OPEN

NPG Asia Materials (2016) 8, e310; doi:10.1038/am.2016.144
www.nature.com/am

ORIGINAL ARTICLE

All-nanocellulose nonvolatile resistive memory
Umberto Celano1,2,5, Kazuki Nagashima3,5, Hirotaka Koga4, Masaya Nogi4, Fuwei Zhuge3, Gang Meng3,
Yong He3, Jo De Boeck1, Malgorzata Jurczak1, Wilfried Vandervorst1,2 and Takeshi Yanagida3
Single-use disposable nonvolatile memory devices hold promise for novel applications in internet of everything (IoE) technology
by storing the health status of individual humans in daily life. However, conventional memory devices are not disposable because
they are mostly composed of non-renewable, non-biodegradable and sometimes toxic materials, causing serious damage toecological systems when they are released to the environment. Here, we demonstrate an environment-friendly, disposablenonvolatile memory device composed of 99.3 vol.% nanocellulose. Our memory device consists of a nanocellulose-based
resistive-switching layer and a nanopaper substrate. The device exhibited nonvolatile resistive switching with the capability ofmultilevel storage and potential scalability down to the single nanoﬁber level (ca. 15 nm). The biodegradability of our memory
device was conﬁrmed by burying it in natural soil for 26 days.
NPG Asia Materials (2016) 8, e310; doi:10.1038/am.2016.144; published online 16 September 2016

INTRODUCTION
Recently, the internet of everything (IoE), which enables
information transfer between individual humans, animals, thingsand places through the internet environment, has attracted muchattention as a novel electronic technology next to the internet ofthings.1–3 IoE technology offers real-time monitoring and analysisof the health status of individual humans in daily life. However,some data cannot be obtained if the target temporarily leaves theinternet environment. Light-weight, single-use and disposablenonvolatile memory devices hold promise for novel applications
in IoE technology because they can store these data temporarily
when the target enters a noninternet environment, and they can bedisposed of when they complete this function. Conventional
memory devices are not disposable because they are mostly madeof non-renewable, non-biodegradable and sometimes toxic
materials, causing serious damage to ecological systems when they
are emitted to the environment.4 Therefore, the development of
nonvolatile memory using renewable, biodegradable, nontoxic andenvironment-friendly materials is strongly required for disposable
IoE electronic devices.
A paper-based memory device is one of the most promising options
because cellulose is the most abundant renewable material. Amongpaper-based electronics, devices using so-called nanopaper haverecently attracted increasing attention. Nanopaper is made of nanocellulose, for example, cellulose nanoﬁbers or cellulose nanorods
obtained by mechanically ﬁbrillated wood pulp. Nanopaper has
foldability, a relatively high resistance to water and solvents, surface
ﬂatness and high optical transparency.5–9 Furthermore, nanopaper has
an extremely low thermal expansion coefﬁcient (6 × 10 − 6/K)

compared with other polymer materials that are used for biodegradable electronics.10–16 Therefore, nanopaper has recently received
much attention as a substrate for ﬂexible electronics.9,17–29 To
date, various electronic devices including organic/inorganic
thin-ﬁlm transistors,9,21–23 light-emitting diodes,24 solar cells,23,25,26
antennas27,28 and triboelectric generators29 have been demonstrated
on nanopaper substrates. However, a nonvolatile memory, which isthe essential component for IoE technology, has not been demonstrated on a nanopaper substrate so far. Here, we demonstrate anenvironment-friendly, disposable nonvolatile memory device
composed of 99.3 vol.% nanocellulose. Our memory device consistsof a nanocellulose-based resistive-switching layer and a nanopaper
substrate. The device exhibited electrochemical redox-based nonvolatile resistive switching with the capability of multilevel storage andpotential scalability down to the single nanoﬁber level (ca. 15 nm).
The disposability of our memory device was also demonstrated byexamining the biodegradability of the device in natural soil.
MATERIALS AND METHODS
Preparation of cellulose nanoﬁbers and nanopaper
Cellulose nanoﬁbers with a width of ca. 15–20 nm were obtained fromsoftwood chips (Cryptomeria japonica).5,8 Brieﬂy, the softwood chips weretreated with sodium chlorite and potassium hydroxide. Then, an aqueous
suspension of the as-prepared wood pulp (0.5 wt%, 2 l) was subjected to ahigh-pressure water-jet system (Star Burst, HJP-25005E, Sugino Machine,
Japan) equipped with a ball-collision chamber. The pulp suspension was ejectedfrom a nozzle with a diameter of 0.15 mm under a high pressure of 245 MPa
with 50 repetition cycles. For the preparation of nanopaper, the cellulose
nanoﬁber suspension was cast onto an acrylic plate, oven-dried at 50 °C for
1 day and then peeled off from the plate. Nanopaper memory devices were1
imec, KU Leuven, Belgium; 2Department of Physics and Astronomy (IKS), KU Leuven, Belgium; 3Institute for Materials Chemistry and Engineering, Kyushu University, Fukuoka,
Japan and 4The Institute of Scientiﬁc and Industrial Research, Osaka University, Osaka, Japan
5These authors contributed equally to this work.
Correspondence: Dr K Nagashima or Professor T Yanagida, Institute for Materials Chemistry and Engineering, Kyushu University, 6-1 Kasuga-Koen, Kasuga, Fukuoka 816-8580, Japan.
E-mail: kazu-n@cm.kyushu-u.ac.jp or yanagida@cm.kyushu-u.ac.jp
Received 9 April 2016; revised 11 July 2016; accepted 28 July 2016

