exists in an inner leaflet of the cell
membrane. Apoptosis induces the translocation of PS
from the inner to the outer leaflet without changing
the permeability of the cell membrane. This translocation can be detected via binding with fluorescently
labeled Annexin V (Fig. 4). After the neurons were
exposed to STS, we monitored the binding of theAnnexin to the neurons (Fig. 5). The Annexin V-positive ratio without exposure to STS ranged from 0%
to 10% during 180 min of observation, whereas theratio with the exposure to STS increased more than
20%. This indicates that PS translocation induced by
STS occurred around 120 min after exposure to STS.
These results obtained by SICM and fluorescencemicroscopy show that apoptosis induces a reductionin cellular volume and subsequent membrane blebbing. Moreover, the membrane blebbing had a similar
onset time to the PS translocation. It has been difficult
to observe the time series of morphological changes
in living cells, especially that of soft and deformablestructures. A non-contact imaging technique such as
SICM provides us with the morphological details ofneurons in the early stages of apoptosis.

10With STS

Without STS

5

0

60

120
Time (min)180

Fig. 5. Ratios of cells binding to Annexin V.

insight into the relationship between individual biological events and applications to information technologies and medical technologies.
References
[1]

[2]

Vol. 14 No. 8 Aug. 201615

0

5. Future perspectives
The Molecular and Bio Science Research Group at
NTT Basic Research Laboratories is focusing on fabricating a nanobiodevice based on biological systems.
To achieve this, it is essential to understand how
many biological systems are orchestrated insideorganisms as well as to investigate individual biological materials and cells. The combination of conventional and new imaging techniques such as fluorescence microscopy and SICM will provide new

20

[3]
[4]
[5]

N. Morone, T. Fujiwara, K. Murase, R. S. Kasai, H. Ike, S. Yuasa, J.
Usukura, and A. Kusumi, “Three-dimensional Reconstruction of the
Membrane Skeleton at the Plasma Membrane Interface by Electron
Tomography,” J. Cell Biol., Vol. 174, No. 6, pp. 851–862, 2006.
R. J. Driscoll, M. G. Youngquist, and J. D. Baldeschwieler, “Atomicscale Imaging of DNA Using Scanning Tunneling Microscopy,”
Nature, Vol. 346, No. 6281, pp. 294–296, 1990.
G. Binning, C.F. Quate, and C. Gerber, “Atomic Force Microscope,”
Phys. Rev. Lett., Vol. 56, No. 9, pp. 930–933, 1986.
P. K. Hansma, B. Drake, O. Marti, S. A. Gould, and C. B. Prater, “TheScanning Ion-conductance Microscope,” Science, Vol. 243, No. 4891,
pp. 641–643, 1989.
A. Tanaka, R. Tanaka, N. Kasai, S. Tsukada, T. Okajima, and K.
Sumitomo, “Time-lapse Imaging of Morphological Changes in aSingle Neuron during the Early Stages of Apoptosis Using Scanning
Ion Conductance Microscopy,” J. Struct. Biol., Vol. 191, No. 1, pp.
32–38, 2015.

4

Feature Articles

Aya Tanaka

Research Scientist, Materials Science Laboratory, NTT Basic Research Laboratories.
She received her B.S., M.S., and Ph.D. inchemistry from Hokkaido University in 2004,
2005, and 2008. During 2008–2009, she was a
post-doctoral fellow at Newcastle University,
UK. In 2009, she joined NTT basic ResearchLaboratories and engaged in research on fabrication of nanobiodevices. She is a member of theSociety of Polymer Science, the Chemical Society of Japan, and the Japan Society of Applied
Physics.

5

NTT Technical Review

Feature Articles: Forefront Research on Bio-soft
Materials

On-chip Graphene Biosensor
Yuko Ueno
Abstract

This article describes our recent research on an on-chip biosensor that uses the surface of graphene,
an atomically thin carbon sheet modified with specific DNA (deoxyribonucleic acid) molecules, whichbinds selectively to a target molecule. Biologically important proteins such as cancer markers can bedetected simply by adding a sample solution smaller than 1 microliter to the sensor chip. Quantitative
detection of multiple proteins on a single graphene biosensor chip is demonstrated by using a microchannel configuration. Our findings have the potential to lead to a simple assay technique for disease markers.
Keywords: graphene, biosensor, protein

1. Introduction
Graphene is an atomically thin two-dimensional
sheet consisting of a hexagonal honeycomb lattice ofcarbon atoms. It has been attracting great attention
owing to its unique properties, which include
extremely high mechanical strength, thermal conductivity, electronic mobility, and thermal and chemical
stability [1]. Graphene was isolated in 2004 by Profs.
Geim and Novoselov of the University of Manchester, who won the 2010 Nobel Prize in Physics for theirpioneering work. The mass production of graphene
was difficult at that time but has progressed greatly. It
is now possible to obtain a monolayer of graphene
several tens of centimeters square by using synthetic
techniques typified by the chemical vapor deposition
process.
At the same time, graphene oxide (GO), the mostwidely known chemical derivative of graphene, isalso attracting considerable interest. GO is an oxidized form of graphene, and it also has an atomicallythin sheet-like structure, which contains nanometersized graphene-like domains. In GO, many of thebonds between the carbon atoms in graphene are broken and link with oxygen to form carbon-oxygenbonds. GO is prepared very differently from graphene
and is rather easy to mass-produce. We can chemically synthesize GO by oxidizing graphite powder instrong acid. This yields a large quantity of GO in a
glass container; however, it is difficult to obtain synthesized GO larger than a millimeter square. There1

fore, GO is not suitable as a replacement for graphene
in electronic materials that require high mobility, but
is applicable to optical devices and sensing materials[2].
On the surface of graphene (or the graphene-likedomains in GO), energy transfer occurs when molecules are located close to the surface. The energy
transfer yield depends on the degree of molecularinteraction between the adsorbed molecules and thegraphene surface. For example, when a fluorescentmolecule such as a dye is located very close to thegraphene surface, the dye does not exhibit fluorescence. Here, graphene works as an excellent acceptor
for fluorescence resonance energy transfer (FRET)
over the entire visible wavelength region. By using
the energy transfer reaction on a graphene surface, we
can visualize biological/chemical reactions by converting those invisible molecular behaviors into themeasurable physical quantities such as light and electricity. This makes graphene a promising material fora novel biosensor.
We have proposed and developed a unique type ofbiosensor, which works on a graphene surface, bymodifying it with a specific deoxyribonucleic acid
(DNA) called an aptamer for the detection of biologically important proteins such as cancer markers(aptasensor, aptamer-based biosensor). We call asolid surface functionalized by biological moleculesa biomolecular interface. First, we achieved biomolecular interfaces using GO and confirmed their usefulness for protein detection. We also fabricatedNTT Technical Review

Feature Articles

Target protein is selectively
adsorbed by aptamer.

Graphene quenches emission
energy of fluorescence dye.
Linker connects
graphene and aptamer.

Fluorescence isobserved when dye
leaves graphenesurface.

Aptamer has strong
affinity to graphene.
Fluorescence dye does not emit
fluorescence when located close tographene surface.
(a) Without target protein(b) With target protein

Fig. 1. Design of protein detection system with graphene aptasensor.

on-chip sensors by combining the system with microfluidic techniques. Then we demonstrated protein
detection by replacing GO with graphene to achieve
an aptasensor with better performance than one withGO.
2. Mechanism of graphene aptasensor
Our graphene aptasensor uses a graphene surface
modified with an aptamer, which is a selected singlestrand (ss) oligonucleotide that binds to a specifictarget. One end of the aptamer is labeled with a fluorescent dye, and the other end is connected to apyrene linker molecule, which shows a strong affinity
to the graphene (graphene-like domains in the case ofGO). Thus, the aptamer is firmly fixed to the graphene surface. The graphene aptasensor detection
process is as follows. In the initial stage, the dyeconjugated aptamer is adsorbed on the graphene surface via physical adsorption (π-π interactions), andthus, the dye is located close to the graphene surface.
Here, the fluorescence of the dye is well quenched bygraphene via FRET and is barely observable(Fig. 1(a)). If the target of the aptamer is present in
the system, the aptamer forms a complex with thetarget and leaves the graphene surface. At the sametime, the dye molecule also leaves the graphene surface, and the dye recovers its fluorescence (Fig. 1(b)).
We can detect the target molecule by using the fluorescence [3].
Aptamers have a wide variety of targets and offer
many advantages as molecular recognition probes.
Aptasensors are generally versatile because they canbe extended to the detection of many different targets
by replacing the aptamers. We confirmed the versatilVol. 14 No. 8 Aug. 2016ity of our on-chip graphene aptasensor by using it todetect three different proteins, namely, thrombin (ablood clotting marker), prostate specific antigen(PSA; a cancer marker), and hemagglutinin (an antigenic glycoprotein found on the surface of influenza
viruses), simply by changing the aptamers but retaining the same sensor composition. Moreover, aptamers can be flexibly designed without loss of bioactivity. Thus, we can design and construct various kindsof biomolecular probes by conjugating additional
functions with an aptamer. Lastly, aptamers are
chemically stable. We confirmed that the aptasensors
that had been stored at a normal temperature andpressure for more than a month operated normally as
if they had just been made.
3. Multichannel configuration for quantitative
detection and linear-array for multiple
protein detection
We first studied the sensing performance of ouraptasensor by using GO and confirmed their usefulness for protein detection. An advantage of using an
on-chip sensor is that it enables us to realize a parallel
analysis system such as a sensor array. A multichannel system allows us to make a precise quantitative
sample-reference comparison. It also enables us toperform simultaneous observations of the sample andreference signals, which can be used to eliminate theeffect of fluorescence degradation caused by laser
exposure and other noises. This is an advantage of our
on-chip graphene aptasensor. Moreover, the on-chip
aptasensor requires no out-of-chip processes such aslabeling or mixing the sample, and thus, the humanerrors that can occur during the sample preparation2

Feature Articles

100 µg/mL
PSA 100 µg/mL

50 µg/mL

Albumin 50 µg/mL

30 µg/mL

Pure water

20 µg/mL
Pure water

500 µm

100 µm100

500 µmFluorescence intensity(arb. units)(a) Selectivity

100 µm100

Fluorescence intensity(arb. units)(b) Concentration dependence

Fig. 2. Selectivity and concentration dependence of PSA detection.

procedure can be minimized. We fabricated an onchip multichannel aptasensor by placing a polydimethylsiloxane sheet with microchannels on a solid substrate, and we formed the graphene aptasensor on its
surface. The target protein was detected in about aminute simply by adding a sample solution smaller
than 1 microliter to the sensor chip. We measured thefluorescence emitted from the graphene surfaces
located in each microchannel simultaneously andcompared the intensities of the channels. For comparison, a commercially obtained ELISA (enzymelinked immunosorbent assay) kit—one of the mostcommon protein detection methods—requires asample volume of at least 100 microliters and anassay time of three hours.
We can fabricate a microchannel with the desired
design using photolithographic techniques. The number of channels is also variable. Here we prepared an
on-chip aptasensor with a triple microchannel configuration. We measured the fluorescence images
when PSA, human albumin solution, and water were
injected into the top, middle, and bottom channels,
respectively (Fig. 2(a)). Albumin, the most abundant
protein in human blood plasma, caused no change inthe fluorescence intensity, just as with water. This
proved the selectivity of our aptasensor for PSA
detection. We also examined the dependence of thefluorescence intensity on the PSA concentration(Fig. 2(b)). We used a quintuple microchannel configuration and injected PSA solution with four different concentrations and water as a reference into eachmicrochannel. The results showed that the fluorescence intensity became weaker as the PSA concentration decreased.
3

We fabricated a 2×3 linear-array aptasensor byusing two different aptamers for different targets,
namely thrombin and PSA, which we labeled with
red and green fluorescent dyes, respectively(Fig. 3(a)). Bright fluorescence was only observed in
the areas where we assumed that the correct aptamertarget pair had been formed (Fig. 3(b)). The simultaneous detection of multiple target molecules on asingle chip was successfully demonstrated [4].
4. Molecular design for enhanced sensitivity
The most interesting feature of aptasensors is that
we can design and construct various kinds of biomolecular probes by incorporating additional functions
with an aptamer. We can improve the sensitivity of an
on-chip GO aptasensor by modifying an aptamer
with an ssDNA spacer (Fig. 4(a)). The strategy was
to increase the distance between the fluorescence dye
and the graphene surface, which is crucial for FRETbased sensors, when forming a complex with the target protein. We fabricated a 2×3 linear-array GO
aptasensor by using three different probes and introducing ssDNA spacers with 0, 10, and 20 thymine
segments between the aptamer and the dye. The fluorescence intensity increased significantly withincreases in the spacer length (Figs. 4(b) and 4(c)).
The limit for thrombin detection was about 1 nM,
which corresponds to the in vivo concentration range
during blood clotting by using the probe with 20 thymine segments, the best design in our present study.
The results showed that introducing an ssDNA spacer
at the correct position is an effective way of enhancing sensor sensitivity [5, 6].
NTT Technical Review

Feature Articles

Aptamer pattern

Sample solution

Control

Thrombin
aptamer-red dye

PSA
aptamer-greendye

PSA

Thrombin
500 µm(a) Layout of 2×3 linear array sensor(b) Fluorescence image of protein detection

DNA spacer
Fluorescence dye

T0

T10

T20

AptamerThrombin

Pure water

500 µm(a) Design of molecular probe(b) Dependence of spacer length

Fluorescence intensity (arb. units)

Fig. 3. Demonstration of simultaneous multiple protein detection.

