nature electronics
Article
https://doi.org/10.1038/s41928-024-01189-x
Permeable, three-dimensional integrated
electronic skins with stretchable hybrid
liquid metal solders
Received: 16 August 2023
Accepted: 22 May 2024
Qiuna Zhuang 1,10, Kuanming Yao 2,10, Chi Zhang3, Xian Song1,
Jingkun Zhou 2, Yufei Zhang 1, Qiyao Huang1,4, Yizhao Zhou5, Xinge Yu
& Zijian Zheng 1,3,4,8,9
2,6,7
Published online: xx xx xxxx
Check for updates
The development of wearable and on-skin electronics requires high-density
stretchable electronic systems that can conform to soft tissue, operate
continuously and provide long-term biocompatibility. Most stretchable
electronic systems have low-density integration and are wired with
external printed circuit boards, which limits functionality, deteriorates
user experience and impedes long-term usability. Here we report an
intrinsically permeable, three-dimensional integrated electronic skin.
The system combines high-density inorganic electronic components with
organic stretchable fibrous substrates using three-dimensional patterned,
multilayered liquid metal circuits and stretchable hybrid liquid metal solder.
The electronic skin exhibits high softness, durability, fabric-like permeability
to air and moisture and sufficient biocompatibility for on-skin attachment
for a week. We use the platform to create wireless, battery-powered and
battery-free skin-attached bioelectronic systems that offer complex
system-level functions, including the stable sensing of biosignals, signal
processing and a­na­ly­sis, e­le­ctrostimulation and wireless communication.
Soft and stretchable integrated electronic systems that offer continuous sensing and intervention capabilities are of potential use in
intensive care1, rehabilitation2, closed-loop diagnosis and treatment3
and virtual reality/augmented reality4. Recent progress in stretchable
electronics has been achieved by developing novel materials5–11 and
architectures12–14 for stretchable electronics. In particular, structural
approaches based on lateral-strain-tolerant island–bridge engineering (for example, buckle, serpentine and spring structures)15,16 and
vertical-strain-isolation engineering17–19 have enabled conventional
rigid integrated circuit (IC) components to be integrated with stretchable polymeric substrates. Such stretchable hybrid electronics have the
advantages of mature IC design and manufacture as well as matching
the mechanical properties of soft organs and tissues20–23.
To achieve stretchable electronics with high density and multiple
functions, it is necessary to develop three-dimensional (3D) stretchable
integrated systems24. The development of 3D stretchable electronics
is, however, still in its early stages and only a few systems have been
reported24–28 (Supplementary Table 1). One challenge is creating robust
Laboratory for Advanced Interfacial Materials and Devices, School of Fashion and Textiles, The Hong Kong Polytechnic University, Hong Kong SAR,
China. 2Department of Biomedical Engineering, City University of Hong Kong, Hong Kong SAR, China. 3Department of Applied Biology and Chemical
Technology, The Hong Kong Polytechnic University, Hong Kong SAR, China. 4Research Institute for Intelligent Wearable Systems (RI-IWEAR), The Hong
Kong Polytechnic University, Hong Kong SAR, China. 5College of Information Science and Electronics Engineering, Zhejiang University, Hangzhou, China.
6
Hong Kong Centre for Cerebro-Cardiovascular Health Engineering (COCHE), Hong Kong Science Park, Hong Kong SAR, China. 7Hong Kong Institute for
Clean Energy, City University of Hong Kong, Hong Kong SAR, China. 8Research Institute for Smart Energy (RISE), The Hong Kong Polytechnic University,
Hong Kong SAR, China. 9State Key Laboratory of Ultra-precision Machining Technology, The Hong Kong Polytechnic University, Hong Kong SAR, China.