All-nanocellulose nonvolatile resistive memory
U Celano et al2

Figure 1 Structural information of a nanopaper memory device. (a) Schematic illustration of the constituent material of a nanopaper memory device. The
nanopaper was made of softwood chips, and the ﬁeld emission scanning electron microscopy (FESEM) image shows the surface morphology of the nanopaper
substrate. The nanopaper memory device consists of 99.3 vol.% nanocellulose. (b) Photograph of the nanopaper memory device. (c) Atomic force microscopy(AFM) image of the resistive-switching layer deposited onto an indium tin oxide (ITO)/nanopaper substrate. The thickness of the resistive-switching layer is ca.
100 nm. (d) Absorption spectra of the nanopaper substrate and resistive-switching layer/ITO/nanopaper substrate. No signiﬁcant change in transmittance wasobserved after the deposition of the ITO electrode and the resistive-switching layer.

fabricated using a thick nanopaper substrate (~30–50 μm). First, an indium tinoxide (ITO) bottom electrode with a thickness of 100 nm was deposited byradiofrequency sputtering at 1 Pa with Ar/O2 mixed gas (100 p.p.m. O2
in Ar). The deposition conditions were optimized to achieve good transparency
and high conductivity. The resistivity of the layer was ~ 3 × 10 − 3 Ωcm that wasmeasured by a standard four-probe measurement. Then, a resistive-switchinglayer was created by the drop-casting method, followed by the ITO deposition.
For the deposition, we placed a droplet (10 μl) of the cellulose nanoﬁber
aqueous suspension (0.02 wt%) on top of the ITO layer, followed by drying at
40 °C for 15 min. After the water evaporated, the thin cellulose nanoﬁber sheet
was left on the ITO layer. The thickness of the resistive-switching layer was
100 nm that was controlled by the number of drop cycles, as shown in
Supplementary Information S1. The thickness of the nanopaper was measured
by a surface proﬁler. Three cycles of drop casting led to a layer thickness of~ 100 nm. Finally, the 100 nm-thick Ag top electrodes were deposited using
radiofrequency sputtering and patterned by a metal mask. For the measurementof the total transmittance of the nanopaper, we used a ultraviolet–visible
spectrometer with an integrating sphere (U-3900, Hitachi High-Tech., Tokyo,
Japan). The measurement was performed using light in the wavelength range of
200–800 nm. Atomic probe microscopy (AFM) was utilized to characterize thesurface morphology of the nanopaper.

performed using a Pt-coated conductive cantilever. Before the measurement,
the cellulose nanoﬁber aqueous solution (0.02 wt%) was diluted to a 1:1000
ratio by deionized water and then dropped onto a 100 nm thick Ag-coated Si
substrate. For the electrical measurement, the voltage was applied to the Agelectrode whereas the conductive cantilever was grounded. The measurement
was performed in atmospheric air conditions at room temperature. On theother hand, the sample for the nanogap measurement was fabricated by 30 kVelectron beam (EB) lithography (JEM-7001F, JEOL, Tokyo, Japan and SPG-724
Nano printer, SANYU Electron, Tokyo, Japan). First, the contact electrode andmarker pattern were patterned onto a Si substrate with a 100 nm SiO2 capping
layer. The ZEP520A-7 (Zeon Corporation, Tokyo, Japan), which was used as an
EB resist, was spin-coated at 5000 r.p.m. for 90 s, followed by baking at 180 °C
for 2 min. After the EB lithography, the pattern was developed by immersion in
ZED-N50 (Zeon Corporation) for 90 s, and the Ti/Pt electrode was depositedby radiofrequency sputtering at 1 Pa. Next, the diluted cellulose nanoﬁber
aqueous suspension was dropped onto the pre-patterned substrate. Afterobserving the single cellulose nanoﬁber by ﬁeld emission scanning electron
microscopy, the nanoelectrode pattern was overlaid by EB lithography. For
making the contact to the cellulose nanoﬁber, Pt was utilized to avoid possible
oxidization at the contact interface. The transport property of a single cellulose
nanoﬁber was determined in vacuum conditions (~1 × 10 − 3 Pa) to minimize
the contribution of adsorbed water.