200
150

1:4:6

100
50
0
T0

T10

T20

(c) Comparison of fluorescence intensity

Fig. 4. Enhanced sensitivity by using molecular probe modified with DNA spacer.

5. Aptasensor built on continuous single-layer
graphene surface
In our GO aptasensor, we used a pyrene linker thatshowed a strong affinity with the graphene-likedomain to immobilize the aptamer. However, the surface area of the graphene-like domain reached atmost 50% that of a GO flake. By changing the platform from GO to graphene, we were able to achievea solid surface with a 100% graphene structure.
We prepared a PSA aptasensor on a commerciallyavailable single-layer graphene surface fixed on asolid substrate. We confirmed that the substrate wasVol. 14 No. 8 Aug. 2016

almost fully covered with a single layer of graphene
by observing optical and Raman images before measuring the sensing performance. The fluorescenceintensity was initially small (Figs. 5(a) and 5(b), A),
increased steeply after PSA was added (B), and thendecreased rapidly as water was added to dilute thesample solution (C - E). It is noteworthy that the fluorescence intensity was almost homogeneous in theobserved area of more than 100 µm × 100 µm. The
maximum fluorescence intensity was larger than thatof a GO aptasensor prepared in the same manner;namely, the ratio reaches 3.3. Thus, we can improve
the sensitivity by changing the sensing platform from4

Feature Articles

A

B

C

D

E

50 µm
(a) Microscope image and fluorescence images for PSA detection

Fluorescence intensity(arb. units)

Fluorescence intensity(arb. units)

B
C

D
A
E

0

200

400
Time (s)600

800(b) Change in fluorescence intensity of graphene
aptasensor with PSA concentration0

5 10 15 20 25 30 35
PSA concentration (µg/mL)(c) Plot of average fluorescence intensity
against PSA concentration

Fig. 5. PSA detection using graphene aptasensor.

GO to graphene. An investigation of the mechanism
behind the improvement in sensitivity is ongoing [7].

[4]References

[5]

[1]
[2]

[3]

C. N. R. Rao and A. K. Sood, “Graphene: Synthesis, Properties, andPhenomena,” 1st ed., Wiley-VCH, Weinheim, 2013.
K. Furukawa and Y. Ueno, “Biosensing on a Graphene Oxide Surface,” NTT Technical Review, Vol. 11, No. 8, 2013.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201308fa7.html
K. Furukawa, Y. Ueno, E. Tamechika, and H. Hibino, “Protein Recognition on a Single Graphene Oxide Surface Fixed on a Solid Support,”

[6]

[7]

J. Mater. Chem. B, Vol. 1, No. 8, pp. 1119–1124, 2013.
Y. Ueno, K. Furukawa, K. Matsuo, S. Inoue, K. Hayashi, and H.
Hibino, “On-chip Graphene Oxide Aptasensor for Multiple Protein
Detection,” Anal. Chim. Acta, Vol. 866, pp. 1–9, 2015.
Y. Ueno, K. Furukawa, K. Matsuo, S. Inoue, K. Hayashi, and H.
Hibino, “Molecular Design for Enhanced Sensitivity of a FRET Aptasensor Built on the Graphene Oxide Surface,” Chem. Commun., Vol.
49, No. 88, pp. 10346–10348, 2013.
Y. Ueno, K. Furukawa, A. Tin, and H. Hibino, “On-chip FRET Graphene Oxide Aptasensor: Quantitative Evaluation of Enhanced Sensitivity by Aptamer with a Double-stranded DNA Spacer,” Anal. Sci.,
Vol. 31, No. 9, pp. 875–879, 2015.
K. Furukawa, Y. Ueno, M. Takamura, and H. Hibino, “Graphene
FRET Aptasensor,” ACS Sens., Vol. 1, No. 6, pp. 710–716, 2016.

Yuko Ueno

Senior Research Scientist, Supervisor, NTT
Basic Research Laboratories.
She received her B.S., M.S., and Ph.D. inchemistry from the University of Tokyo, in 1995,
1997, and 2002. She joined NTT Integrated
Information & Energy Systems Laboratories in
1997. She was a post-doctoral fellow at the University of California at Berkeley and at Lawrence
Berkeley National Laboratory, Berkeley, CA,
USA, from 2004 to 2005. She received the
Encouragement Award (award for a young scientist) from the Japan Society for Analytical Chemistry (JSAC) in 2008. She is a member of JSAC,
the Chemical Society of Japan, the Spectroscopical Society of Japan, the Japan Society ofApplied Physics, the Surface Science Society ofJapan, and the Society for Chemistry and MicroNano Systems.

5

NTT Technical Review

Feature Articles: Forefront Research on Bio-soft
Materials

Conductive Composite Material for
Vital Data Measurement
Tetsuhiko Teshima, Shingo Tsukada,and Hiroshi Nakashima
Abstract

As the interest in healthcare has grown in recent years, the technologies needed to monitor vital data
have been attracting a lot of attention with a view to disease diagnosis and prevention as well as lifestyle
improvement. At NTT Basic Research Laboratories, we are working on the research and developmentof conductive composite materials by coating or mixing conductive polymer with biocompatible substrates. We utilize these materials as electrodes for measuring biological signals from, for example, theskin or the heart, with high biocompatibility and sensitivity. We report here on composite electrodes
made of conductive polymer (PEDOT:PSS) and silk substrates that are used when measuring biomedical
signals via human tissue.
Keywords: biocompatible material, conductive polymer, vital data

1. Introduction
In recent years, rapid diagnosis and early-stage
treatment of disease have become important with theaim of reducing the risk of contracting serious diseases. In particular, there is a strong need to develop
methods for continuously monitoring heart rate andfor measuring electrocardiographic (ECG) waveforms in order to prevent heart diseases such as heart
attack and arrhythmia. Conventionally, metal electrodes and metal-plated fibers have been widely used
to measure heart rate and ECG waveforms. However,metal-based electrodes lack flexibility and biocompatibility. As a result, the obtained signals are often
affected by high noise, and the patient’s skin may beallergic or sensitive to metal. Furthermore, electrolyte
paste is needed to attach medical electrodes to theskin surface, and this may induce a rash and/or itching during long-term use.
Our group has focused on the use of PEDOT:PSS(poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)) as an electrode material that is non-toxic andstable in wet conditions. This molecule, which consists of a π electron-conjugated compound (PEDOT)
and a polyelectrolyte (PSS), is widely used as a trans1

parent electrode for touchscreens and displays, and isa replacement for rare metals such as indium tin oxide(ITO). We have been taking advantage of the highhydrophilicity and biocompatibility of PEDOT:PSS
to carry out research and development of electrodes
designed to measure the action potential of nerve
cells [1]. In this article, we introduce biocompatible
electrodes that we achieved by integrating
PEDOT:PSS with a base substrate such as fibers,
without using a metal-based electrode and electrolyte
paste.
2. Conductive silk fiber
The high hydrophilic property of PEDOT:PSS molecules means that they swell into a gel in humid environments. Therefore, these molecules exhibit reduced
mechanical stiffness, water resistance, and processability, leading to limitations in their use. We avoided
these limiting factors by immobilizing PEDOT:PSS
on the surface of silk fibers in an electrochemicalmanner [2].
Images of silk bundles coated with PEDOT:PSS are
shown in Fig. 1(a). We can clearly observe themicrometer-scale fine structure of silk fibers thatNTT Technical Review

Feature Articles

PEDOT:PSS/silk electrode(w/o electrolyte paste)

Commercially available
medical metal electrode(w/ electrolyte paste)

Commercially available
silver-plated nylon
electrode

Base line oscillation in ECG signals by respiratory body motion
20 µm(a) Optical microscope (above)
and SEM (below) imagesof silk fiber bundle coatedwith PEDOT:PSS(b) ECG measurement of rats using PEDOT:PSS/silkelectrodes, conventional metal-based medical electrodes,
and silver-plated nylon electrodes

SEM: scanning electron microscope

Fig. 1. PEDOT:PSS/silk composite electrode.

were thinly and uniformly coated with PEDOT:PSS.
When we employed PEDOT:PSS/silk electrodes for
ECG measurements with experimental animals, electrodes located on the surface of the skin provided us
with ECG data that were comparable to data obtainedwith conventional metal-based electrodes (Fig. 1(b)).
In contrast, it is difficult for the silver-plated nylon
electrodes to make good contact with the skin surface. There was also a baseline oscillation in the ECG
signals caused by respiratory body motion, which
made it difficult to obtain a stable measurement of the
ECG waveform. The water-repellent surface of conventional metal-based electrodes caused this difficulty, whereas the PEDOT:PSS/silk composite electrodes with high hydrophilicity meant that the moisture was rapidly absorbed by the fiber, as shown in
Fig. 2.
The hydrophilic electrodes tended to absorb sweat
and water vapor from the skin, which increased theirflexibility and adhesion, making it possible to perform stable measurements. The PEDOT:PSS coating
technology is not only applicable to silk fibers but
also to a wide range of fibrous materials including
polyester and nylon.

Vol. 14 No. 8 Aug. 2016

3. Conductive silk hydrogel electrodeTo broaden the application range of the PEDOT:PSS/
silk composite, we are developing silk substrates withvarious shapes by solubilizing and molding silk
fibers. When this technique was applied to a solutionof silk fibroin obtained by chemically treating silk
fibers, the solution retained its biocompatibility,hydrophilicity, and processability (Fig. 3(a)). The
water solubility of both silk fibroin and PEDOT:PSS
enabled them to mix and disperse well, yielding composites with high hydrophilicity.
The fibroin protein gelation process with alcohol
produced conductive silk-hydrogel electrodes with
arbitrary shapes. We employed this process to fabricate a cell-electrode interface for electrical measurements and for the activation of an adherent cellthrough a photolithographic technique, as shown in
Fig. 3(b). A thin-film electrode was formed into thedesired shape applicable for cell morphology andfunction, and it possessed sufficient stiffness to withstand the cell traction force (Fig. 3(c)). By culturing
the cells on the surface of films, we were able to collect the cell-laden films and manipulate them into thedesired position. When the electrodes were inserted
inside the silk hydrogel electrodes, voltage was
2

Feature Articles(a) Silver-plated nylon electrode(b) PEDOT:PSS/silk composite electrodeFig. 2. Hydrophobic silver-plated nylon electrode (a) and hydrophilic PEDOT:PSS/silk composite electrode (b).

(a) Silk fibroin protein solubilized
in solution(b) Micro-patterned PEDOT:PSS/silk films(c) Manipulation of cell-laden film

Fig. 3. Micrometer-sized PEDOT:PSS/silk fibroin electrodes.

applied to the entire targeted film. By using thismethod, we electrically activated the cells to specifically stimulate the voltage-dependent channels
expressed on the cells. In the future, we will employ
this method to measure the cell action potential ofvarious cells and tissues.
The addition of a photocurable hydrogel to the basematerial enabled us to produce a large-scale silkhydrogel based electrode by light irradiation. An
3image of PEDOT:PSS/silk hydrogel arrays that were
patterned on extremely flexible silk hydrogel thinfilm with high hydrophilicity and biocompatibility isshown in Fig. 4(a).
Changing the shape of the mold to fill the precursor
during light irradiation made it possible to build an invivo implantation hydrogel electrode with the desired
three-dimensional shape such as a fiber or a suction
cup. The fabricated hydrogel electrodes exhibited
NTT Technical Review

Feature Articles(b) Attachment of silk hydrogelelectrodes on three-dimensional
heart tissue-like structures(a) Thin silk hydrogel electrode withembedded PEDOT:PSS/silk hydrogel
arraysFig. 4. Large-scale PEDOT:PSS/silk composite electrodes.

high breaking strength, flexibility, and stretchability
both in air and water in the tensile test because of
their high viscoelastic properties. When these electrodes were introduced into the brain tissue of experimental animals, their structure was retained inside
the tissue over a long period of time without damaging it. This facilitated the stable recording of brain
electrical signals over a period of more than one
month.
The PEDOT:PSS/silk hydrogel electrodes proposed in this study exhibited little cell toxicity, shapestability, and little change of conductivity even in the
body. Thus, we are now working on optimizing thestructure of these biocompatible electrodes so thatthey adhere stably to the surface of pulsating tissue
such as the heart (Fig. 4(b)).
4. Development and practical applicationof “hitoe”
By taking advantage of the fibrous form of conductive silk fibers, we developed electrode-integrated
innerwear that can record the heart rate and ECG
waveforms when the subject wears it. The composite
electrodes embedded inside the innerwear are locatedso that they measure the potential difference between
two points on the body surface, thus obtaining biological signals with a high signal-to-noise ratio, asshown in Fig. 5(a). The obtained data were sent via
Bluetooth* from a transmitter to a smartphone. The
ECG waveform shown in Fig. 5(a) indicates not only
a sharp QRS waveform, but also a clearly identified
P/T waveform and the heart rate estimated from thenumber of R wave peaks. The electrodes in the innerwear were attached to the body without the need forelectrolyte paste, thus enabling long-term, almost
Vol. 14 No. 8 Aug. 2016noiseless, and stable measurement of ECG waveforms.
To further improve the adhesion to the skin and the
moisture-retaining ability of the electrodes, we jointly developed with Toray Industries, Inc. the functional material “hitoe” by employing nanometerscale fibers. The diameter of a single nanofiber isabout 700 nm, and the continuous layers of
PEDOT:PSS are highly impregnated within the gaps
between nanofibers (Fig. 5(b)). Since the ultra-thin
nanofibers increased the contact area and moisturizing effect on the skin, the sensitivity with which bioelectrical signals were detected was greatly improved.
In addition, we achieved high functionality by integrating required components. This allowed us to rearrange electrodes, control the wearing pressure,employ electrical wiring suitable for use in clothing,and include a structure to prevent short circuits
caused by perspiration and rain.
As hardware for measuring heart rate, biomedical
signal measuring innerwear incorporating “hitoe” isbeing marketed—in collaboration with GOLDWIN
INC. and NTT DOCOMO—as a way to monitor theload imposed on the body during exercise. We can
expect the combination of heart rate data obtained
during exercise with traveling distance and calorie
consumption measurements to improve sports performance and to enhance training management efficiency. Moreover, the monitoring of a wearer’s
physical condition using “hitoe” will be demonstrated for use as a labor management tool for professionals engaged in dangerous work, long-distance bus/truck
drivers, and night workers, or as safety management*

The Bluetooth word mark and logos are registered trademarks
owned by Bluetooth SIG, Inc.