10
These authors contributed equally: Qiuna Zhuang, Kuanming Yao.
e-mail: xingeyu@cityu.edu.hk; tczzheng@polyu.edu.hk
1
Nature Electronics
Article
interfaces between the rigid components and stretchable circuits in 3D
space28. Another challenge is that 3D stretchable electronics are typically fabricated using multilayered sequential casting of elastomeric
films, such as polydimethylsiloxane (PDMS), and the bonding of rigid
chips. This usually results in the thickness of the entire 3D stack being
higher than 1 mm, which deteriorates conformability and stretchability compared with single-layer electronic skins. In addition, thick- or
thin-film-based 3D stacks lack permeability29,30, which is important
for wearer comfort and long-term biocompatibility31–33. Recently, we
reported a permeable and stretchable multilayered device using a
liquid metal (LM) fibre mat33. However, there remain technical challenges related to integrating functional electronic components into
such fibre mats, which restricts the development of highly integrated,
stretchable and permeable electronics.
In this Article, we report a permeable, 3D integrated electronic
skin (P3D-eskin) that combines high-density inorganic electronic
components with organic stretchable fibrous substrates using 3D patterned, multilayered LM circuits and hybrid liquid metal (hLM) solder.
Our P3D-eskin replaces impermeable and rigid printed circuit boards
with a skin-like stretchable, soft and breathable design form factor as
well as maintains complex system-level and continuous functions such
as data acquisition, signal processing and analysis, intervention and
wireless communication with a mobile device. The P3D-eskin consists
of a micropatterned permeable and stretchable multilayered circuit
board made of LM and fibre mats. We use a stretchable hLM solder
that reliably maintains a stable electrical interface between the rigid IC
components and soft LM interconnects at up to 1,500% strain without
electrical failure. We achieve 3D integration among the different layers by engineering the vertical penetration of LM to form stretchable
vertical interconnect accesses (VIAs).
The P3D-eskin exhibits high air and moisture permeability and
prevents skin inflammation over long-term skin attachment. Compared with a stretchable PDMS-based electronic skin (PDMS-eskin),
the system-level thickness is reduced by ~54% and rigidity, by ~60%.
Our electronic skin also exhibits advanced, complex and monolithic
system-level integration compared with previously reported permeable electronic systems that avoid using external printed circuit boards
(Supplementary Table 2). We use the platform to fabricate bioelectronic devices that can continuously record and wirelessly transmit
multiposition physiological signals.
https://doi.org/10.1038/s41928-024-01189-x
As shown in Fig. 1a and Supplementary Fig. 1, the P3D-eskin typically
consists of four stretchable and permeable layers including a base LM
circuit layer, a top LM circuit layer, a paste mask layer bonded with rigid
electronic components using a stretchable hLM solder and an encapsulation layer. Eutectic gallium-based alloys are selected as the LM due to
their high stretchability and low modulus as a liquid34–38, high electrical
conductivity39–42, excellent biocompatibility43–46 and patternability47–49.
The detailed fabrication procedures of P3D-eskin are illustrated in
Extended Data Fig. 1 and Methods. Briefly, we first fabricated the base
circuit layer (40–100 μm) and the top circuit layer (40–100 μm) made
of LM micropatterns on the stretchable fibrous mat using a combination of photolithography, pattern transfer and stencil printing process
(Methods, Supplementary Note 1 and Supplementary Figs. 2 and 3).
The micropatterned LM served as the stretchable antenna, interconnects, pads and contacts, whereas the vertical electrical connections
between the base and top layer were achieved using LM VIAs. Subsequently, we bonded the rigid electronic components onto the LM
circuits using an hLM comprising a combination of partially oxidized
liquid metal (oLM) and LM. We printed the oLM on the paste mask layer
made of thin fibrous poly(styrene-block-butadiene-block-styrene)
(SBS; 15–30 μm), which was previously deposited on the top circuit
layer (Fig. 1b). Pins of rigid electronic components (Supplementary
Table 3), including light-emitting diodes (LEDs), microcontroller unit
(MCU), oscillator, multiplexer (MUX), current mirror, digital–analogue
converter (DAC), operational amplifier (OP-AMP), high-voltage module
(HV, 20 V) and low-dropout regulator (LDO, 3.3 V), were adhered onto
the printed oLM pads. We then applied additional LM pastes on the
pin/oLM interfaces (Fig. 1c). Finally, the encapsulation layer (~50 μm),
also made of a permeable but waterproof SBS mat, was directly electrospun to conformally cover the entire 3D hybrid electronic circuit.
