Reconfigurable Logic Devices Connected with LaserSintered Liquid Metal Nanoparticles
Edward L. White, Jennifer C. Case, and Rebecca K. Kramer
School of Mechanical Engineering
Purdue University
West Lafayette, Indiana, USArebeccakramer@purdue.edu
Abstract — In this paper we demonstrate the use of liquid
metal nanoparticles as electrical traces between logic gates. When
first deposited, films of nanoparticles are non-conductive, despite
their metallic composition. Nanoparticle films can be coalesced
into conductive traces using an infrared laser. We demonstrate theutility of this approach by using laser-patterned films to connect
NAND gates in different circuits, creating an OR gate and a lightsensitive thermostat. We also show that patterned films can beremoved, making the devices re-usable and re-programmable.
Liquid metal nanoparticle-based devices may be used to create
environmentally responsive devices in the future.
Keywords—flexible electronics; liquid metal electronics; reconfigurable logicI.

INTRODUCTION

Although many different types of sensors exist for different
physical phenomena, the information collected by these sensors
is generally processed by fixed logic devices. In contrast to thisapproach, we hypothesize that a new class of environmentallyresponsive electronic circuits that adjust functionality andprogramming in response to changes in environmental
conditions are possible [1]. Our research is based on liquidmetal alloys, which can be used to create electrical connections
in highly deformable systems [2,3]. Multiple approaches topatterning liquid metals have previously been demonstrated,
including injection into microchannels, ablation of a film ofnative liquid metal, and extrusion of liquid metal onto a surface[4]. Reconfigurable devices made from liquid metals have also
been demonstrated [5,6]. These devices use the movement ofliquid metals within microchannels to achieve adjustablefunctionality.
Instead of using pre-patterned channels, we sought anapproach that would allow for the rapid fabrication of arbitrary
conductive pathways. Further, we sought a patterning methodthat would be responsive to external stimuli. We have previously
demonstrated that particle-filled inks based on organic carrier
solvents [7] can be deposited on a surface in a non-conductive
state using ink-jet printing [8] and spray printing [9]. During
synthesis, the metal particles develop an oxide layer that
insulates the particles from one another. This oxide layer
remains on the particles after they are deposited onto a substrate,
producing a non-conductive film of particles. To convert thefilm of non-conducting particles into a continuous conductive
trace, this oxide layer must be removed or ruptured, allowing theliquid metal within the particle to contact its neighbors.

978-1-5090-1012-7/17/$31.00 ©2017 IEEEFig. 1. Illustrations and images of the fabrication sequence. a) A copper-cladpolyimide film (Pyralux). b) A copper pattern is etched into the film andelectrical components are soldered to the circuit. c) Blue painter’s tape isapplied over the electrical components to mask them during the nanoparticle
dispersion application. d) The liquid metal dispersion is applied. e) An IR laser
is used to coalesce the liquid metal nanoparticles into a conductive film. The
laser does not penetrate completely through the full thickness of the particle
layer, maintaining a high resistance between copper traces (>10KΩ) relative
to the trace on the right. f) Photograph of flexible PCB with electronic
components attached, corresponding to the illustration in b). g) Final laser
patterned device, corresponding to the illustration in e).

Previously, we demonstrated mechanical sintering of liquid
metal particles via application of pressure to the particle film
surface [8,9].
In the present work, we replace mechanical force withinfrared (IR) laser radiation to sinter the liquid metal particles
and create conductive traces by rupturing the electrically-insulating oxide shells. As a demonstration of this technique, weused laser radiation to coalesce particles deposited on a flexible
PCB substrate, connecting traditional semiconductor NAND
gates to create multi-gate logic devices. NAND gates areuniversal logic elements, and any other logic function can bemade from combinations of NAND gates. This concept is a
stepping stone towards a broader goal of building electronic
components that not only sense changes in environmental
conditions, but can alter their basic functionality in response toexternal stimuli.
II.

FABRICATION

Fabrication of the devices occurred in four phases. First,
standard electronic components were soldered to a patterned
flexible PCB substrate, as shown in Fig. 1(a,b,f). Second, liquid
metal nanoparticle ink was prepared according to the procedurepreviously described by Boley, et al. [8]. To summarize thisprocedure, a room temperature alloy of gallium and indium wassonicated in ethanol to create nanoparticles. The resulting ink
retained the physical properties of the ethanol, making it sprayprintable. Third, masking tape was used to cover the electronic
devices, and the nanoparticle ink was sprayed onto the circuit.
The ink was sprayed onto the surface at a density of 0.07mL/cm2using multiple passes to prevent pooling, as shown in Fig. 1(c,d).
The masking tape was not adhered to the flexible substrate, andinstead had an “overhang” off of the top of the electroniccomponents, as shown in Fig. 1(c). Fourth, an IR laser (VLS2.30, Universal Laser Systems) was used to activate thenanoparticles, causing the individual particles to coalesce into a
conductive trace as shown in Fig. 1(e,g).
Key to this process were the feathered edges of thedispersion film. A gradually decreasing thickness of
nanoparticle ink was deposited under elevated masking tape, asshown in Fig. 1(d). The laser did not penetrate completely
through the full particle layer, maintaining a high resistance
between copper traces (>10KΩ), but a low resistance throughthe trace (<100Ω). The thinner film thickness at the featherededges allowed the laser to connect the conductive traces on theupper surface of the dispersion to the copper substrate.
After laser patterning, the dispersion could be removed fromthe surface using a caustic bath, as shown in Fig. 2. Soaking thedispersion film in 1.0M sodium hydroxide for a few seconds
removed the oxide layer from the nanoparticles, causing them todetach from the surface [1]. Sodium hydroxide was selected
because it is able to quickly strip the nanoparticle film without
damaging the underlying copper traces and electroniccomponents. After cleaning, the logic device can be re-sprayedand re-patterned according to the sequence described above.
III.