Electrical transport measurement of nanopaper memory andcellulose nanoﬁber

Electrode removal and biodegradation experiments

The current–voltage (I–V) characteristics of the nanopaper memory devices
were obtained using a semiconductor parameter analyzer (Keithley 4200-SCS,
TFF Corporation, Tokyo, Japan). The experiments were carried out inatmospheric air conditions at room temperature. For all cases, the current
compliance was employed to prevent the devices from destructive breakdown
phenomena. For the single cellulose nanoﬁber measurement, we employedboth a conductive-AFM (C-AFM) and a nanogap measurement. C-AFM was

The removal of the electrode from the nanopaper memory was conducted invarious solvents such as propanol, acetone, ethanol, N,N-dimethylformamide
and water. Sonication was applied at 28 kHz for 5 min. After the sonication
process, the residual electrode was evaluated. For the biodegradation experiment, we utilized natural soil. Then, 4 l of soil was collected in September at
34:49:30 north latitude and 135:31:28 east longitude and kept in a plastic bag.
The temperature and the humidity of soil were well controlled to be 22 ± 2 °C

NPG Asia Materials

All-nanocellulose nonvolatile resistive memory
U Celano et al3
and 92 ± 3%, respectively. As control experiments, nanopaper memory devices
on a Si substrate and a polyethylene terephthalate substrate were also examined.

RESULTS AND DISCUSSION
Figure 1a shows the schematic image of a nanopaper-based memory
device. Although our previous nanopaper-based nonvolatile memory
device was fabricated on a Si substrate and/or Al foil,30 the nanopaper
resistive-switching layer was created on a nanopaper substrate in thisstudy. The thickness of the nanopaper substrate was 30 μm. A ﬁeld
emission scanning electron microscopy image showed the surface
morphology of the nanopaper. A smooth surface with a densely
packed nanoﬁber structure was found. Then, the 100 nm ITO thinﬁlm, which was used as an electrode, was sputtered onto thenanopaper substrate. The resistive-switching layer was created bydrop-casting an aqueous solution of cellulose nanoﬁbers. Although
our previous study utilized Ag nanoparticle-decorated cellulose
nanoﬁbers for the resistive-switching layer,30 we utilized pure cellulose
nanoﬁbers in this study. Because the Ag nanoparticles have a large size
distribution and are locally aggregated in the resistive-switching layer,
it is in principle difﬁcult for the Ag nanoparticle-containing nanopaper
to obtain a uniform resistive-switching property when the device
becomes smaller. On the other hand, our novel resistive-switchinglayer in this study is simply composed of cellulose nanoﬁbers without
Ag nanoparticles, allowing a uniform performance of resistive switching even when the device becomes smaller. The thickness of theresistive-switching layer was 100 nm. This thickness was chosen to
secure the electrical insulation and also to minimize the operating
voltage for reaching the critical electric ﬁeld of the initial forming
process. Finally, a Ag electrode was patterned onto the resistiveswitching layer by a metal mask. A photograph of the fabricatednanopaper memory device is shown in Figure 1b. Remarkably, ournanopaper memory device has a 99.3% volume fraction of nanocellulose. This high-volume fraction was achieved by the fabrication of athin conductive electrode layer on a ﬂat surface nanopaper substratethat was unfeasible on regular paper. In general, a large part ofsemiconductor devices is composed of the substrate material. Therefore, the use of a nanopaper substrate plays a crucial role in obtaining
such a high-volume fraction of nanocellulose. Although a paper
substrate has previously been used for electronic device applications,no previous devices on regular paper have a cellulose volume fractionof 450%.31 This is because the surface roughness of regular paper is a
few tens of micrometers, and therefore electronic devices on regularpaper need an electrode of several tens of micrometer thickness toguarantee the continuity of the electrical conduction. Alternatively, thevolume fraction of the electrode can be drastically reduced while
maintaining its conduction property by using a ﬂat surface nanopaper
substrate. Figure 1c shows an AFM image of the resistive-switchinglayer/ITO/nanopaper substrate. The root-mean-square surface roughness of our nanopaper memory device was 7.41 nm, and this is 2–3
orders of magnitude smaller than that of the one on regular paper.
Thus, the use of a ﬂat surface nanopaper substrate plays a crucial rolefor obtaining the high-volume fraction of cellulose by reducing thevolume of the electrodes. Note that in addition to the high-volume