The microporous fibrous structure of the electrospun fibre mat allows
air and moisture (water vapour) to pass through it (Supplementary
Fig. 4a), whereas the intrinsic hydrophobicity of the SBS fibre mat (Supplementary Fig. 4b), which shows a large water contact angle of 127°
(Supplementary Fig. 4c), can repel water droplets. Therefore, permeability and waterproofness are achieved at the same time.
The P3D-eskin was extremely soft (Fig. 1d) and highly stretchable,
showing a stable electrical function under a large tensile strain of
550% (Fig. 1e). It offers wireless, continuous and comfortable physiological monitoring and intervention of the human body through a
mobile device interface (Supplementary Video 1). Importantly, because
the P3D-eksin was fabricated based on the porous and fibrous substrate, interlayer and encapsulation, it also has very high permeability (Fig. 1f) compared with impermeable, 3D stretchable electronics
made with elastic thin films and bulks. The air and moisture permeabilities of P3D-eskin reached 177 mm s−1 and 676 g m−2 day−1, which is
15-fold and 44-fold higher than medical tapes and 3-fold and 22-fold
higher than commonly used wound dressing, respectively (Fig. 1g;
Methods provides details). The waterproofness of the P3D-eskin system was assessed using a standard rain test. After spraying water
onto the front side of the P3D-eskin system for 2 min, no observable
water was found on the blotting paper (Supplementary Fig. 5 and
Supplementary Video 2). Further, we also tested the stability of the
P3D-eskin system in water and artificial sweat (pH 4.7 ± 0.1). We fabricated an LED-embedded P3D-eskin system and immersed it in both
liquids. Electrical stability was indicated by the stable luminance of
the LEDs in water and in artificial sweat (Supplementary Fig. 6 and
Supplementary Video 3).
The P3D-eskin possesses high chronic biocompatibility; the skin
area covered by the P3D-eskin maintained inflammation-free characteristics during one week of on-skin attachment. As a reference, we
also fabricated a similar 3D eskin using thin PDMS as the substrate,
interlayer and encapsulation material (Supplementary Fig. 7). The
PDMS-eskin was thicker by ~54% and more rigid by ~60%, and hardly
showed any permeability to air and a poor moisture permeability below
50 g m−2 day−1 due to the compact and thin-film type of layout following the conventional spin-coating and casting processes. Although
PDMS is known as a biocompatible material, the impermeability of
Fig. 1 | P3D-eskins. a, Exploded schematic of a typical P3D-eskin. LM
microelectrodes are adopted as the reliable interface between the soft, rough
fibre mat substrate and the rigid components. VIAs are used for interlayer
electrical connections. The key components in each layer include an MCU,
oscillator, MUX, current mirror, DAC, OP-AMP, HV (20.0 V) and LDO (3.3 V). The
dashed lines indicate the distribution and positions of VIAs in the system. b,
Digital image of permeable 3D LM circuits and oLM pads. c, Digital images of the
soft and stretchable P3D-eskin with hLM solder. d,e, Digital images showing the
stable electrical performance of the bent (d) and stretched (550% strain) (e) P3Deskin. f, Schematic showing the permeability of P3D-eskin to air and moisture.
g, Air and moisture permeabilities of several wearable substrates including P3Deskin, PDMS-eskin reference, wound dressing, medical tape and cotton fabric.
Bar height, mean; error bars, standard deviation (s.d.); n = 5 independent tests. h,
Digital images showing the skin status after attaching P3D-eskin and PDMS-eskin
for one week. The area covered by P3D-eskin was inflammation free, whereas that
covered by PDMS-eskin displays serious skin erythema.