4

Feature Articles

Smartphone

Transmitter
Conductive wire

Bluetooth
ECG

(−)

P

R
T

bpm
Heart rate

(+)
Q

(GND)

S

Electrodes(a) Electrode-integrated innerwear with loaded Bluetooth transmitter

Conductive polymer

Nanofibers

Advanced
processing technology
PEDOT:PSS

PET microfibers(15 µm)Fiber diameter: 700 nm

PEDOT:PSSSkin

PET nanofibers(700 nm)Skin(b) Functional wearable electrode “hitoe”

GND: ground
PET: polyethylene terephthalate

Fig. 5. Development and practical application of “hitoe”.

hardware for the prevention of work-related accidents
such as heat stroke and heart attack.
5. Summary
If we are to measure vital data accurately, the electrode components should be in close contact with thetarget tissue at all times. The PEDOT:PSS/silk composite electrodes that our group has been working onoffer high electrical conductivity, biocompatibility,and flexibility. Therefore, the electrodes are capableof measuring biological signals with long-term stability even when attached to the skin and in vivo tissue.
5

By integrating them with innerwear, we have developed wearable bioelectrodes for measuring heart rateand ECG waveforms with high precision withoutburdening the wearer. Simply by wearing these bioelectrodes, we can easily undertake our own healthmanagement for longer periods regardless of whether
we are engaged in normal activity or are exercising or
sleeping.
Furthermore, by solubilizing silk fibers, we produced PEDOT:PSS/silk composite electrodes withthe desired shapes. For example, we fabricatedmicrometer-sized thin electrode films in order toapply voltage to cells, and millimeter-sized implantable
NTT Technical Review

Feature Articles

electrodes to record electrical signals from tissue.
Since these electrodes are derived from aqueous silk
fibroin solution, they can be blended with any typesof additives such as ethylene glycol to increase theconductivity or cell differentiation/growth factors
corresponding to the types of cells and tissues. Currently, we are further developing these technologies
to realize a complex and three-dimensional multielectrode array. We expect them to be applicable as invivo implantation electrodes that are attached to thesurface of tissues and that measure signals from mul-

Tetsuhiko Teshima

Researcher, NTT Basic Research Laboratories.
He received his B.E. in agriculture, M.E. inbiology, and Ph.D. in information science andtechnology from the University of Tokyo in
2009, 2011, and 2014. During 2011–2014, he
was a research fellow of the Japan Society for the
Promotion of Science. He joined NTT Basic
Research Laboratories in 2014 and is currently
working on a silk fibroin-based biocompatible
interface with skin and in vivo tissue to monitor
vital data. He is a member of the Materials
Research Society (USA) and the Japan Society ofApplied Physics (JSAP).

tiple points within them, in order to obtain basic
knowledge of the profound information contained
inside the human body.
References[1][2]

T. Nyberg, A. Shimada, and K. Torimitsu, “Ion Conducting Polymer
Microelectrodes for Interfacing with Neural Networks,” J. Neurosci.
Meth., Vol. 160, No. 1, pp. 16–25, 2007.
S. Tsukada, H. Nakashima, and K. Torimitsu, “Conductive Polymer
Combined Silk Fiber Bundle for Bioelectrical Signal Recording,”
PLOS ONE, Vol. 7, No. 4, p. e33689, 2012.

Hiroshi Nakashima

Senior Research Scientist, Supervisor, NTT
Basic Research Laboratories.
He received his B.E., M.E., and Ph.D. inapplied chemistry from Waseda University,
Tokyo, in 1995, 1997, and 2002. He joined NTT
Basic Research Laboratories in 1997. Since then,he has been engaged in research on the synthesis
and control of the optoelectrical properties of(semi) conductive polymers and nanobiomaterials. His current work includes research on biosignal monitoring using a hybrid material composedof conductive polymer and textile fiber. He is amember of the Chemical Society of Japan and
JSAP.

Shingo Tsukada

Senior Distinguished Researcher, NTT Basic
Research Laboratories.
He received his M.D. from Toyama Medical
and Pharmaceutical University and his medical
license in 1990. He received a Ph.D. in medicine
from University of Tsukuba, Ibaraki, in 2003. He
was a visiting researcher at University of California at San Diego during 2003–2005. He joined
NTT Basic Research Laboratories in 2010 as a
Research Specialist and has been a SeniorResearch Scientist since 2013. He has been
studying the mechanism and activity control ofsignal transduction of brain cells. His current
interests are the detection of biomedical signals
using novel wearable-type and implant-typebioelectrodes based on the composites of conductive polymers with various fibers and textiles.
He is a member of the Society for Neuroscience,
the Physiological Society of Japan, JSAP, theJapan Neuroscience Society, the Japanese Circulation Society, and the Japanese Orthopaedic
Association.

Vol. 14 No. 8 Aug. 20166

Regular Articles

Substrate-transfer Technique Usingh-BN for GaN-based High-power
Transistors
Masanobu Hiroki, Kazuhide Kumakura,and Hideki Yamamoto
Abstract

We transferred AlGaN/GaN (aluminum gallium nitride/gallium nitride) high electron mobility
transistors from a sapphire substrate to a material with high thermal conductivity using a substratetransfer technique that involves the use of an h-BN (hexagonal boron nitride) release layer. We succeededin suppressing the self-heating effect and obtained good power performance in direct current
characteristics. The transfer technique can overcome thermal problems in power transistors.
Keywords: GaN, high-power transistors, epitaxial lift-off

1. Introduction
Gallium nitride (GaN) and its alloys are attractive
materials for short-wavelength light-emitting devices
and for high-power electronic devices due to theirwide energy gap. Blue light-emitting diodes, blueviolet laser diodes, and high electron mobility transistors (HEMTs) have been widely used for room illumination, Blu-ray Disc* players, radio frequency
amplifiers in Long Term Evolution mobile base stations, and in other applications. Other III-nitride
semiconductor materials such as indium nitride(InN), aluminum nitride (AlN), boron nitride (BN),and their alloys are also attractive for new applications of III-nitride semiconductors. Indium nitride
has a narrow bandgap (0.7 eV) and has been reported
to have an electron mobility as high as 4400 cm2/Vs[1]. Thus, InN and its alloys are attractive for infrared
light-emitting devices and high-speed electronic
devices. Aluminum nitride and BN have an extremely
wide bandgap of around 6 eV, which enables the fabrication of ultraviolet-light-emitting devices andhigh-power transistors. We recently succeeded in
growing high-quality single-crystal hexagonal boron
nitride (h-BN) and found that it can be used as a1

release layer for nitride semiconductors [2, 3].
Epitaxial lift-off (ELO) and substrate-transfer techniques enable new ways to use semiconductor devices. These techniques have been used to fabricate a
flexible GaAs (gallium arsenide)-based solar cell [4].
In addition, they help to reduce fabrication costs
because we can reuse substrates. With III-nitride
semiconductors, however, ELO from substrates isdifficult because of their strong atomic bonding withthe substrate as well as the lack of a suitable etchantfor selective etching. We recently proposed using an
h-BN release layer for ELO of III-nitride layers [3].
One of the benefits of the ELO and substrate-transfer
techniques is the enhanced heat dissipation in GaNbased electrical devices. For HEMT on sapphire, thetemperature rises significantly in operation with highcurrent levels since a sapphire substrate widely used
for the growth of GaN has low thermal conductivity(k) of 40 W/m K. This self-heating degrades device
performance and reliability. By transferring thedevices to a material with high thermal conductivity, we
can suppress the temperature rise in the active region ofdevices. This article describes the enhancement of*Blu-ray Disc is a trademark of the Blu-ray Disc Association.

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B
N

Van der Waals
force bonding

Fig. 1. Crystal structure of h-BN.

heat dissipation for aluminum gallium nitride/gallium nitride (AlGaN/GaN) HEMTs transferred to acopper plate with a thermal conductivity of 390 W/m
K [5, 6].
2. Transfer technique for AlGaN/GaN HEMTs
using h-BN ELOFirst, we present our ELO technique based on theh-BN release layer. The crystal structure of h-BN isshown in Fig. 1. The h-BN crystal has a layered structure like graphite, and each layer is weakly bonded byvan der Waals forces. Thus, the layers can be easily
exfoliated. Moreover, single-crystal GaN layers canbe grown on an AlN or AlGaN nucleation layer (NL)
deposited on h-BN. Thus, the GaN layers can bereleased from sapphire substrates by mechanicalforces.
The transfer process for AlGaN/GaN HEMT is
illustrated in Fig. 2. First, the h-BN layer is grown on
a C-plane sapphire substrate by metal organic vapor
phase epitaxy (MOVPE). Next, a 100-nm-thick AlN
NL is grown on the h-BN layer. Subsequently, an
AlGaN/GaN heterostructure is grown by MOVPE at
a temperature of 1000°C at a pressure of 300 Torr.
These are the same as the conventional conditions forthe direct growth on sapphire substrate. The heterostructure consists of a 3-μm-thick GaN buffer layerand 27-nm-thick AlGaN barrier layer. Two-dimensional electron gas (2DEG) with a 1 × 1013 cm−2
density is induced by a polarization charge at the
AlGaN/GaN heterointerfaces, which forms a conductive path from the source to the drain.
AlGaN/GaN HEMTs were fabricated using conventional photolithography and lift-off techniques.
Vol. 14 No. 8 Aug. 2016

The AlGaN barrier outside the active region was
etched away by inductively coupled plasma etching
to remove the 2DEG conductive channel for electricalisolation between devices (mesa isolation). We usedelectron-beam evaporation for metal deposition andrapid thermal annealing to form the ohmic contacts.
The source/drain ohmic electrodes consisted of titanium (Ti; 15 nm)/ aluminum (Al; 80 nm)/ nickel (Ni;30 nm)/gold (Au; 50 nm) annealed at 850°C for 30 s.
The Schottky gate electrode was Ni (50 nm)/Au (100nm). The gate length, source-drain spacing, and gate
width were 1.5, 6, and 100 μm, respectively.
The next step in the process is to release AlGaN/
GaN HEMTs from the sapphire substrate and transfer
them to a copper plate. For that purpose, we firstcoated the sample surface with PMMA (polymethyl
methacrylate) and adhered the sample to a glass plate.
Then we mechanically released it from the sapphire
substrate. For transfer to a copper plate, we used
Au-Au thermocompression bonding [7]. We deposited Ti/Au on the back side surface of the HEMT, andwe deposited Au on the copper plate. The two Au
surfaces were brought into contact with each other,and then a pressure of approximately 20 MPa wasapplied at about 200°C. The cross-sectional transmission electron microscopy (TEM) image of the transferred HEMT is shown in Fig. 3. The TEM image
reveals that the GaN layer strongly adhered to thecopper layer at the Au-Au thermocompression interface.
3. Enhancement of heat dissipation of AlGaN/
GaN HEMTs using substrate-transfer technique
Typical drain current (I d )–drain bias (V ds )2

Regular Articles

S

G
AlGaN

Mesa
isolation

AlGaN

D

2DEG

GaN

GaN

AlN

h-BN

Sapphire sub.

Sapphire sub.