fraction of nanocellulose, our nanopaper memory device also has ahigh optical transparency. Because the cellulose nanoﬁbers are laterally
stabilized by hydrogen bonds and the interstices between the
nanoﬁbers are small enough, the light scattering of the nanopaper isdrastically reduced and high optical transparency can be realized.5
Figure 1d shows the total transmittance of the nanopaper memory as afunction of the wavelength. The transmittance of the nanopaper was
ca. 80% at a wavelength of 600 nm, and this value was maintainedeven after depositing the ITO electrode and the resistive-switchinglayer. Thus, we successfully fabricated a nanopaper memory device
that has a high-volume fraction of nanocellulose.
Next, we show the resistive-switching phenomena of the nanopaper
memory device. Figure 2a shows the typical I–V curves of thenanopaper memory. The measurement was performed at room
temperature in atmospheric air conditions. In this measurement, adevice size of 200 × 200 μm was used. The voltage was applied to the
Ag top electrode, whereas the ITO bottom electrode was grounded.
Initially, the device was highly insulative, and its resistance was
5.3 × 1010 Ω (purple line). An abrupt increase of current was observed
when the voltage was increased to 1.5 V. A current compliance of
1 mA was applied to prevent destructive electrical breakdown of thedevice. This is a so-called forming process that is used to initialize theresistive memory device.32–35 After the forming process, the device
showed a low-resistance state (LRS), with a resistance of 63 Ω
(red line). The LRS was maintained until the negative voltage wasapplied. When the voltage reached − 0.5 V, the current suddenly
dropped, and the device showed a high-resistance state (HRS). This is
the so-called RESET process. After the RESET process, the device
showed a resistance of 3.3 × 109 Ω (blue line). The LRS was seen again
when a positive voltage of 1.0 V was applied. This is the so-called SET
process. Note that the large ON/OFF memory window (a resistanceratio of over 107) in our nanopaper memory is because of the highly
insulative property of cellulose nanopaper (Supplementary
Information S4). The LRS and the HRS were sequentially observedby the SET and RESET processes after initiating the device via theforming process. Thus, our nanopaper memory device showed apolarity-dependent bipolar resistive-switching behavior.32 Next, we
performed retention measurements to examine the nonvolatility of thenanopaper memory device. Figure 2b shows both the LRS and HRS
resistances as a function of time. The voltage of 0.1 V was applied only
when the resistances were read out. Although a slight variation in the
HRS resistance was seen, both the HRS and LRS resistances weremaintained for 104 s while keeping a resistance ratio of over 107,
clearly indicating the nonvolatility of the nanopaper memory device.
Thus, we successfully demonstrated nonvolatile resistive switching in
our nanopaper memory device.
We further examined the multilevel memory effect of our nanopaper memory device. Because our nanopaper memory has a large
ON/OFF memory window, the capability of the multilevel memory
effect is expected by exploiting the intermediate resistance states in thememory window. The multilevel memory effect is an importantfeature for increasing the density of the data storage, as the memory
density can be multiplicatively increased without increasing the device
footprint. Figure 2c shows the LRS and HRS currents when we apply
various compliance currents (50 μA, 500 μA and 1 mA) during theSET processes. The readout voltage was 0.1 V. Three distinct LRSs
appeared according to the levels of the compliance current. Thesestates were maintained for 42 × 103 s, as shown in Figure 2d.
Thus, we demonstrated the capability of multilevel memory in our
nanopaper memory device. Next, we examined the scalability of thenanopaper memory device. Because the nanopaper was composed of acellulose nanoﬁber network, the minimum unit of our memory device
is a single cellulose nanoﬁber. Therefore, we performed a single
cellulose nanoﬁber measurement. Figure 3a shows a schematic
illustration of the single cellulose nanoﬁber measurement. The
C-AFM was utilized for characterizing the transport property of asingle cellulose nanoﬁber. The cellulose nanoﬁbers were developed
onto a Ag thin ﬁlm-coated Si substrate. The details of the control of
the number density of the cellulose nanoﬁbers are provided in
NPG Asia Materials