Design and fabrication of P3D-eskins
Nature Electronics
Article
https://doi.org/10.1038/s41928-024-01189-x
a
b
Permeable but waterproof superstrate
oLM pads
Current mirror
Electronic components
3D LM circuits
OP-AMP
DAC
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LED
MCU
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MUX
Oscillator
oLM paste mask layer
3 mm
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P3D-eskin
After peeling off the
eskins
PDMS-eskin
2 cm
Article
multilayered PDMS-eskin resulted in skin erythema under the on-skin
attachment test (Fig. 1h).
hLM solders for reliable 3D electrical interfaces
To achieve high stretchability and stability of P3D-eskin, it is a critical
challenge to ensure a seamless interface among the different vertical
layers that provide necessary electrical insulation and connection,
as well as stable interfaces between the soft LM and rigid electronic
chips that can endure large deformations. To address this challenge,
we formulated two different kinds of LM ink, namely, pristine LM and
oLM (Extended Data Fig. 2), serving as a stretchable hLM solder for
the 3D circuit.
The pristine LM showed high fluidity but low wettability to the
fibrous SBS substrate (Supplementary Fig. 8) and it was used for the
fabrication of stretchable circuit antennas, interconnects and VIAs on
the base and top circuit layers. As such, the base and top circuits retain
high in-plane stretchability and out-of-plane insulation, unless they
were connected with VIAs. However, connecting the rigid pins with
the pristine LM resulted in poor stretchability; the pin/LM interface
fell apart during stretching due to the dewetting of LM (Fig. 2a). In
contrast, the wettability of oLM was much higher because oxidation
reduces the surface tension of LM50–54. oLM was, therefore, chosen to
print on the paste mask layer as contact pads, providing good adhesion between the soft LM circuits underneath and the rigid pins of the
electronic components. Nevertheless, due to the low stretchability of
oLM, the pin/oLM interface also broke apart when the 3D circuit was
stretched (Fig. 2b).
We, therefore, developed the ultrastretchable hLM solder, which
made use of the wettability advantage of oLM as well as the stretchability attribute of LM. As shown in Fig. 2c, an additional pristine LM paste
was applied at the pin/oLM interface to form the hLM solder (Fig. 2c).
Importantly, the hybrid connection method reduced the stress concentration factor (the ratio of the maximum stress to average stress,
that is, σmax/σavg) at the interface between the rigid chip and soft SBS
by 30% (Fig. 2d and Supplementary Fig. 9), compared with those using
either single-component pristine LM or oLM as the connection material
(Supplementary Table 4). As a result, the hybrid connection provided
outstanding interfacial stability even under large tensile strains. The
resistance of a 100 Ω rigid microresistor bonded with an hLM solder
showed negligible change when the circuit was stretched to 1,500%
strain (Fig. 2e). In contrast, the same circuit using either LM or oLM as
the solder failed when stretched to less than 50% strain.
Figure 2f shows the schematic of the 3D circuit using the hLM
solder. It should be noted that due to its high wettability, the oLM
spontaneously penetrated through the thin paste mask (SBS) layer to
connect with the LM circuit traces underneath (in this specific case,
the top LM circuit layer). As a consequence, a vertical electrical connection between the pin/oLM and the LM 3D circuit underneath was
formed. At the same time, the top LM circuit layer was connected with
the base LM circuit layer using stretchable LM VIAs. As shown in Fig. 2g,
there was no obvious interfacial gap among the different layers of the
P3D-eskin because all the fibrous SBS mats were deposited using the
electrospinning method. The interfaces remained seamless during
stretching or bending deformation.
Fig. 2 | Reliable 3D hybrid interfaces using ultrastretchable hLM solder. a–c,
Schematic and SEM images showing the electrical interfaces of rigid components
using pristine LM (a), oLM (b) and hLM solder (c). Scale bars, 200 μm. d, FEA of
stress distribution of the electrical interface using ultrastretchable hLM solder.
e, Electrical resistance of the electrical interfaces of microresistors (0603,
~100 Ω) using pristine LM, oLM and hLM solder. f, Schematic of the 3D electrical
connection and interfaces between the rigid IC and ultrastretchable hLM solder.