(a) Growth by MOVPE(b) Fabrication of HEMT

Tape

S

Glass plate
S
PMMA

AlN

h-BN

G

D

G
AlGaN

D

AlGaN
GaN
GaN
AlN

Ti

AlN
Au

Copper plate

Sapphire sub.

(c) Release from a sapphire substrate(d) Au-Au thermocompression
bonding to a copper plate

PMMA: polymethyl methacrylate

Fig. 2. S chematic views of h-BN ELO and transfer processes of AlGaN/GaN HEMT. (a) Growth of AlGaN/GaN
heterostructures on AlN NL deposited on h-N/sapphire. (b) Fabrication of AlGaN/GaN HEMTs by conventional
photolithography and lift-off technique. The labels S, G, and D indicate source, gate, and drain electrodes,
respectively. (c) Mechanical ELO of the HEMTs from sapphire substrate. (d) Transfer of the HEMTs to a copper plate
by Au-Au thermocompression bonding.

characteristics of the AlGaN/GaN HEMT before
release from the substrate and after transfer to thecopper plate are shown in Fig. 4. Good pinch-off andsaturation characteristics were obtained even after thetransfer, indicating that no degradation took place inthe transfer process. The device performance significantly improved after transfer. The maximum Id at agate bias (Vgs) of 1 V significantly increased from0.55 A/mm to 0.68 A/mm. Before release, a large
3

reduction in Id vs Vds was observed in the saturation
region at a large positive gate bias (Vgs). At Vgs = 1 V,
the Id decreased by 25% as the Vds increased from
5 V to 20 V. In contrast, the decrement in Id vs Vds was
as small as 3% for the transferred HEMT. The negative slope is commonly attributed to self-heating in
the device operation [8]. The transfer from the sapphire to the copper plate improves the heat dissipation efficiency and thereby suppresses the decrease in
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Drain

Carbon coat

Gate

Source

GaN

Au
Cu

1 µm
Cu: copper

Fig. 3. C ross-sectional TEM image of an AlGaN/GaN HEMT transferred to a copper plate by Au-Au thermocompression
bonding. White and grey layers on the sample surface consist of carbon, which was coated for surface protection in
TEM measurements. The difference in the colors is caused by the purity of carbon.

1.0

After transfer (on Cu)
Before release (on sapphire)

Id (A/mm)

0.8

Vgs:
+1 V

0.6
−1 V
+1 V
−1 V
−3 V
−3 V

0.4
0.2
0.0
0

−5 V
−5 V
5

10

15

20

Vds (V)

Fig. 4. Id–Vds characteristics of AlGaN/GaN HEMT before release and after transfer.

drain current under high-bias operation.
We also estimated the temperatures in the active
region of HEMTs using micro-Raman spectroscopy.
The temperatures of the devices can be estimatedfrom the peak shift in the A1 phonon mode [9, 10].
The temperature maps of an HEMT on a sapphire
substrate and another HEMT transferred to copper
are shown in Fig. 5. The highest temperature wasobserved at the center of the channel at the gate edge
on the drain side. For the HEMT on sapphire (before
its release), the temperature exceeded 200°C at a
power dissipation (P) level of 0.68 W. In contrast, forthe transferred HEMT, the temperature was only
Vol. 14 No. 8 Aug. 2016

110°C at P = 1.54 W. The channel temperature as afunction of P in the HEMTs is plotted in Fig. 6 before
their release and after their transfer. The channel temperature is the average one in the center region of thechannel at the gate edge on the drain side where thehighest temperature was observed. A linear approximation revealed that the thermal resistances of theHEMTs before their release and after their transfer
were 28 and 6.5°C mm/W, respectively, indicating a
one-fourth reduction. The transfer to copper significantly improved the heat dissipation in the HEMTs.

4

Regular Articles

200
60

150

Drain

Source

40

20

(µm)

(µm)

40

S

100
20

G

S

G
D

D

0

50

0

Gate

0°C

−20

0
(a) AlGaN/GaN HEMT

4
(µm)

8

−4

0

4

8

(µm)

(b) Before release(c) After transfer

Fig. 5. ( a) Schematic view of AlGaN/GaN HEMT. The dotted rectangle corresponds to the measurement area by microRaman spectroscopy. Temperature maps of (b) an HEMT on sapphire (before release) at a power dissipation of 0.68
W and (c) another HEMT transferred to a Cu plate at a power dissipation of 1.54 W. The white regions in each map
represent the source, gate, and drain electrodes from left to right.

200

Temperature (°C)

Before release (on sapphire)
150
After transfer (on Cu)
100

50

0.0

0.5
1.0
Power dissipation (W)

1.5

Fig. 6. T emperature at the gate edge on drain side as a function of power dissipation in HEMTs before release and after
transfer.

4. Conclusion
We transferred AlGaN/GaN HEMTs from a sapphire substrate to a copper plate using the ELO technique with an h-BN release layer. The negative slopeof Id caused by the self-heating effect was reduced
5

after the transfer. We also observed the temperaturesin the active HEMTs using micro-Raman spectroscopy and found that the temperature rise was suppressed after the transfer to the copper plate. The
temperature at P = 1.54 W was 110°C for the HEMT
after transfer, whereas it exceeded 200°C at P = 0.68
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W before the release. These results indicate that thetransfer technique is effective for enhancing the heatdissipation in GaN-based devices.
References
[1]
[2]
[3]
[4]
[5]

A. G. Bhuiyan, A. Hashimoto, and A. Yamamoto, “Indium Nitride(InN): A Review on Growth, Characterization, and Properties,” J.
Appl. Phys., Vol. 94, No. 5, pp. 2779–2808, 2003.
Y. Kobayashi, T. Akasaka, and N. Kobayashi, “In-situ Monitoring of
GaN MOVPE by Shallow-angle Reflectance Using Ultraviolet Light,”
J. Cryst. Growth, Vol. 195, No. 1, pp. 187–191, 1998.
Y. Kobayashi, K. Kumakura, T. Akasaka, and T. Makimoto, “Layered
Boron Nitride as a Release Layer for Mechanical Transfer of GaNbased Devices,” Nature, Vol. 484, pp. 223–227, 2012.
C.-W. Cheng, K.-T. Shiu, N. Li, S.-J. Han, L. Shi, and D. K. Sadana,“Epitaxial Lift-off Process for Gallium Arsenide Substrate Reuse andFlexible Electronics,” Nat. Commun., Vol. 4, Article no. 1577, 2013.
M. Hiroki, K. Kumakura, Y. Kobayashi, T. Akasaka, T. Makimoto,

Masanobu Hiroki

Senior Research Scientist, Thin-Film Materials
Research Group, NTT Basic Research Laboratories.
He received a B.S. and M.S. in physics and a
Ph.D. in engineering from Osaka University in
1998, 2000, and 2010. He joined NTT Basic
Research Laboratories in 2000. He moved to
NTT Photonics Laboratories in 2003 and to NTT
Basic Research Laboratories in 2012. His current
research interests include crystal growth of IIInitride semiconductors and their electronic
device applications. He is a member of the Japan
Society of Applied Physics (JSAP).

and H. Yamamoto, “Suppression of Self-heating Effect in AlGaN/
GaN High Electron Mobility Transistors by Substrate-transfer
Technology Using h-BN,” Appl. Phys. Lett., Vol. 105, 193509, 2014.
[6] M. Hiroki, K. Kumakura, and H. Yamamoto, “Enhancement of
Performance of AlGaN/GaN High-electron-mobility Transistors by
Transfer from Sapphire to a Copper Plate,” Jpn. J. Appl. Phys., Vol.
55, No. 5S, 05FH07, 2016.
[7] T. Shimatsu and M. Uomoto, “Atomic Diffusion Bonding of Wafers
with Thin Nanocrystalline Metal Films,” J. Vac. Sci. Technol. B., Vol.
28, 706, 2010.
[8] R. Gaska, A. Osinsky, J. W. Yang, and M. S. Shur, “Self-heating in
High-power AlGaN-GaN HFETs,” IEEE Electronic Device Lett., Vol.
19, No. 3, pp. 89–91, 1998.
[9] F. Demangeot, J. Frandon, M. A. Renucci, O. Briot, B. Gil, and R. L.
Aulombard, “Raman Determination of Phonon Deformation
Potentials in α-GaN,” Solid State Commun., Vol. 100, No. 4, pp.
207–210, 1996.
[10] T. Batten, J.W. Pomeroy, M.J. Uren, T. Martin, and M. Kuball,“Simultaneous Measurement of Temperature and Thermal Stress in
AlGaN/GaN High Electron Mobility Transistors Using Raman
Scattering Spectroscopy,” J. Appl. Phys., Vol. 106, 094509, 2009.

Hideki Yamamoto

Executive Manager of Materials Science Laboratory, NTT Basic Research Laboratories.
He received his B.S., M.S., and Ph.D. in chemistry from the University of Tokyo in 1990, 1992,
and 1995. His principal research areas are thinfilm growth, surface science, and superconductivity. He has been working at NTT laboratories
and NTT Corporation since 1995. He was a visiting scholar at Geballe Laboratory for Advanced
Materials, Stanford University, USA, during
2004–2005. He is a member of JSAP, the Physical Society of Japan, the Surface Science Societyof Japan, the American Physical Society, and theMaterials Research Society.

Kazuhide KumakuraGroup Leader of Thin-Film Materials ResearchGroup, Materials Science Laboratory, NTT Basic
Research Laboratories.
He received his B.E., M.E., and Ph.D. in electrical engineering from Hokkaido University in
1993, 1995, and 1998. He joined NTT Basic
Research Laboratories in 1998. His current
research interests include crystal growth of
nitride-based semiconductors and their optoelectronic and magnetic device applications. He is amember of JSAP.

Vol. 14 No. 8 Aug. 20166

Regular Articles

Path Accommodation Design Engine
for Simply and Reliably DesigningMulti-layer Transport Networks
Akihiro Kadohata, Fumikazu Inuzuka,
Wataru Kawakami, Atsushi Watanabe,
and Mitsuhiro Teshima
AbstractCore and metro transport networks continue to be simplified with the adoption of 100-Gbit/s packet
transport systems. In line with this, the engineering work involved in path accommodation design andmanagement is being carried out as a smooth flow, which is referred to as creating a flow-throughoperation. This article describes a multi-layer transport path accommodation design engine that takes
into account the reliability demanded by each service in order to accommodate optical and multigranular electrical paths from a few megabits per second to 100 Gbit/s.
Keywords: PTS, optical path, multi-layer, accommodation design

1. Introduction
To flexibly keep up with future increases in communication traffic, core and metro networks are
evolving into multi-layers integrating optical andelectrical paths on the 100-Gbit/s packet transportsystem (100G-PTS) [1]. Furthermore, to simplify
networks while improving their quality, reliability,
and interoperability, it is necessary not only to design
networks in such a way that the closed paths in eachlayer of the optical-path or electrical-path layers areaccommodated—which we refer to as accommodation design—but also to achieve a way to select routes
that takes into consideration both path layers, theassignment of resources (i.e., wavelength and bandwidth used for paths), and the accommodation designof the optimal paths for each service on the basis ofvarious conditions such as redundancy and reliabilityof routes.

1

2. Multi-layer transport path accommodation
design engine
The multi-layer transport path accommodation
design engine (hereinafter, design engine) was developed by NTT Network Innovation Laboratories andis positioned as part of a logical design function
block, as shown in Fig. 1. An overview of the design
engine is shown in Fig. 2. The assignment of wavelength or bandwidth and the route of optical or electrical paths are automatically designed in consideration of source and destination nodes as well as various design conditions, and the results are returned tothe operator.
In developing the design engine, we assumed ausage scenario in which an operator handles theaccommodation design of paths. The following suppositions were made regarding application scenarios
involving automatic design: (i) routes and resources
should be designed as a whole; (ii) routes should bedesigned by operators themselves, but resources
should be designed automatically; (iii) routes and
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Path design and management function
Creating a flow-through operation of path engineering work
Setup

DesignLogical design

OAM

Physical design

Path-route design/
path-resource design

Multi-layer transport pathaccommodation design engine

Facility design/ equipmentinstallation design (circuit
pack etc.)

Electrical path layer
Optical path layer…

Statusnotification

Pathregistration

Equipment
registration

Equipment management

100G-PTS

Simplification of the network
OAM: operations, administration and management

Fig. 1. Image of application of the multi-layer transport path accommodation design engine.

resources should be designed by operators themselves, but delay times and other elements should be
calculated; and (iv) not only optimal design results(best matched to the design conditions) but also quasi-optimal design results (approximately matched tothe design conditions) should be looked at and judged
in an integrated manner. Moreover, we considered
requirements such as providing designs applicable tospecific design policies for each service, future
expandability such as the addition of equipment andequipment vendors, and easy of embedding in network operation systems.
The design engine has the following features: (1)functional partitioning based on the usage scenario;(2) multiple candidate routes in accordance with thedesign policy; (3) flexibility and expandability thanks
to a configuration file that sets various design conditions and software startup parameters when adding
other network elements and network equipment vendors; and (4) easy implementation of embedded
equipment in network operation systems. These fourfeatures are first described here in terms of the entire
design engine, and the primary functions and key
features of the design engine are then explained.