All-nanocellulose nonvolatile resistive memory
U Celano et al
4

Figure 2 Electrical characterization of nanopaper memory. All of the measurements were performed in atmospheric air conditions at room temperature.
(a) Current–voltage (I–V) curves of nanopaper memory devices. The device initially showed an insulative pristine state (purple). After the ﬁrst soft breakdown
process called the forming process, the device showed reversible resistance switching between the low-resistance state (LRS; red) and the high-resistancestate (HRS; blue). (b) Retention characteristics for both the LRS and HRS of the nanopaper memory. The readout voltage was 0.1 V. No signiﬁcantdegradation of the resistance state was observed for at least 104 s, with the resistance ratio maintained at 4107, indicating the nonvolatility of thenanopaper memory device. (c) Multilevel memory effect of the nanopaper memory device. The compliance current was varied (50 μA, 500 μA and 1 mA) tocreate different LRSs. The readout voltage was 0.1 V. Distinct LRSs were observed according to the range of the compliance current. (d) Retention
characteristics of resistance states for multilevel operation. The readout voltage was 0.1 V.

Supplementary Information S2. A Pt-coated cantilever was used as anelectrode. Figure 3b shows a topographic AFM image of the cellulose
nanoﬁbers. The maximum height of the cellulose nanoﬁbers was ca.
15–20 nm, consistent with the width of a cellulose nanoﬁber.36 The
insulative property of the cellulose nanoﬁber was conﬁrmed by the
C-AFM image (Supplementary Information S3) and a nanogap
measurement using a single cellulose nanoﬁber (Supplementary
Information S4). Figure 3c shows the I–V curves of the single cellulose
nanoﬁber measurement. The series resistance formed by the AFM tip
and the ﬁlament limits the current ﬂow in the range of hundreds ofnA.37 The measurement was performed at room temperature inatmospheric air conditions. After the forming process at a voltage of
0.3 V, the cellulose nanoﬁber exhibited bipolar resistive switching. No
signiﬁcant difference in switching behavior was observed after 20
switching cycles, indicating the reliability of the switching of the single
cellulose nanoﬁber. Figure 3d shows topographic AFM images before
and after the resistive switching with various switching cycles. After theswitching, the image commonly showed a decrease in the sample
height. This is well consistent with the electrochemical Ag ﬁlamentmodel34,35 (see Supplementary Information S5) because a void shouldbe created after the rupture of the Ag ﬁlament in the RESET process.
Based on the Ag ﬁlament model, we analyzed the electrical conductionof the single cellulose nanoﬁber. Considering the resistivity of Ag(1.59 × 10 − 6 Ωcm)38 and the sample height of 15 nm, the diameter ofthe Ag ﬁlament can be estimated to be ca. 12 nm. This size is reasonable because the diameter of the cellulose nanoﬁber is ca. 15–20 nm.36
NPG Asia Materials

Thus, these results highlight the excellent scalability of the nanopaper
memory device down to the single cellulose nanoﬁber level.
Finally, we demonstrate the disposability of the nanopaper memory
device. Before the disposal, the Ag electrode should be removed fromthe device because Ag has an antibacterial effect39,40 that suppresses
the fungus-based biodegradation of the nanopaper in the naturalenvironment. We found that the removal of the Ag electrode can beachieved by simply applying sonication in water (details are discussed
in Supplementary Information S6). Next, we examined the biodegradability of the nanopaper memory device using natural soil. Our
biodegradation-based disposal process using natural soil has someadvantages with respect to the data security and the energy consumption compared with the techniques used in a previous work.31 The
previously reported shredding process cannot be utilized when thedevice size becomes smaller than the blade spacing of the shredder,whereas the biodegradation process can be applied for any device
scale. The burning process has high reliability for data destruction
owing to the incineration of the device, but it consumes a highamount of energy to achieve the burning temperature. In contrast, thebiodegradation of nanopaper may proceed at room temperature innatural soil, allowing us to reduce the energy consumption drastically.
Figure 4a shows photographs of the nanopaper memory device after it
is buried in natural soil. The details of the experiment can be seen in
Supplementary Information S7. The temperature and the humidity ofsoil were controlled to be 22± 2 °C and 92± 3%, respectively. As
control experiments, nanopaper memory devices on Si andAll-nanocellulose nonvolatile resistive memory
U Celano et al
5

Figure 3 Examination of resistive switching using a single cellulose nanoﬁber. (a) Schematic illustration of the single cellulose nanoﬁber measurement
system. To obtain the single cellulose nanoﬁber, the solution, created by the dilution of a 0.02 wt% cellulose nanoﬁber aqueous suspension 1:1000
with deionized (DI) water, was dropped onto the Ag-coated silicon substrate. The characterization of the resistive switching for the single cellulose
nanoﬁber was performed by conductive-atomic force microscopy (C-AFM) with a Pt-coated Si cantilever. (b) Topographic AFM image of cellulose
nanoﬁbers on Ag/Si substrate. The nanocellulose was raveled out to the single nanoﬁber level. (c) The current–voltage (I–V) curves taken from a single
cellulose nanoﬁber. The measurement was performed in atmospheric air conditions at room temperature. For the measurement, the voltage wasapplied to the Ag bottom electrode, and the Pt-coated cantilever was grounded. Similar to the resistive switching in the nanopaper memory, reversible
resistive switching was seen in the single cellulose nanoﬁber. Similar switching characteristics were obtained after the 10th and 20th switchingcycles. (d) Topographic AFM images of cellulose nanoﬁber before and after switching. Before the observation, the resistance switching was continued
until the switching events diminished.

polyethylene terephthalate substrates were also examined. After 5 days,
the nanopaper memory layer was no longer observed on all devices,whereas the substrate did not show any signiﬁcant changes. The
degradation of the nanopaper substrate started after 15 days and wascomplete after 26 days. Because the cellulose can be partiallyhydrolyzed in a high humidity environment and decomposed by
microorganisms that have a cellulose-splitting enzyme, cellulase, thebiodegradation of the nanopaper memory device might progress innatural soil. Similar biodegradation phenomena were reported for
TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl)-oxidized nanopaper.41
On the other hand, the Si and polyethylene terephthalate substrates
were stable even after 26 days, without showing degradation. To
conﬁrm the bacterial effect in natural soil, we conducted a biodegradation test by simply keeping our sample in room-air conditions(humidity 50–60%), humid-air conditions (humidity 100%) andwater, as shown in Supplementary Information S8. In water, theresistive-switching layer was fully removed with the electrode inseveral minutes, whereas the nanopaper substrate was fully maintained
after 35 days. On the other hand, in humid-air conditions and roomair conditions, both the resistive-switching layer and the nanopaper
substrate partially (humid-air condition) and fully (room-air condition) remained after 35 days. We further examined the temperatureeffect in both the humid-air and water conditions. At 90 °C, there