oLM serves as the contact pads, whereas pristine LM serves as the patterned
in-plane interconnects, VIAs and additional contact paste. g, Cross-sectional SEM
Nature Electronics
https://doi.org/10.1038/s41928-024-01189-x
As a proof of concept of the stable 3D interfaces, we attached
different kinds of rigid electronic component, including microresistors, metal–oxide–semiconductor field-effect transistors (MOSFETs) (Supplementary Video 4) and LEDs to the stretchable 3D LM
circuit and then tested their performances under large strains. Connecting to different microresistors ranging from 100 Ω to 1 MΩ, the
resistance of the circuit showed negligible change when stretched
to 1,500% strain (Fig. 2h) and remained stable during 1,000 cycles
of stretch–release tests (Fig. 2i). The stable brightness of the LED
during the stretching process also indicated the constant resistance of the stretchable circuit (Supplementary Fig. 10). The stretchable p-type (Fig. 2j and Supplementary Fig. 11a) and n-type (Fig. 2k
and Supplementary Fig. 11b) MOSFET circuits also showed stable
transfer and output characteristics under large strains up to 500%.
We further fabricated stretchable logic circuits including the
clock-controlled switch (Fig. 2l,m), inverse gate, NOT-OR (NOR) gate
and 3D switch array with the MOSFETs (Extended Data Fig. 3). These
logic circuits could normally operate in the logic output states under
various strains.
After storing for eight months, the initial electrical resistance of
the LM circuit before cycling increased slightly from 0.33 to 0.42 Ω.
During the stretching tests, samples previously stored in air showed
similarly high electrical stability and robustness. The electrical resistances only increased by 0.119 and 0.083 Ω for the freshly made sample
and stored sample after the cycling tests, respectively (Supplementary
Fig. 12a). Additionally, the electrical interfaces between the LM circuits
and microresistors also possessed high stretchability and electrical
stability after storage for eight months (Supplementary Fig. 12b). The
failure modes of the solder joints after long-term repeated cycling tests
are discussed in Supplementary Note 2 and Supplementary Fig. 13.
To assess any leakage concerns when the LM 3D circuits are pressed
on the arm before mounting the components, the LM 3D circuits did
not leak onto the skin even with a high pressure of up to 50 kPa (Supplementary Fig. 14a). After the on-skin pressing test, the LM 3D circuits
remained intact without any merging of the lines (Supplementary
Fig. 14b and Supplementary Video 5). Additionally, the P3D-eskin system with the coverage of the superstrate was still well encapsulated at
a large strain of 850% (Supplementary Fig. 15).
Long-range wireless transcutaneous
electrostimulation system
Figure 3a illustrates the block diagram of a wireless transcutaneous
electrostimulation and electrophysiological sensing system fabricated
based on the P3D-eskin platform. It was equipped with a Bluetooth
Low Energy (BLE) 5.1 built-in MCU and matched with a 2.4 GHz BLE
LM antenna (planar inverted F-shaped antenna), which was capable of
providing stable wireless control and data transmission functions with
the mobile device at a distance of up to 15 m (Fig. 3b). The embedded
electrostimulating electrodes could generate high-voltage electrical pulses with precisely controlled current intensity, frequency and
duty cycle for delivering electrical stimulations to the user’s/animal’s
body. By controlling the on/off period of the MUX, the generated d.c.
high voltage was transformed into periodical pulses with precisely
controlled frequency (1–100 Hz) and duty cycle (1–10%) (Fig. 3c).
images showing a rigid microchip integrated with the 3D LM circuit at 0 and 50%
strain. The interfaces between the rigid chip and LM circuit are well maintained
under the large tensile strain, and the LM circuit is stretched in 3D space. h,i,
Electrical resistances of a series of highly stretchable microresistor-integrated
3D LM circuits. The electrical resistances show outstanding stability when the
circuits are stretched to 1,500% for 1,000 cycles. j,k, Transfer characteristics of
stretchable p-type (j) and n-type (k) MOSFETs. l,m, Digital image (l) and logic
outputs (m) of a stretchable logic circuit (clock-controlled switch) fabricated
with stretchable MOSFETs. FPC, flexible printed circuit.
Article
https://doi.org/10.1038/s41928-024-01189-x
Importantly, because of the high permeability of P3D-eskin, the generation of electrical pulses did not show any signal drift or electrical
failure (Fig. 3d,e) even when the entire P3D-eskin was steamed on top
of boiling water (Fig. 3f and Supplementary Video 6). In contrast, water
droplets accumulated on the surface of the impermeable PDMS-eskin
(Supplementary Fig. 16).