Vol. 14 No. 8 Aug. 2016

2.1 Functional partitioning based on usage scenario
The design engine is composed of three function
blocks configured in accordance with the abovedescribed design scenarios concerning the operator as
shown in Fig. 2: a route-design function block (forcalculating candidate routes from a source node to a
destination node); a resource-design function block(for assigning wavelengths to optical paths andassigning bandwidth to electrical paths); and an optimal route-selection function block (for selecting
routes and ranking them according to evaluation priorities).
2.2 Multiple candidate routes in accordance withdesign policy
When a source node and a destination node are
input, the design engine calculates optimal candidate
routes and multiple quasi-optimal candidates. The
multiple candidate routes are prioritized in accordance with a priority order set by means of the configuration file, ordered as optimal candidate routes
and quasi-optimal candidate routes, and output as
response results. This ordering of multiple candidates
can be imagined as displaying multiple candidates
2

Regular Articles(i) Automatic design ofroute and resources• Source and destination nodes• Bandwidth• Evaluation priorities(iii) Optimal path selection(when both the route andresources are predetermined)• Route• Resources(ii) Automatic design ofresources (when the routeis predetermined)• RouteAssignment ofwavelength/bandwidth

Search for route
Route

Setting/change

Route &
resources

RMI

Resource-design
function block

RMI

RMI

RMI

Route-design
function block

Reply• Best candidate• Quasi-optimalcandidate

Optimal routeselection
function block
Selection andsorting based on
evaluated item

Configuration file

Reference
Database(Network and resource status information)
RMI: Remote Method Invocation

Fig. 2. Overview of multi-layer transport path accommodation design engine.

(e.g., trains) via a transfer-information app on smartphones in accordance with a display priority set by
the user in terms of elapsed time, transport charge,
and number of transfers. One usage scenario is thatthe operator can not only select optimal candidates;
the operator has the freedom to select from multiple
candidates including quasi-optimal ones wheneverthey please.
 lexibility and expandability by a configura2.3 F
tion file
If the types of equipment and the equipment vendors are different, the values of parameters used forbandwidth calculations and design conditions may
differ. Moreover, the design conditions may change
when the equipment is modified. As for the design
engine, flexibility and expandability are achieved byresponding to changes in the design conditions andparameters by using the configuration file. Utilizing
the path-route-design function block makes it possible to flexibly change design conditions such as theupper limit of delay times of end-to-end services andthe upper limit of the number of nodes passed
3

through. Moreover, it is possible to parameterize theevaluation priority used for ordering optimal candidates by the optimal route-selection function blockusing configuration files. For example, it is possible
to set the order of priority such as delay times of endto-end services and the number of nodes passed
through as an evaluation priority. Setting the weighting of evaluation priority in this manner makes it
possible to output optimal candidate paths in response
to services and usage purpose.
2.4 B uilt-in easy implementation in network
operation system
The design engine is implemented on the basis of a
network model, path model, and object model specified in international standard TMF513 [2]. Furthermore, it adopts RMI (Remote Method Invocation)—an API (application programming interface) generally used with Java—as an interface, and it can be easily implemented by embedding it in the network
operation system.

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Function 2: Handling cases where source(or destination) node of primary paths
differs from that of secondary paths

Function 1: Route design for electrical
path/optical path layers

Electrical path layer
A

A’Electrical pathC

Primary and secondary
nodes are different.

B
Z’Z

Function 3: Independent designof mutual links, conduit lines, andnodes of primary and secondary
paths

Function 4: Design ofthird route

Virtual link 1(Optical path 1)

Virtual link 2
Same conduit lines(Optical path 2)

Third path
Secondary path

Virtual link 3(Optical path 3)
Primary and secondary
nodes are different.
Optical path layer

Primary path

Fig. 3. Path-route-design function.

3. Path-route-design function block
The path-route-design function block provides integrated path designs in accordance with a variety ofpath-route-design conditions and the state of physical, optical, and electrical layers. To put that concretely, it makes it possible to select routes on thebasis of design conditions such as the building thatthe system will be accommodated in, the number ofnodes, links, and conduit lines for the physical layer,
the delay time of paths in path layers, and the delaytime difference between paths of the currently usedsystem and the backup system. The path-route-design
function block has four primary functions, which aredescribed below and shown in Fig. 3.
3.1 Batch design for multi-layer paths
With multi-layers (which mediate both opticalpaths and electrical paths) as targets, paths aredesigned in batches. For this batch design, first, optical paths that can accommodate electrical paths aresearched for. An optical path is deemed to be a virtual link in the electrical-path layer; a virtual link that
can accommodate the bandwidth number of the electrical path is searched for, and candidate routes are
selected in order of the fewest number of virtual links
passed through. As for the path accommodation
design for nodes A’ to Z’ in Fig. 3, candidate routes
Vol. 14 No. 8 Aug. 2016

are selected in order of paths via virtual link 3 andpaths via virtual links 1 and 2. In contrast to the previously described example, in cases where there are no
optical paths that can accommodate electrical paths,
it is possible to design multi-layers as a whole bydesigning an optical path afresh and designing anelectrical path on the virtual links. In addition, theassignment of a wavelength for the optical path at thattime is executed by calling the resource-design function block (described below). This function makes it
possible to independently design routes in the respective layers for optical and electrical paths.
3.2 Design when source (destination) nodes ofprimary and secondary paths differ
Path routes can be designed even if the paths do nothave the same source or destination node for a redundancy path, which consists of primary and backup
paths. In this way, the case in which the currently
used system and backup system of the client equipment are installed at physically separate locations (orwhen equipment at the same location is installed
separately) can also be handled as a redundant configuration.
3.3 Independent designs for nodes, links, andconduit lines of primary and secondary paths
It is possible to design routes that avoid duplication
4

Regular Articlesof transit nodes and transit links of the primary pathand the secondary path. It is also possible to designroutes that avoid duplication of conduit lines that
contain the respective transit links of the primary pathand the secondary path.
3.4 D esign of third path for primary and secondary paths
Reliable design that can handle changes in network
configuration due to factors such as major disasters
and fluctuations in the set number of nodes is possible. For example, when the existing primary or secondary path is disrupted, route design involving athird path [1] to rebuild a redundant configuration is
implemented in order to avoid the disrupted points.
4. Resource-design function block
The resource-design function block provides wavelength and bandwidth for the routes calculated by thepath-route-design function block or the routes set by
the operator in advance. In the design of resources for
assigning wavelengths to optical paths, a wavelengthcontinuity constraint is imposed on the links routed;
that is, the same wavelength must be used between
source and destination nodes. The concept of opticalpaths was advocated by NTT research & development laboratories in 1992; therefore, an algorithm forcarrying out path accommodation design to allocate
optical-path routes and wavelengths was proposed[3].
An algorithm called least fragmentation (LF) [4],which was devised by NTT Network Innovation
Laboratories, is implemented in the resource-design
function block. When multiple candidate wavelengths can be assigned to candidate routes, the LF
algorithm selects a single wave number that minimizes fragmentation (i.e., a state in which wavelengths utilized for certain routes are fragmented)
while considering the wavelength-utilization status ofadjacent links on the candidate path routes. To putthat more concretely, the wavelength-utilization status of links is expressed as two values: “0” (false) for
unused and “1” (true) for used. The value of an exclusive OR (XOR) between adjacent links is taken as alinear measure, and the wave number that minimizes
that XOR value is selected as the wavelength to be
assigned. In this manner, the wavelength allocated to
candidate routes is calculated in a short time—even in
the case of a large-scale network—by a simple logic
operation including the candidate path route and thewavelength-utilization status of adjacent links on that5path.
As a conventional wavelength-assignment algorithm, the first fit (FF) algorithm (Fig. 4) for assigning wavelength in ascending order of wave number
does not account for the wavelength-utilization statusof adjacent links on the candidate route. Accordingly,
to accommodate a new path request in the network(i.e., source node PTS-A and destination node PTS-C
in Fig. 4), it is necessary to add links between nodes,
for example, between nodes PTS-A and PTS-B in
Fig. 4. In contrast, with the LF algorithm, the wavelength is selected so that the same wavelength can be
assigned to maintain continuity for as many links as
possible; consequently, new requests for paths can beaccommodated in the network without having to add
links in response to those requests, as shown in theexample in Fig. 4.
In addition to the LF and FF algorithms, a randomalgorithm (which randomly selects allocable wavelengths) can be used. The results of a comparativeevaluation of the three algorithms—on the basis of
the number of paths that can be accommodated inrelation to network configuration (i.e., scale)—areshown in Fig. 5. Over the entire network scale range,
the number of paths that can be accommodated islargest when the LF algorithm is used. It can thus beconcluded that the LF algorithm can effectively utilize network resources to the maximize extent possible.
5. Conclusion
We developed a multi-layer transport path accommodation design engine that achieves optimal pathdesign and satisfies the required conditions for various services. From here on, we will continue toresearch and develop technologies for optical-pathaccommodation design while aiming to create coreand metro networks for economically providing a
multitude of broadband services.

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1
Wavelength 2
number 3
4

PTS-A
PTS-B
PTS-C
PTS-D
Solid line: wavelength currently in use
Assumed number of fiber wavelengths
Dotted line: unused wavelength
to be accommodated: 4
Paths are newly extended: PTS-B PTS-C
Proposed method:
Conventional method:
Least fragmentation (LF)
First fit (FF)

PTS-A

PTS-B

PTS-C

PTS-D

PTS-A

PTS-B

PTS-C

PTS-D

Wavelength allocation in ascendingorder wavelength 2

Wavelength allocation in view of immediate
environment* wavelength 4*Wavelength-usage state
A path is newly extended: PTS-A PTS-C
between A-B and C-D

PTS-A

PTS-B

PTS-C

PTS-D

PTS-A

No useable wavelength between A to C
Expansion of fiber is necessary.

PTS-B

PTS-C

PTS-D

Wavelength 2 can be assigned.

Fig. 4. Improved accommodation efficiency by LF algorithm.

MxN grid (MxN nodes)
N

Cost 266 (26 nodes)

NSF (14 nodes)

Japan (12 nodes)

Number of paths that can beaccommodated

M

Comparison of number of paths that can be accommodated
when network resources (fibers etc.) are fixed.
450
Random
400
FF
350
LF
300
250
200
150
100
50
0
6x6

5x6 Cost266 5x5

4x5

NSF Japan

Network scale
Large

Small

NSF: National Science Foundation

Fig. 5. Quantitative evaluation of LF algorithm.

Vol. 14 No. 8 Aug. 20166

Regular Articles

References
[1]

[2]

T. Kawasaki, M. Inami, Y. Sasakura, D. Shimazaki, M. Horiguchi, andK. Koda, “Development of 100-Gbit/s Packet Transport System,”
NTT Technical Review, Vol. 13, No. 3, 2015.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201503ra1.html
TMF513: Multi-Technology Network Management (MTNM)
Business Agreement Release 3.5, 2007.

Akihiro Kadohata

Research Engineer, The First Promotion Project, NTT Network Innovation Laboratories.
He received a B.S. in applied physics from
Tokyo University of Science in 2005, an M.S. inearth and planetary science from the Universityof Tokyo in 2007, and a Ph.D. in electrical engineering and computer science from Nagoya
University, Aichi, in 2016. In 2007, he joined
NTT Network Innovation Laboratories, where hehas been researching photonic network architecture and design. He is a member of the Instituteof Electronics, Information, and Communication
Engineers (IEICE). He received the Young Engineers Award from IEICE in 2013.

Fumikazu Inuzuka

Senior Research Engineer, The First Promotion
Project, NTT Network Innovation Laboratories.
He received a B.E. and M.E. in applied physics
from the Tokyo University of Agriculture andTechnology in 2002 and 2004. In 2004, he joined
NTT Network Innovation Laboratories, where heis engaged in research on photonic transport andprocessing systems and photonic networking
systems. He is a member of IEICE.

Wataru Kawakami

Senior Research Engineer, Supervisor, The
First Promotion Project, NTT Network Innovation Laboratories.
He received his B.E., M.E., and Ph.D. in electrical engineering from Kyoto University in
1986, 1988, and 1997. He joined NTT Telecommunication Networks Laboratory in 1988 andstudied optical signal processing. He was withNTT EAST from 1999 to 2008, where he developed IP (Internet protocol) network services(including the FLET’S service). He also workedon the development of NGN (Next Generation
Network) at NTT during 2008–2010.

7

[3]

[4]

O. Ishida, A. Hirano, Y. Sone, A. Kadohata, and A. Watanabe, “Highly
Efficient Photonic Networking Technology,” NTT Technical Review,Vol. 9, No. 8, 2011.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201108fa4.html
Y. Sone, A. Hirano, A. Kadohata, and O. Ishida, “Efficient Routing
and Wavelength Assignment Algorithm Minimizes Wavelength
Fragmentations in WDM Mesh Networks,” Proc. of OECC 2011 (the16th Opto-Electronics and Communications Conference), pp. 176–
177, Kaoshsiung, Taiwan, July 2011.