were no signiﬁcant differences in the biodegradation effect comparedwith that in the same test performed at room temperature. This clearly
indicates that the biodegradation of our nanopaper memory wascaused by a bacterial effect in the natural soil. Figure 4b shows theresidual area ratio of the substrates after being buried in the soil. The
degradation of the nanopaper substrate started after 12 days, whereas
the Si and polyethylene terephthalate substrates fully remained after
26 days. These results indicate that we successfully demonstrated
environment-friendly, disposable nonvolatile memory devices that arealmost entirely composed of nanocellulose.
CONCLUSIONSIn this study, we demonstrate an environment-friendly, disposablenonvolatile memory device composed of 99.3 vol.% nanocellulose.
Our memory device consists of a nanocellulose-based resistive-switching layer and a nanopaper substrate. The device exhibited Ag ﬁlamentbased electrochemical redox nonvolatile resistive switching with thecapability of multilevel storage and potential scalability down to thesingle nanoﬁber level (ca. 15 nm). The biodegradability of ourmemory device was conﬁrmed by burying it in natural soil for 26 days.
These results indicate the potential of environment-friendly, disposable nonvolatile memory applications for developing a single-use
monitoring device for health diagnosis.
NPG Asia Materials

All-nanocellulose nonvolatile resistive memory
U Celano et al
6

Figure 4 Biodegradability of the nanopaper memory device. (a) Time series of photographs for the nanopaper memory on the nanopaper substrate,polyethylene terephthalate (PET) substrate and Si substrate after burying in the natural soil. The test soil was collected in September at 34:49:30 northlatitude and 135:31:28 east longitude. The soil (4 l in total amount) was kept in a plastic bag to minimize the variation of the water concentration of the soil
all through the biodegradation test. The humidity in the plastic bag was controlled to be ca. 90%. (b) Time evolution of residual area ratio (%) of nanopaper
memory devices on nanopaper substrate (red), PET substrate (yellow) and Si substrate (purple). A signiﬁcant decrease in the residual area ratio was obtained
only for the nanopaper substrate, whereas the other substrates fully remained.

CONFLICT OF INTEREST
The authors declare no conﬂict of interest.

ACKNOWLEDGEMENTS
This study was supported by a Grant-in-Aid for Scientiﬁc Research (S)(No. 26220908). UC was supported by a PhD grant from the Agency for
Innovation by Science and Technology (IWT).
Author contributions: UC, KN, HK, MN and TY conceived the nanopaper
memory architecture. UC designed the device, performed the resistiveswitching measurements for both the nanopaper memory and the single
cellulose nanoﬁber, analyzed the data and wrote the manuscript. KN conducted
the single nanoﬁber measurement using the nanogap method, the Ag electroderemoval experiment and the biodegradation experiment, analyzed the data andwrote the manuscript. FZ, GM and YH supported the biodegradation
experiment. JDB, MJ and WV analyzed the data and revised the manuscript.
HK, MN and TY supervised the project and revised the manuscript.

1 Ghoneim, M. T. & Hussain, M. M. Review on physically ﬂexible nonvolatile memory forinternet of everything electronics. Electronics 4, 424–479 (2015).

NPG Asia Materials

2 Atzori, L., Iera, A. & Morabito, G. The internet of things: a survey. Comp. Netw. 54,
2787–2805 (2010).
3 Miorandi, D., Sicari, S., Pellegrini, F. D. & Chlamtac, I. Internet of things: vision,
applications and research challenges. Ad Hoc Netw. 10, 1497–1516 (2012).
4 Suckling, J. & Lee, J. Redeﬁning scope: the true environmental impact of smartphones?
Int. J. Life Cycle Assess. 20, 1181–1196 (2015).
5 Nogi, M., Iwamoto, S., Nakagaito, A. N. & Yano, H. Optically transparent
nanoﬁber paper. Adv. Mater. 21, 1595–1598 (2009).
6 Nogi, M. & Yano, H. Optically transparent nanoﬁber sheets by deposition of transparent
materials: a concept for a roll-to-roll processing. Appl. Phys. Lett. 94, 233117(2009).
7 Nogi, M., Kim, C., Sugahara, T., Inui, T., Takahashi, T. & Suganuma, K. High thermal
stability of optical transparency in cellulose nanoﬁber paper. Appl. Phys. Lett. 102,
181911 (2013).
8 Yagyu, H., Saito, T., Isogai, A., Koga, H. & Nogi, M. Chemical modiﬁcation of cellulose
nanoﬁbers for the production of highly thermal resistant and optically transparent
nanopaper for paper devices. ACS Appl. Mater. Interfaces 7, 22012–22017 (2015).
9 Huang, J., Zhu, H., Chen, Y., Preston, C., Rohrbach, K., Cumings, J. & Hu, L. Highly
transparent and ﬂexible nanopaper transistors. ACS Nano 7, 2106–2113 (2013).
10 Yano, H., Sugiyama, J., Nakagaito, A. N., Nogi, M., Matsuura, T., Hikita, M.
& Handa, K. Optically transparent composites reinforced with networks of bacterial
nanoﬁbers. Adv. Mater. 17, 153–155 (2005).
11 Ho, M.-P., Wang, H., Lau, K.-T. & Leng, J. Effect of silk ﬁber to the mechanical andthermal properties of its biodegradable composites. J. Appl. Polym. Sci. 127,
2389–2396 (2013).
12 Saravaman, D. Spider silk-structure, properties and spinning. J. Text. Apparel Technol.
Manag. 5, 1–20 (2006).