The output voltage of the DAC of the P3D-eskin was controlled by
sending the setting commands on the mobile device (Supplementary
d
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Nature Electronics
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Article
https://doi.org/10.1038/s41928-024-01189-x
a
b
Stimulation generator
20 V
booster
HV
MUX
BLE
DAC
MCU
OP-AMP
Current
mirror
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regulator
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Fig. 3 | Wireless transcutaneous electrostimulation and electrophysiological
sensing using P3D-eskin. a, Brief block diagram of the sensing system and
the customized mobile app. Electrical stimulations can be delivered to the
bicep femoris on a rat, and the corresponding EMG signals were recorded
using the LM microelectrode. b, Digital image showing the long-range wireless
communication of the sensing system at a distance of 15 m. c, Generated
stimulation pulses with controlled duty cycle ranging from 1% to 10%, frequency
fixed at 100 Hz. d,e, Generated stimulation pulses with controlled repetition
frequencies ranging from 5 Hz to 100 Hz in the dry state (d) and steamed state
Nature Electronics
(e). f, Digital image showing the P3D-eskin was steamed on top of boiling water.
g, Generated stimulation current pulses with controlled current intensity
under different wirelessly sent commands (0×40 to 0×60) at the dry and
steamed states (fixed load, 1 kΩ). h, Digital image of the wireless transcutaneous
electrostimulation and electrophysiological sensing system based on the
P3D-eskin platform. The system was attached to the bicep femoris of a rat. i,
Evoked EMG response signals under stimulation frequencies of 1, 5 and 10 Hz.
j, Spectrogram of the EMG signals in response to the electrostimulation input
generated by the P3D-eskin at a frequency of 5 Hz.
Article
https://doi.org/10.1038/s41928-024-01189-x
a
b
d
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Cu antenna
Cu antenna at 50% strain
Pa
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30
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Inflammatory after exercise
n
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T 30 °C; RH 70%
38
30
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4
1 cm
Phase (°)
6
Q factor
Inductance (µH)
g
1 cm
Impedance (kΩ)
Substrate
0
8
Time (h)
Fig. 17). The generated current pulses passed from the skin-interfacing
LM electrostimulating electrodes, through the body and into the current mirror of the current control module, where the current intensity
was precisely set in the range from 0 to 2 mA (Fig. 3g). The actual current
intensity was transformed into a safely sensible voltage for the MCU by
Nature Electronics
an OP-AMP described in the literature4. The electrostimulation output
waveforms and controlled current intensities under stretching states
were very consistent and stable, demonstrating system-level stability
when stretching (Supplementary Fig. 18). We utilized the P3D-eskin to
deliver electrostimulations to the bicep femoris of a rat (Fig. 3h). We
Article
https://doi.org/10.1038/s41928-024-01189-x
Fig. 4 | Battery-free stretchable NFC system based on the P3D-eskin platform.
a, Exploded schematic of battery-free P3D-eskins using NFC technology. The
system included a stretchable LM antenna, stretchable printed LM microcircuit
integrated with microchips (MCU for NFC, analogue–digital converter and
sensors) and permeable fibre mats as the encapsulation and substrate. b,c,
Digital images of the NFC P3D-eskins before (b) and after (c) the encapsulation
of permeable superelastic fibre mat. d,e, Digital images of the stretchable NFC
antennas using conventional serpentine Cu coil (d) and intrinsically stretchable
LM coil (e). Both antennas are stretched at 50% strain. f, FEA of the stress
distribution of stretchable antennas under biaxial stretching (50% strain) using
Cu serpentine and intrinsically stretchable LM. g, Inductance of serpentine Cu
antenna (five turns) and intrinsically stretchable LM antenna (ten turns) with the
same coverage area as a function of frequency at various strains. h–j, Q factor (h),
phase (i) and impedance (j) of the stretchable LM antenna at various strains. k,l,
Digital images of the skin inflammatory states after covering with P3D-eskin (k)
and PDMS-eskin (l) for 30 min exercise. m,n, Thermal imagery of an adult body
using 40 NFC P3D-eskin arrays, depicting multiposition body temperature in
cool/dry (m) and warm/humid (n) environments. o, Continuous temperature
monitoring of an adult body during sleeping using P3D-eskin, PDMS-eskin and
gold standard (commercial infrared thermal imager). The error band (shade) in
the figure stands for s.d., and the scatter value represents the mean value.