Atsushi Watanabe

Senior Research Engineer, Supervisor, The
First Promotion Project, NTT Network Innovation Laboratories.
He received a B.S. and M.S. in information
science from Ibaraki University in 1989 and1991. In 1991, he joined NTT Transmission
Systems Laboratories, where he engaged inresearch on traffic control in asynchronous transfer mode networks. Since 1993, he has been
involved in research and development of optical
path-based photonic networks. He is currently
the Engineering Assistant Manager of the Electronics Device & System Business Unit, Broadband System & Device Business Group, NTT
Electronics Corporation. He received the Young
Engineer Award in 1997 from IEICE, of which heis a member.

Mitsuhiro Teshima

Director, Senior Research Engineer, Supervisor, The First Promotion Project, NTT Network
Innovation Laboratories.
He received a B.E. and M.E. from Tokyo Institute of Technology in 1989 and 1991. He joined
NTT Transmission System Laboratories in 1991,
where he has been researching frequency control
of semiconductor lasers, optical cross-connects,
and metropolitan and access area network technologies. From 2003 to 2007, he developed adigital television transport network system at
NTT Communications. Since 2007, he has been
engaged in research on high speed access control
and transport technologies at NTT Network
Innovation Laboratories. He is currently theManager of the Electronics Device & System
Business Unit, Broadband System & Device
Business Group, NTT Electronics Corporation.
He received the Young Engineers Award fromIEICE in 1998 and the APCC-IEEE ComSoc
APB Joint Award from the Asia-Pacific Conference on Communications and IEEE (Institute ofElectrical and Electronics Engineers) AsiaPacific Board in 2001.

NTT Technical Review

Global Standardization Activities

Trajectory of ITU-T Standardization
and Key Issues at the ITU-T CJKCTO Meeting
Masakatsu Fujiwara
Abstract

The ITU-T (International Telecommunication Union - Telecommunication Standardization Sector)
CTO (chief technology officer) meeting for the CJK (China, Japan, and Korea) region was held in Seoul,
Korea, on March 21, 2016. This article presents the main subjects of discussion at the meeting, namely,
5G (fifth-generation mobile communications system), IoT (Internet of Things), video, and other areas.
Keywords: ITU-T, IoT, 5G

1. Introduction
The International Telecommunication Union (ITU)
Telecommunication Standardization Bureau (TSB)
Director CJK (China, Japan, and Korea) CTO (chief
technology officer) Consultation Meeting (hereinafter, “CJK CTO Meeting”) serves as a consultative
forum to the ITU’s TSB Director. The participants ofthe meeting are tasked with applying feedback fromthe telecommunications industry to the ITU Telecommunication Standardization Sector (ITU-T)’s standardization activities. The meeting hosts CTOs andother representatives of major private sector firms
from China, Japan, and Korea and fosters debate anddiscussion on relevant topics.
Discussions were held on expectations for future
initiatives by ITU-T, predominantly centering on thefifth-generation mobile communications system (5G)
and the Internet of Things (IoT), the key technological sectors the ITU-T intends to focus on, and themeans of implementing such initiatives. A communiqué was prepared as a collection of proposals made to
ITU-T. The following sections summarize the future
standardization issues, which are all areas of greatinterest to the industry participants of the CJK CTO
Meeting.
The CTO Meeting was established in 2008 by
Resolution 68 of the World Telecommunication Stan1

dardization Assembly (WTSA), with the TSB Director assigned hosting and operations tasks. The meeting has been held annually since 2008. The CJK CTO
Meeting, limited to the three CJK countries, was firstlaunched by the current TSB Director Chae-sub Lee
in April 2015. On the basis of the number of ITU-T
personnel and articles contributed, the CJK region
currently represents the majority at ITU-T.
The March 2016 CJK CTO Meeting was the second
session of the event, following the first session in
April of last year. The meeting is intended to increasecooperation around standardization among the three
countries. At the meeting, CJK representatives
exchanged opinions on future issues concerning standardization. These opportunities for collegial interaction are also beneficial from the standpoint of crafting
better standardization strategies. For this meeting, theroster was largely made up of representatives of theprivate sector firms listed in Table 1, Director Chaesub Lee of the TSB, and other members (Photo 1).
2. Thrust of meeting
This meeting explored four core subjects: networks
in support of 5G, IoT and its applications, the futureof video, and WTSA-16.

NTT Technical Review

Global Standardization Activities

Table 1. Participating organizations at CJK CTO Meeting.
Alibaba Group
China

FiberHome Technologies Group
Huawei Technologies Co., Ltd.
ZTE Corporation
Fujitsu Limited
KDDI Corporation
NEC CorporationJapan

National Institute of Information andCommunications Technology
NTT
NTT DOCOMOElectronics and Telecommunications Research InstituteKT Corporation

Korea

LG Uplus Corporation
Samsung Electronics Co., Ltd.
SK Telecom Co., Ltd.

Photo 1. Meeting participants.

2.1 Networks in support of 5G
The participants expressed their gratitude for theefforts of FG IMT-2020, a Focus Group (FG) established in May 2015 to discuss the issues of the IMT(International Mobile Telecommunications)-2020
system (5G). CJK members confirmed their intention
to actively contribute to the activities of this group.
The participants agreed that future key technicalVol. 14 No. 8 Aug. 2016

issues for networks supporting 5G include: 1) end-toend network management, 2) network architectureand fixed-mobile convergence, 3) network softwarization and network slicing, including for fronthaul and backhaul, and 4) information-centric networking. In terms of the next steps for 5G standardization, the meeting called for collaboration withother standards-developing organizations (SDOs)
2

Global Standardization Activities

such as 3GPP (3rd Generation Partnership Project) so
as to avoid duplication of efforts, and emphasized theimportance of partnering with open-source organizations.
2.2 IoT and its applications
In addition to praising the 2015 establishment of
the new Study Group (SG), SG20, the participants
were in agreement about SG20’s responsibility—pursuant to its enactment of international standards on
IoT—to promote the growth of related technologies,
including machine-to-machine (M2M) telecommunications and ubiquitous sensor networks, and to fostera climate in which stakeholders can collaborate todevelop new technologies. The meeting also underscored the importance of improving collaborationand harmonization between SGs and other SDOs inorder to prevent overlaps in issues such as oneM2M
and other standards being explored by other groups.
2.3 The future of video
Given the rapid increase in the proportion of video
data in network traffic and the forecast that this traffic
will continue to increase markedly, the participants
agreed that achieving image quality, stability, andavailability is crucial to future networks. In addition,
the participants agreed that with the presumed
appearance of new video formats, among them realtime and immersive super realistic video, there is a
growing need for networks to properly scale andhandle these types of content, and a new host of key
issues surrounding these technologies, such as the
optimization of bandwidth consumption.

2.4 WTSA-16
It was announced that the WTSA-16 assembly willtake place in Yasmine Hammamet, Tunisia, from
October 25 to November 3, 2016. It was alsoannounced that prior to the assembly, the CxO meeting would be held on October 23, and CTO participants were asked to attend. Because WTSA-16 will
involve a rethinking of the structure of SGs and theselection of chairs and vice-chairs, as well as debate
around methodologies used at ITU-T, the CJK CTO
Meeting called for a contribution of articles regarding
the proposals made by CTO members.
3. Future outlook
The first session of the CJK CTO Meeting in 2015
reported in its communiqué a plan to take greater
steps towards involvement with the advance of IoT;
evidence of this fact was seen in the formation immediately thereafter of SG20, a new Study Group focusing on the standardization of IoT protocols. Thissuggests that the TSB intends to emphasize the outcome of the meeting, and the communiqué would
further indicate that this is an area of active interest
for ITU-T. Details have been published on ITU-T’s
website and can be read by interested parties [1].
Reference[1]

TSB Director CJK CTO Consultation Meeting Communiqué, Mar.
2016.
http://www.itu.int/en/ITU-T/tsbdir/cto/Documents/160321/Final_
communique.pdf

Masakatsu Fujiwara

General Manager, NTT R&D European Representative Office, NTT Research and Development Planning Department.
He received a B.E. in electrical engineeringand an M.E. in applied systems science from
Kyoto University in 1994 and 1996, and an
M.B.A. from Cornell University, USA, in 2005.
After joining NTT Network Service Systems
Laboratories in 1996, he was engaged in researchand development (R&D) of key technologies for
network management systems during 1996–2007
and 2010–2013. He was involved in R&D strategy planning at NTT Research and Development
Planning Department during 2007–2010. He has
been with NTT R&D European Representative
Office in Frankfurt, Germany, since 2013.

3

NTT Technical Review

Practical Field Information about
Telecommunication Technologies
Practical Field Information about Telecommunication Technologies

Recent Case Study of Fault in
IP Phone User System
Abstract

This article describes a problem a customer was having with disconnects in inter-office calling using
IP-VPN (Internet protocol virtual private network) services and how we rectified it. This is the thirtysixth article in a series on telecommunication technologies. This month’s contribution is from the Network Interface Engineering Group, Technical Assistance and Support Center, Maintenance and Service
Operations Department, Network Business Headquarters, NTT EAST.
Keywords: IP phone, IP-VPN, VoIP-GW

1. Introduction
With the spread of FLET’S HIKARI NEXT andother Internet protocol (IP) access services, individual customers of voice calling services have been
increasingly shifting to IP phone services from conventional calling services such as subscriber telephone (analog line) and INS-Net 64 (ISDN: integrated
services digital network) services. Corporate customers have also been shifting to user systems that perform outside calling using IP phone services andinter-office extension calling using IP-VPN (virtualprivate network) services with the aim of reducing
communication costs.
In this article, we introduce a case study involving
occasional disconnects in inter-office calling using
IP-VPN services; extension calling using the IP network.
2. Background of problem
The customer who experienced the problem has
been using an IP-VPN service to make voice calls
between the head office and branch offices by private
branch exchange (PBX)-based IP extension calling.
When both the voice-over-IP gateway (VoIP-GW)
and PBX installed in the head office and branch
offices were upgraded, a problem would occasionallyoccur in which an IP extension incoming call would
disconnect on being answered, thereby terminating1

the call (Fig. 1). This event was not restricted to specific offices—it would occur at any office that used IP
extension calling. Replacing the entire VoIP-GW or
PBX package substrates did not solve the problem, so
the Technical Assistance and Support Center was
consulted to troubleshoot the problem.
3. On-site troubleshooting
Packet capture equipment was installed in the IP
interval on the head-office side that had been experiencing a high frequency of disconnections in order toexamine the conditions under which the event
occurred. This equipment was used to collect andanalyze IP packet traffic and PBX logs at the time ofthe event occurrence (Fig. 2).
The following results were obtained from thisanalysis. (1) Packet capture data revealed that thehead-office VoIP-GW on the call-terminating sidetransmitted a BYE disconnect signal to the call-originating side (Fig. 3). (2) A putt-putt sound was output
from the head-office VoIP-GW at the time of the disconnect, coinciding with the regeneration of voice
data as indicated in packet capture data. (3) A surveyof PBX logs over a period of about one month
revealed that a disconnection (incomplete call) occurring within two seconds of answering an incoming
call occurred 65 times on multiple phone sets. It wastherefore concluded that the event was not caused bya defect in a phone set or by the intentional termination
NTT Technical Review

Practical Field Information about Telecommunication Technologies

Branch office

Head office*Upgraded

PBX

VoIP-GWIP

ONU

AnalogONU*Upgraded

IP-VPN

IP*UpgradedCalloriginating

AnalogVoIP-GW

PBX*Upgraded

Disconnected
on answeringCallterminating

IP extension calling

ONU: optical network unit

Fig. 1. Example of event occurrence.

IP-VPNONU

Head officeIP

AnalogVoIP-GW
PBX
Logs

IP packet captureCallAnalyze terminating

Fig. 2. On-site troubleshooting layout.

Call-originatingside (branch office)Call-terminatingside (head office)Connection
requestConnecting
Calling
Answersuccessful
Answeracknowledged
Voice data
Disconnect
Disconnect
successful

Fig. 3. Call control sequence at time of event.

of calls.
The above results suggested that the cause of thisproblem lie in either the VoIP-GW or PBX in the head
office where the IP extension call terminated. With
Vol. 14 No. 8 Aug. 2016

this in mind, the Technical Assistance and Support
Center constructed a pseudo-environment of customer facilities to carry out more detailed testing.

2

Practical Field Information about Telecommunication Technologies

Branch officepseudo-environment

Head office pseudo-environmentCallterminating

IP-VPNONU

VoIP-GW

ONU

AnalogStandalone
phone

VoIP-GW
PBX

Call
originatingCallterminating
PBX-connectedphone

Checks electrical operation in analog interval using waveform recorderFig. 4. Schematic of reproducibility test.