All-nanocellulose nonvolatile resistive memory
U Celano et al7
13 Hwang, S.-W., Tao, H., Kim, D.-H., Cheng, H., Song, J.-K., Rill, E., Brenckle, M. A.,
Panilaitis, B., Won, S. M., Kim, Y.-S., Song, Y. M., Yu, K. J., Ameen, A., Li, R., Su, Y.,
Yang, M., Kaplan, D. L., Zakin, M. R., Slepian, M. J., Huang, Y., Omenetto, F. G.
& Rogers, J. A. A physically transient form of silicon electronics. Science 337,
1640–1644 (2012).
14 Dagdeviren, C., Hwang, S.-W., Su, Y., Kim, S., Cheng, H., Gur, O., Haney, R.,
Omenetto, F. G., Huang, Y. & Rogers, J. A. Transient, biocompatible electronics andenergy harvesters based on ZnO. Small 20, 3398–3404 (2013).
15 Hwang, S.-W., Song, J.-K., Huang, X., Cheng, H., Kang, S.-K., Kim, B. H., Kim, J.-H.,
Yu, S., Huang, Y. & Rogers, J. A. High-performance biodegradable/transient electronics
on biodegradable polymers. Adv. Mater. 26, 3905–3911 (2014).
16 Kim, D.-H., Viventi, J., Amsden, J. J., Xiao, J., Vigeland, L., Kim, Y.-S., Blanco, J. A.,
Panilaitis, B., Frechette, E. S., Contreras, D., Kaplan, D. L., Omenetto, F. G., Huang, Y.,
Hwang, K.-C., Zakin, M. R., Litt, B. & Rogers, J. A. Dissolvable ﬁlms of silk ﬁbroin for
ultrathin conformal bio-integrated electronics. Nat. Mater. 9, 511–517 (2010).
17 Koga, H., Nogi, M., Komoda, N., Nge, T. T., Sugahara, T. & Suganuma, K. Uniformly
connected conductive networks on cellulose nanoﬁber paper for transparent paper
electronics. NPG Asia Mater. 6, e93 (2014).
18 Nge, T. T., Nogi, M. & Suganuma, K. Electrical functionality of inkjet-printed silver
nanoparticle conductive tracks on nanostructured paper compared with those on plastic
substrates. J. Mater. Chem. C 1, 5235–5247 (2013).
19 Toivonen, M. S., Kaskela, A., Rojas, O. J., Kauppinen, E. I. & Ikkala, O. Ambient-driedcellulose nanoﬁbril aerogel membranes with high tensile strength and their use foraerosol collection and templates for transparent, ﬂexible devices. Adv. Funct. Mater.
25, 6618–6626 (2015).
20 Kang, W., Yan, C., Foo, C. Y. & Lee, P. S. Foldable electrochromics enabled bynanopaper transfer method. Adv. Funct. Mater. 25, 4203–4210 (2015).
21 Fujisaki, Y., Koga, H., Nakajima, Y., Nakata, M., Tsuji, H., Yamamoto, T., Kurita, T.,
Nogi, M. & Shimidzu, N. Transparent nanopaper-based ﬂexible organic thin-ﬁlm
transistor array. Adv. Funct. Mater. 24, 1657–1663 (2014).
22 Jung, Y. H., Chang, T.-H., Zhang, H., Yao, C., Zheng, Q., Yang, V. W., Mi, H., Kim, M.,
Cho, S. J., Park, D.-W., Jiang, H., Lee, J., Qiu, Y., Zhou, W., Cai, Z., Gong, S. & Ma, Z.
High-performance green ﬂexible electronics based on biodegradable cellulose
nanoﬁbril paper. Nat. Commun. 6, 7170 (2015).
23 Zhu, H., Fang, Z., Preston, C., Li, Y. & Hu, L. Transparent paper: fabrications,
properties, and device applications. Energy Environ. Sci. 7, 269–287 (2014).
24 Hsieh, M.-C., Kim, C., Nogi, M. & Suganuma, K. Electrically conductive lines on
cellulose nanopaper for ﬂexible electrical devices. Nanoscale 5, 9289–9295 (2013).
25 Fang, Z., Zhu, H., Yuan, Y., Ha, D., Zhu, S., Preston, C., Chen, Q., Li, Y., Han, X.,
Lee, S., Chen, G., Li, T., Munday, J., Huang, J. & Hu, L. Novel nanostructured paperwith ultrahigh transparency and ultrahigh haze for solar cells. Nano Lett. 14,
765–773 (2014).
26 Nogi, M., Karakawa, M., Komoda, N., Yagyu, H. & Nge, T. T. Transparent Conductive
Nanoﬁber Paper for Foldable Solar Cells. Sci. Rep. 5, 17254 (2015).
27 Inui, T., Koga, H., Nogi, M., Komoda, N. & Suganuma, K. A miniaturized ﬂexible
antenna printed on a high dielectric constant nanopaper composite. Adv. Mater. 27,
1112–1116 (2015).
28 Nogi, M., Komoda, N., Otsukac, K. & Suganuma, K. Foldable nanopaper antennas fororigami electronics. Nanoscale 5, 4395–4399 (2013).
29 Zhong, J., Zhu, H., Zhong, Q., Dai, J., Li, W., Jang, S.-H., Yao, Y., Henderson, D.,
Hu, Q., Hu, L. & Zhou, J. Self-powered human-interactive transparent nanopaper
systems. ACS Nano 9, 7399–7406 (2015).