whereandare the two inputs. The operation of theprogrammed device is shown in Fig. 3. In this configuration,
only three of the six NAND gates on the device were utilized.
The remaining unused NAND gates could have been used toconstruct an additional OR gate, or any combination of gates
requiring three NANDs.
B. Complex Example: Light-Sensitive Thermostat
As an example of a multi-gate logic system that can be
fabricated with this approach, we created a light-sensitive
thermostat. In this example, the device used two different
temperature set points depending on the illumination condition.
In practice, this might represent a case where it is desired to keepa room warmer during the day than at night. Thermal and lightsensors were implemented on a daughterboard attached to thelogic device. The desired truth table is shown in Table I. Using

RESULTS

A. Simple Example: OR Gate
As an example of how NAND gates can be used to emulate
any other logic unit, we wired the device as an OR gate. The
functionality of an OR gate can be re-written using the NAND
operator:∨Fig. 2. Cleaning process to remove the nanoparticle film. a) A laser-patterned
device showing conductive patterns. b) The device is placed in 1.0M NaOHbath. c) After a few seconds, the dispersion has detached from the surface andformed into a few small liquid metal balls. d) Cleaning is finished by wiping
the surface with a paper towel with NaOH, followed by rising with water anddrying. The device is now ready to be re-sprayed with nanoparticle ink.

∧∧

∧

(1)

Fig. 3. Operation of the device as a two-terminal OR gate. Three NAND
cells are required to form an OR gate. In this case, the three gates in theupper left triangle of the device are used. Input A (highlighted with a redcircle) drives the upper left NAND cell, input B (highlighted with a bluecircle) drives the lower left cell, and the output (highlighted with a cyancircle) is driven by the upper middle cell. The output state of each cell isshown with an LED.

∧TABLE I.
DESIRED OUTPUT STATES OF A LIGHT-SENSITIVETHERMOSTAT. THE DEVICE IS PROGRAMMED TO HOLD A HIGHERTEMPERATURE WHEN EXPOSED TO LIGHT.

LLLHLHLLHHLLLL

HHLHHHHL

xN/A∨IV.

∧(2)

whereis the lower temperature set point,is the highertemperature set point, and is the state of the light. Since
NAND gates are universal, the equation above can be recast into
an expression using only NANDs:
∧∧

∧∧∧

∧

∧

(3)

From this expression, it can be seen that four NAND cells
were required to implement the desired operation. The patterned
dispersion and circuit operation are shown in Fig. 4. Thisexample demonstrates how to transition from a general Boolean
formulation of a logic device to a NAND-only formulation, andhow the resulting logic operation is implemented in a working
device using laser-patterned conductive traces in a nanoparticle
film.

both AND and OR functions, the output F of the circuit needs tobe:∧∧

In this paper, we have demonstrated the deposition andpatterning of a liquid metal nanoparticle ink to create
reconfigurable laser-activated conductive traces. As an example,
we have demonstrated using nanoparticle ink as a laserprogrammable conductive layer between NAND logic gates,
and have demonstrated how an array of NAND gates can be used
to construct a useful logic device. This same approach could beapplied to arbitrary types and numbers of devices, creating both
functionally and physically flexible electronics. In the future, we
propose to use alternative coalescence mechanisms, besides
laser activation, to create environmentally-responsive electronicdevices.

∧Noting that the temperature terms only appear in negatedform, we designed the thermistor circuit such that the output ishigh when the temperature is below the set point and low when
it is above. Because of the circuit design, we also had a negatedlight input. Thus, we simplify the logic function into the final
form:

CONCLUSIONACKNOWLEDGMENT
This work is partially supported under the NASA Early
Career Faculty Program, Space Technology Research Grants
Program (Grant NNX14AO52G). ELW is supported by theNational Science Foundation Graduate Research Fellowship(Grant DGE-1333468). JCC is supposed by the NASA NSTRF
Program (NNX15AQ75H).
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Fig. 4.
A programmable logic device with corresponding
thermistor/photoresistor daughterboard. The left column shows theoperation of the circuit with light off, the right column shows operation
with light on. The lower set point, , was set to 40ͦᵒC and the upper set, was set to 60ᵒC. The actual temperature is shown in the inset inpoint,
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in both illumination conditions. The connections from the daughterboard(blue), (white), andto the logic device are, from top to bottom(yellow).

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