4. Reproducibility testing
To find the cause of this event, the Technical Assistance and Support Center conducted a reproducibility
test by preparing VoIP-GW and PBX equipment and
IP-VPN services as used by the customer and constructing an environment that emulated the system
environment in which the event was occurring. Furthermore, to troubleshoot both the VoIP-GW andPBX at the event occurrence, a standalone phone set
was prepared to check whether there were any differences between the call-terminating operation whenconnecting the standalone phone to the VoIP-GW andthe call-terminating operation when connecting the
PBX to the VoIP-GW. It was also decided to check forany changes in electrical operation in the analog
interval below the VoIP-GW using a waveformrecorder and to check the electrical operation between
the VoIP-GW and terminals connected below the
VoIP-GW when terminating a call (Fig. 4).
5. Results of reproducibility testing(1)	A reproducibility test was conducted several
times when directly connecting a standalone
phone set to the VoIP-GW and terminating acall, and not a single disconnection event
occurred.
(2)	A reproducibility test was also done several
times when directly connecting the PBX to theVoIP-GW and terminating a call, and in thiscase, a call-disconnection event occurred on
multiple occasions immediately after answering
the incoming call.
3

Consequently, in reproducibility testing using the
PBX, the technicians examined the voltage waveform
between the layer 1 and 2 (L1–L2) lines during normal answering of an incoming call and the voltage
waveform between the L1–L2 lines when a call was
disconnected immediately after answering. First, fora normal connection, the VoIP-GW transmits a calling signal to the PBX on terminating the incoming
call, and the PBX-connected phone goes off-hook. At
this time, the polarity between L1–L2 is depolarized,
and the call progresses (Fig. 5).
However, at the time of the event occurrence, theL1–L2 loop state cannot be maintained after depolarization of L1–L2 polarity when the PBX-connected
phone goes off-hook, and as a result, the circuit isreleased (disconnected), and the call is terminated(Fig. 6).
Additionally, on examining these waveforms indetail, it was apparent that the time from off-hook to
depolarization was 50–575 ms when connecting normally, but 606–640 ms whenever the event occurred.
The above reproducibility test therefore showedthat this event in which an incoming call failed toconnect would occur whenever the time from offhook to depolarization was 606–640 ms. In view ofthis fact, the settings related to the analog connection
between the VoIP-GW and PBX were examined. It
was found that the period for detecting the closingof the VoIP-GW TEL port (analog telephone port)
circuit due to a terminal off-hook operation was set
to 640 ms as a default value in the VoIP-GW. Meanwhile, on the PBX side, the guard time setting turned
out to be 600 ms as a default value. This setting servesto prevent an unstable waveform generated at
NTT Technical Review

Practical Field Information about Telecommunication Technologies

Waveform in normal operation
Off-hook

Call terminating

220 msCall in progress
CallingsignalDepolarization

Fig. 5. Voltage between L1–L2 lines in normal connection.

Waveform at time of event
Off-hook

Call terminating

610 ms0V

Disconnect
CallingsignalDepolarization

Fig. 6. Voltage between L1–L2 lines at time of event.

depolarization after the off-hook operation frombeing falsely recognized as an on-hook operation.
From these results, it was confirmed that changing
any of the following settings in the VoIP-GW or PBX
could avoid the problem of an incoming call being
disconnected.
(1)	In the VoIP-GW, change the default value ofthe period for detecting closing of the TEL
port circuit from 640 ms to 560 ms.
(2)	In the PBX, change the default value of theguard timer for preventing an unstable waveform from being falsely recognized as onVol. 14 No. 8 Aug. 2016

hook from 600 ms to 650 ms.
(3)	Change the PBX on-hook signal detection
method from its default value of polarity
detection or BT detection to BT detection sothat depolarization after the call is off-hook is
not treated as if it were on-hook.
6. Countermeasure
On the basis of the results of this reproducibility
test examining disconnections of incoming calls, theTechnical Assistance and Support Center consulted
4

Practical Field Information about Telecommunication Technologies

Off-hook

Time lag to depolarization
occurs due to VoIP-GW loop
detection period. PBX judges
depolarization occurring morethan 600 ms after off-hook to
be an on-hook operation.

Callterminating
0V

PBX disconnects
CallingsignalDepolarization

Fig. 7. Operation in customer’s equipment at time of event.

with the customer and recommended that the setting
for the PBX on-hook signal detection method bechanged (item (3) above). This effectively eliminatedthe event in which an incoming IP extension call was
disconnected upon answering.
7. Conclusion
The VoIP-GW used by the customer in this casestudy had a period of 640 ms for detecting the closingof the TEL port circuit due to a terminal off-hook
operation. Thus, depending on the timing of this offhook operation, a maximum time lag of 640 ms from
VoIP-GW detection of the off-hook state to L1–L2
depolarization might occur. In the PBX, meanwhile,a time of 600–640 ms from off-hook to L1–L2 depolarization would exceed the guard timer (600 ms),
which is used to prevent an unstable waveform at the5time of depolarization from being falsely recognized
as an on-hook operation. In such a case, it appears
that the PBX would erroneously judge that an onhook operation had occurred and would disconnect
the circuit as a result (Fig. 7).
In this case study, the combination of specific VoIPGW and PBX models resulted in the disconnection ofincoming calls immediately after answering.
When equipment is selected for constructing a newuser system or upgrading an existing one, it is important that system operation be checked beforehand asmuch as possible based on the customer’s usage format. However, if a problem occurs that cannot berectified by a straightforward measure such as replacing equipment, a technique that measures waveforms
and signals to isolate the source of the problem canprovide a shortcut to a solution.

NTT Technical Review

External Awards
The 63rd JSAP Spring Meeting 2016, Poster Award
Winner: Masaaki Ono, Hideaki Taniyama, Masato Tsunekawa,
Eiichi Kuramochi, Kengo Nozaki, and Masaya Notomi, NTT Basic
Research Laboratories
Date: April 1, 2016
Organization: The Japan Society of Applied Physics (JSAP)

For “A Novel Graph-based Topology Control Cooperative Algorithm for Maximizing Throughput of Disaster Recovery Networks.”
Published as: T. Ngo, H. Nishiyama, N. Kato, S. Kotabe, and H.
Tohjo, “A Novel Graph-based Topology Control Cooperative Algorithm for Maximizing Throughput of Disaster Recovery Networks,”
Proc. of IEEE VTC2016-Spring, Nanjing, China, May 2016.

For “Demonstration of Efficient Mode Conversion to Slot Waveguides with λ2/2000 Cross-sectional Area.”
Published as: M. Ono, H. Taniyama, M. Tsunekawa, E. Kuramochi,
K. Nozaki, and M. Notomi, “ Demonstration of Efficient Mode Conversion to Slot Waveguides with λ2/2000 Cross-sectional Area,” The63rd JSAP Spring Meeting, 20p-P4-8, Tokyo, Japan, Mar. 2016.

Best Paper Award
Winner: Hideki Kuribayashi, Tohoku University; Katsuya Suto,
University of Waterloo; Hiroki Nishiyama and Nei Kato, Tohoku
University; Kimihiro Mizutani, Takeru Inoue, and Osamu Akashi,
NTT Network Innovation Laboratories
Date: May 25, 2016
Organization: IEEE International Conference on Communications(ICC) Best Paper Award Selection Committee

ISSJ 10th Anniversary Best Paper Award
Winner: Takeshi Morita and Yunki Hong, Keio University; Shinobu
Saito, NTT Software Innovation Center; Tadashi Iijima and Takahira
Yamaguchi, Keio University
Date: May 14, 2016
Organization: Information Systems Society of Japan (ISSJ)
For “A Support Tool for Production Management Process Modeling Based on SCOR Ontology.”
Published as: T. Morita, Y. Hong, S. Saito, T. Iijima, and T. Yamaguchi, “A Support Tool for Production Management Process Modeling
Based on SCOR Ontology,” JISSJ, Vol. 11, No. 1, pp.13–47, Dec.
2015.
ITU-AJ Award, Encouragement Award: ICT Field
Winner: Kenjiro Arai, NTT Network Service Systems Laboratories
Date: May 17, 2016
Organization: The ITU Association of JapanFor his contribution to standardization in 3GPP (3rd Generation
Partnership Project)/TTC (Telecommunication Technology Committee) call control signaling in IMS (Internet-protocol Multimedia
Subsystem).
Best Paper Award
Winner: Doohwan Lee, Hirofumi Sasaki, Hiroyuki Fukumoto, andTadao Nakagawa, NTT Network Innovation Laboratories
Date: May 17, 2016
Organization: 2016 International Workshop on Smart WirelessCommunications (SmartCom2016) organizing committee
For “Toward Realization of a New Wireless Transmission Technology: Orbital Angular Momentum (OAM) Multiplexing.”
Published as: D. Lee, H. Sasaki, H. Fukumoto, and T. Nakagawa,“Toward Realization of a New Wireless Transmission Technology:
Orbital Angular Momentum (OAM) Multiplexing,” IEICE Tech.
Rep., Vol. 116, No. 29, SR2016-16, pp. 57–58, May 2016.
Best Paper Award
Winner: Thuan Ngo, Hiroki Nishiyama, and Nei Kato, Tohoku University; Satoshi Kotabe and Hiroshi Tohjo, NTT Network Innovation
Laboratories
Date: May 18, 2016
Organization: 2016 IEEE (Institute of Electrical and Electronics
Engineers) 83rd Vehicular Technology Conference (VTC2016Spring) committee

1

For “A Mobility-based Mode Selection Technique for Fair Spatial
Dissemination of Data in Multi-channel Device-to-device Communication.”
Published as: H. Kuribayashi, K. Suto, H. Nishiyama, N. Kato, K.
Mizutani, T. Inoue, and O. Akashi, “A Mobility-based Mode Selection Technique for Fair Spatial Dissemination of Data in Multichannel Device-to-device Communication,” Proc. of IEEE ICC 2016,
Kuala Lumpur, Malaysia, May 2016.
Best Paper Award
Winner: Thuan Ngo, Hiroki Nishiyama, and Nei Kato, Tohoku University; Satoshi Kotabe and Hiroshi Tohjo, NTT Network Innovation
Laboratories
Date: May 25, 2016
Organization: IEEE International Conference on Communications(ICC) Best Paper Award Selection Committee
For “GHAR: Graph-based Hybrid Adaptive Routing for Cognitive
Radio Based Disaster Response Networks.”
Published as: T. Ngo, H. Nishiyama, N. Kato, S. Kotabe, and H.
Tohjo, “GHAR: Graph-based Hybrid Adaptive Routing for Cognitive
Radio Based Disaster Response Networks,” Proc. of IEEE ICC 2016,
Kuala Lumpur, Malaysia, May 2016.
Best Industry Paper Award
Winner: Ho-Jin Song, Toshihiko Kosugi, Hiroshi Hamada, Takuro
Tajima, Amin El Moutaouakil, Hideaki Matsuzaki, and Makoto Yaita,
NTT Device Technology Laboratories; Yoichi Kawano, Tsuyoshi
Takahashi, Yasuhiro Nakasha, and Naoki Hara, Fujitsu Limited; Katsumi Fujii, Issei Watanabe, and Akifumi Kasamatsu, National Institute of Information and Communications Technology
Date: May 27, 2016
Organization: IEEE International Microwave Symposium (IMS)2016 committee
For “Demonstration of 20-Gbps Wireless Data Transmission at
300 GHz for KIOSK Instant Data Downloading Applications withInP MMICs.”
Published as: H.-J. Song, T. Kosugi, H. Hamada, T. Tajima, A. E.
Moutaouakil, H. Matsuzaki, M. Yaita, Y. Kawano, T. Takahashi, Y.
Nakasha, N. Hara, K. Fujii, I. Watanabe, and A. Kasamatsu, “Demonstration of 20-Gbps Wireless Data Transmission at 300 GHz for
KIOSK Instant Data Downloading Applications with InP MMICs,”
Proc. of IMS2016, San Francisco, CA , USA, May 2016.