30 Nagashima, K., Koga, H., Celano, U., Zhuge, F. W., Kanai, M., Rahong, S., Meng, G.,
He, Y., Boeck, J. D., Jurczak, M., Vandervorst, W., Kitaoka, T., Nogi, M. & Yanagida, T.
Cellulose nanoﬁber paper as an ultra ﬂexible nonvolatile memory. Sci. Rep. 4,5532 (2014).
31 Lien, D.-H., Kao, Z.-K., Huang, T.-H., Liao, Y.-C., Lee, S.-C. & He, J.-H. All-Printed
Paper Memory. ACS Nano 8, 7613–7619 (2014).
32 Celano, U., Goux, L., Belmonte, A., Opsomer, K., Franquet, A., Schulze, A.,
Detavernier, C., Richard, O., Bender, H., Jurczak, M. & Vandervorst, W. Threedimensional observation of the conductive ﬁlament in nanoscaled resistive memory
devices. Nano Lett. 14, 2401–2406 (2014).
33 Yanagida, T., Nagashima, K., Oka, K., Kanai, M., Klamchuen, A., Park, B. H.
& Kawai, T. Scaling effect on unipolar and bipolar resistive switching of
metal oxides. Sci. Rep. 3, 1657 (2013).
34 Yang, Y., Gao, P., Gaba, S., Chang, T., Pan, X. & Lu, W. Observation of
conducting ﬁlament growth in nanoscale resistive memories. Nat. Commun. 3,732 (2012).
35 Valov, I., Linn, E., Tappertzhofen, S., Schmelzer, S., Hurk, J., Lentz, F. & Waser, R.
Nanobatteries in redox-based resistive switches require extension of memristor theory.
Nat. Commun. 4, 1771 (2013).
36 Uetani, K. & Yano, H. Nanoﬁbrillation of wood pulp using a high-speed blender.
Biomacromolecules 12, 348–353 (2011).
37 Celano, U., Goux, L., Opsomer, K., Belmonte, A., Iapichino, M., Detavernier, C.,
Jurczak, M. & Vandervorst, W. Switching mechanism and reverse engineeringof low-power Cu-based resistive switching devices. Nanoscale 5, 11187–11192(2013).
38 Serway, R. A. Principles of Physics (Saunders College Pub., London, Fort Worth, Texas,
1998).
39 Davoudi, Z. M., Kandjani, A. E., Bhatt, A. I., Kyratzis, I. L., O’Mullane, A. P.
& Bansal, V. Hybrid antibacterial fabrics with extremely high aspect ratio Ag/AgTCNQ
nanowires. Adv. Funct. Mater. 24, 1047–1053 (2014).
40 Fellahi, O., Sarma, R. K., Das, M. R., Saikia, R., Marcon, L., Cofﬁnier, Y., Hadjersi, T.,
Maamache, M. & Boukherroub, R. The antimicrobial effect of silicon nanowires
decorated with silver and copper nanoparticles. Nanotechnology 24, 495101 (2013).
41 Homma, I., Fukuzumi, H., Saito, T. & Isogai, A. Effects of carboxyl-group counter-ionson biodegradation behaviors of TEMPO-oxidized cellulose ﬁbers and nanoﬁbril ﬁlms.
Cellulose 20, 2505–2515 (2013).

This work is licensed under a Creative Commons
Attribution 4.0 International License. The images orother third party material in this article are included in the article’s
Creative Commons license, unless indicated otherwise in the credit
line; if the material is not included under the Creative Commonslicense, users will need to obtain permission from the license holder toreproduce the material. To view a copy of this license, visit http://
creativecommons.org/licenses/by/4.0/
r The Author(s) 2016

Supplementary Information accompanies the paper on the NPG Asia Materials website (http://www.nature.com/am)

NPG Asia Materials