NTT Technical Review

External Awards

Best Paper Award 2015
Winner: Yasuhiro Fujiwara, NTT Software Innovation Center;
Makoto Nakatsuji, NTT Service Evolution Laboratories; Hiroaki
Shiokawa and Takeshi Mishima, NTT Software Innovation Center;
Makoto Onizuka, Osaka University
Date: June 2, 2016
Organization: The Institute of Electronics, Information and Communication Engineers (IEICE)
For “Fast Ad-hoc Search Algorithm for Personalized PageRank.”
Published as: Y. Fujiwara, M. Nakatsuji, H. Shiokawa, T. Mishima,
and M. Onizuka, “Fast Ad-hoc Search Algorithm for Personalized
PageRank,” IEICE Trans. Inf. & Syst. (Japanese Edition), Vol. J98-D,
No. 5, pp. 774–787, May 2015.
Best Paper Award 2015
Winner: Hideya So and Atsuya Ando, NTT Network Innovation
Laboratories; Tomohiro Seki, Nihon University; Munenari Kawashima, NTT Intellectual Property Center; Takatoshi Sugiyama,
Kogakuin University
Date: June 2, 2016
Organization: IEICE
For “Multiband Sector Antenna with the Same Beamwidth
Employing Multiple Woodpile Metamaterial Reflectors.”
Published as: H. So, A. Ando, T. Seki, M. Kawashima, and T. Sugiyama, “Multiband Sector Antenna with the Same Beamwidth
Employing Multiple Woodpile Metamaterial Reflectors,” IEICE
Trans. Electron., Vol. E97-C, No. 10, pp. 976–985, Oct. 2014.
IPSJ Fellow
Winner: Katsumi Takahashi, NTT Secure Platform Laboratories
Date: June 3, 2016
Organization: Information Processing Society of Japan (IPSJ)
For his contribution to the development and diffusion of privacy
protection technology for the use of personal data.
Outstanding Paper Award
Winner: Ryohei Banno, Susumu Takeuchi, Michiharu Takemoto,and Tetsuo Kawano, NTT Network Innovation Laboratories; Takashi
Kambayashi, NTT-AT IPS Corporation; Masato Matsuo, Japan Patent Office
Date: June 3, 2016
Organization: IPSJ
For “Designing Overlay Networks for Handling Exhaust Data in aDistributed Topic-based Pub/Sub Architecture.”
Published as: R. Banno, S. Takeuchi, M. Takemoto, T. Kawano, T.
Kambayashi, and M. Matsuo, “Designing Overlay Networks for
Handling Exhaust Data in a Distributed Topic-based Pub/Sub Architecture,” Journal of Information Processing, Vol. 23, No. 2, pp. 105–
116, Mar. 2015.
IPSJ Nagao Special Researcher Award
Winner: Yasuhiro Fujiwara, NTT Software Innovation Center
Date: June 3, 2016
Organization: IPSJ

Vol. 14 No. 8 Aug. 2016For his pioneering research on efficient mining techniques for big
data.
Best Paper Award
Winner: Akihiro Shimoda, Keisuke Ishibashi, and Shigeaki Harada,
NTT Network Technology Laboratories; Kazumichi Sato, NTT
Communications; Masayuki Tsujino, NTT Network Technology
Laboratories; Takeru Inoue, NTT Network Innovation Laboratories;
Masaki Shimura, Takanori Takebe, Kazuki Takahashi, Tatsuya Mori,and Shigeki Goto, Waseda University
Date: June 6, 2016
Organization: Technical Committee on Internet Architecture, IEICE
Communications Society
For “Inferring the Number of Accesses to Internet Services Using
DNS Traffic.”
Published as: A. Shimoda, K. Ishibashi, S. Harada, K. Sato, M.
Tsujino, T. Inoue, M. Shimura, T. Takebe, K. Takahashi, T. Mori, and
S. Goto, “Inferring the Number of Accesses to Internet Services
Using DNS Traffic,” IEICE Tech. Rep., Vol. 115, No. 307, IA201563, pp. 129–134, Nov. 2015.
Best Paper Runner-Up Award
Winner: Richard Chen, Takeru Inoue, Toru Mano, Kimihiro Mizutani, Hisashi Nagata, and Osamu Akashi, NTT Network Innovation
Laboratories
Date: June 6, 2016
Organization: Technical Committee on Internet Architecture, IEICE
Communications Society
For “Efficient Network Policy Checking with Multi-dimensional
Graph Traversal Algorithm.”
Published as: R. Chen, T. Inoue, T. Mano, K. Mizutani, H. Nagata,and O. Akashi, “Efficient Network Policy Checking with Multidimensional Graph Traversal Algorithm,” IEICE Tech. Rep., Vol.
115, No. 256, IA2015-34, pp. 25–30, Oct. 2015.
Distinguished Service Award
Winner: Toshio Norimatsu, NTT Network Service Systems Laboratories
Date: June 21, 2016
Organization: The Telecommunication Technology Committee
For his contribution to the formulation of the standardized specifications for ENUM (E.164 number mapping)/DNS (domain namesystem) interfaces that enable number portability.
Distinguished Service Award
Winner: Takashi Kotanigawa, NTT Network Service Systems Laboratories
Date: June 21, 2016
Organization: The Telecommunication Technology Committee
For his contribution to the standardization concerning logicalinterfaces between networks of carriers such as MPSL-TP (multiprotocol label switching - transport profile) and Ethernet.

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Papers Published in Technical Journals andConference Proceedings
Multi-Service Fabric – An Any-vendor SDN Architecture
for Service Provider Network
T. Iwai
SDN & OpenFlow World Congress, Düsseldorf, Germany, October 2015.
Many service providers are now investigating the employment of
NFV (network functions virtualization). For SDN (software-defined
network), however, the scope is usually limited to applications insidetheir datacenter networks. NTT, who announced the “NetroSphere”
concept in which we aim to make maximum use of general-purpose
servers and switches for carrier networks, is now challenging to
replace the high-end core and edge routers to switch clusters controlled as SDN. We named this architecture as Multi-Service Fabric(MSF) architecture and have been developing the network systems
based on the architecture. This architecture targets enabling “anyvendors” to join carrier networks. The presentation covers the architecture, technologies, challenges and the use-cases of MSF.

TMS over V5/hMT+ Disrupts Tactile Direction Discrimination
T. Amemiya, B. Beck, H. Gomi, and P. Haggard
Neuroscience 2015, 156.17, Chicago, IL, USA, October 2015.
Several human imaging studies have found that visual motion area
V5/human medial temporal complex (hMT+) responds to tactile
motion. A multivariate pattern analysis found specific patterns ofactivity in V5/hMT+ corresponding to leftward and rightward tactile
directions. Some studies have also reported activations in the primarysomatosensory cortex (SI) and posterior parietal cortex (PPC) during
tactile motion, but they have not established a causal involvement ofthese areas in tactile direction processing. Here, we created an ecological tactile motion stimulus by varying the direction of a single
object moving across the fingertip. We disrupted activity in SI, PPC(Brodmann’s areas 7/40) and V5/hMT+ using online double-pulse
transcranial magnetic stimulation (TMS) while participants judged
tactile motion direction. TMS over both SI and V5/hMT+, but notover PPC, reduced tactile direction discrimination. Our results demonstrate, for the first time, that V5/hMT+ plays a causal role in tactile
direction processing, extending previous studies that found directionally sensitive patterns of activity in V5/hMT+. Further, our findings
are consistent with a serial model of cortical tactile processing, inwhich processing by higher-order perceptual areas depends upon thequality of input received from the SI. By contrast, our results do notprovide clear evidence that the PPC is causally involved in discriminating tactile direction. This suggests that the pathway for tactile
motion processing is not routed through inferior regions of the PPC.

Optical Linear Blending of Viewing Zones Using Convolution of Iris for Smooth Motion Parallax Autostereoscopic
3D Display
T. Kawakami, M. Date, M. Sasai, and H. Takada
Journal of Display Technology, Vol. 12, No. 2, pp. 143–151, February 2016.
A new autostereoscopic three-dimensional (3D) display is proposed. The use of only a small number of projectors and optical linear

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blending by optical convolution of the iris produces smooth motion
parallax and the same depth regardless of viewing positions andinterpupillary distance by applying the visual effects of dual edge
perception in a depth-fused 3D (DFD) display. This is a breakthroughin overcoming the trade-off between 3D image reality and the number of video sources.

Techniques to Reduce Driving Energy of 1-pixel Displays
H. Manabe, M. Date, H. Takada, and H. Inamura
IEEE Transactions on Industry Applications, Vol. 52, No. 3, pp.
2638–2647, May 2016.
We propose a pair of techniques to lower the energy consumed bydriving 1-pixel liquid-crystal displays (LCDs). The first one employs
multiple capacitors while the other divides the LCD into two to lower
the drive voltage. Simulations show that large capacitance values andmany capacitors reduce the energy consumed, and stacking two thinlow voltage LCDs maximizes the effect by decreasing the overhead
consumed by the microcontroller. Actual polymer-dispersed LCDs
(PDLCDs) were tested to confirm that the techniques worked as
effectively as the simulations implied. The results show that the overall energy consumption of large PDLCDs is reduced more than 70%
using multiple capacitors, and the combination of the two techniques
successfully reduces the driving energy even for small LCDs. While
the polarity reversal of the proposed technique does incur some delay,
it was confirmed by a flicker test that the technique does not degrade
image quality.

Optical Diversity Transmission Using WDM Signal andPhase-conjugate Lights through Multi-core Fiber
M. Koga, M. Moroi, and H. Takara
Optics Express, Vol. 24, No. 9, pp. 9340–9352, May 2016.
This paper proposes a maximum-ratio combining (MRC) scheme
for a wavelength division multiplexing (WDM) signal and phaseconjugate pair (PCP) diversity transmission to cancel nonlinear
phase-shift. A transfer function approximation for nonlinear phaseshift cancellation was formulated. It shows, with the help of numerical calculation, that span-by-span chromatic dispersion compensation is more effective than the lumped equivalent at the receiver. Thisis confirmed in a 2-core diversity 5-channel WDM transmission
experiment over 3 spans of 60-km MCF (multi-core fiber) with 25
Gbit/s-QPSK (quadrature phase-shift keying) PCP. The peak Q-value
was enhanced by 3.6 dB through MRC, resulting in a superior bitratedistance product and optical power density limit compared to twice
the single-core transmission.

Progress in LPC-based Frequency-domain Audio Coder
T. Moriya, R. Sugiura, Y. Kamamoto, H. Kameoka, and N. Harada
APSIPA Transactions on Signal and Information Processing, Vol.
5, e11, May 2016.
This paper describes the progress in frequency-domain linear prediction coding (LPC)-based audio coding schemes. Although LPC
was originally used only for time-domain speech coders, it has been

NTT Technical Review

Papers Published in Technical Journals and Conference Proceedings

applied to frequency-domain coders since the late 1980s. With theprogress in associated technologies, the frequency-domain LPCbased audio coding scheme has become more promising, and it has
been used in speech/audio coding standards such as MPEG-D unified
speech and audio coding and 3GPP (3rd Generation Partnership
Project) enhanced voice services since 2010. Three of the latest
investigations on the representations of LPC envelopes in frequencydomain coders are shown. These are the harmonic model, frequencyresolution warping, and the Powered All-Pole Spectral Envelope, allof which are aimed at further enhancement of the coding efficiency.

Screen Free Floating 3D Image in a Crystal Ball Using
Spatially Imaged Iris and Multiview DFD (Depth Fused 3D)
Technologies
M. Date, T. Kawakami, M. Sasai, and H. Takada
Proc. of SID Display Week 2016 Digest, pp. 146–149, San Francisco, CA, USA, May 2016.
A method for displaying clear floating images in a crystal ball is
proposed. Its symmetric optics can provide clear and natural
360-degree images with smooth motion parallax in horizontal andvertical directions using the directional selectivity of a spatiallyimaged iris method and natural 3D (three-dimensional) images of amultiview DFD display.

Flexible SDN Architecture for Carrier Networks
T. Iwai
Proc. of the 4th Annual Network Virtualization & SDN Europe,
Madrid, Spain, May 2016.
Beyond NFV/SDN, NetroSphere concept & technologies, flexible
SDN network architecture, Multi-Service Fabric (MSF), and thetechnologies and use cases of MSF are proposed.

Vol. 14 No. 8 Aug. 2016

Thickness Modulation and Strain Relaxation in Straincompensated InGaP/InGaP Multiple-quantum-well Structure Grown by Metalorganic Molecular Beam Epitaxy on
GaAs (100) Substrate
M. Mitsuhara, N. Watanabe, H. Yokoyama, R. Iga, and N. Shigekawa
Journal of Crystal Growth, Vol. 449, pp. 86–91, June 2016.
We have investigated the structural features of a strain-compensated InGaP/InGaP multiple-quantum-well (MQW) structure on GaAs
(100) substrate with a band-gap energy of around 1.7 eV for solar cellapplications. In transmission electron microscopy images, noticeable
thickness modulation was observed in the barrier layers for a sample
grown at the substrate temperature of 530 °C. Meanwhile, the X-ray
diffraction patterns indicated that strain relaxation predominantly
occurred in the well layers. Decreasing the substrate temperaturefrom 530 to 510 °C was effective in suppressing both the thickness
modulation and strain relaxation. Additionally, increasing the growthrate of the well layer further suppressed the thickness modulation. In
room-temperature photoluminescence (PL) emission spectra, thesample grown at 510 °C showed approximately 50 times higher PL
peak intensity than the one grown at 530 °C.

Single-source AlGaAs Frequency Comb Transmitter for
661 Tbit/s Data Transmission in a 30-core Fiber
H. Hu, F. Da Ros, F. Ye, M. Pu, K. Ingerslev, E. P. Da Silva, Md.
Nooruzzaman, Y. Amma, Y. Sasaki, T. Mizuno, Y. Miyamoto, L.
Ottaviano, E. Semenova, P. Guan, D. Zibar, M. Galili, K. Yvind, L.
K. Oxenløwe, and T. Morioka
Proc. of Conference on Lasers and Electro-Optics, JTh4C.1, San
Jose, CA, USA, June 2016.
We demonstrate an AlGaAs-on-insulator nano-waveguide-basedfrequency comb with high OSNR (optical signal-to-noise ratio)
enabling a single source to fully load a 9.6-km heterogeneous 30-core
fiber with 661-Tbit/s data achieved by 30xcores, 80xWDM (wavelength division multiplexing), 40 Gbaud, and PDM-16QAM (polarization-division multiplexed 16-level quadrature amplitude modulation).

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