recorded the real-time electromyography (EMG) signals at adjacent
areas on the rat’s bicep femoris. During the stimulation period, the
corresponding EMG responded to various frequencies (1, 5 and 10 Hz)
(Fig. 3i) and matched well with the stimulation input (Fig. 3j), which
proved that electrical stimulations were successfully delivered with
the wireless P3D-eskin system.
with a commercial infrared thermal imager. In the testing conditions,
P3D-eskin possessed an initial smaller cooling load, making a smaller
thermal impact on the skin than the PDMS-eskin (Supplementary
Fig. 23). Additionally, the permeable characteristic of P3D-eskin
allowed better convective heat transfer between the air and skin, which
is closer to the real situation where bare skin can regulate the body
temperature compared with an unbreathable dressing with heat conduction only. Further, the P3D-eskin was softer and more stretchable,
allowing a more conformal contact with the skin, and may, thus, reduce
thermal artefacts with smaller signal fluctuations.
Battery-free P3D-eskin system
We also developed a battery-free P3D-eskin using near-field communication (NFC) technology (Fig. 4a). The NFC P3D-eskin before and
after the final encapsulation is shown in Fig. 4b,c, respectively. It is also
highly flexible and stretchable (Supplementary Fig. 19a).
The LM antenna coils of the NFC outperform serpentine Cu coils
in terms of design compactness, stretchability and electromagnetic
stability. To achieve good stretchability, conventional stretchable Cu
antenna coils were fabricated in the shape of the serpentines, which
greatly reduced the coil density (Fig. 4d). In contrast, due to the intrinsic stretchability of LM, the LM coil was much more compact at the
same occupied area. The turn number of the LM coil was twice that of
the Cu antenna (Fig. 4e and Supplementary Fig. 19b). Moreover, the
Cu antenna only sustained a stretchability of up to 50% strain, at which
point the jump wire and interconnects with the components were
disconnected (Supplementary Fig. 19c), whereas the LM antenna was
well connected even at a strain of 300%. Finite-element analysis (FEA)
results indicated that this is because the stress distribution of the LM
antenna was much more uniform than that of the Cu antenna. The
stress concentration factor of the LM coil was 586-fold lower than that
of the Cu coil (Fig. 4f and Supplementary Table 4). Furthermore, the
LM coil showed higher inductance than the Cu coil (Fig. 4g and Supplementary Fig. 19d,e). The quality (Q) factor (Fig. 4h), phase (Fig. 4i) and
impedance (Fig. 4j) of the LM coil showed high stability under various
tensile strains and working distances (Supplementary Fig. 20), within
a readable frequency of ~13.56 MHz.
We developed a temperature-sensing NFC P3D-eskin to continuously record the temperature distribution of different positions of
the human body (Supplementary Fig. 21). Continuous monitoring
of body temperature during daily activities such as sitting, walking
and exercising could be stably monitored using a customized mobile
application (Supplementary Fig. 22). P3D-eskins offer a high degree of
wearing comfort and biocompatibility with skin health. Wearing the
P3D-eskin during intensive exercise did not lead to the accumulation
of sweat, thereby avoiding skin dampness, allergy and inflammation
(Fig. 4k), whereas the skin covered by the PDMS-eskin showed obvious
skin erythema (Fig. 4l).
Thermal imagery of an adult male body in cool/dry (Fig. 4m) and
hot/humid (Fig. 4n) environments could be depicted by the multiposition physiological temperature mapping using 40 NFC P3D-eskin
arrays. We recorded the body temperature during continuous sleep
monitoring for 8 h (Fig. 4o). Importantly, not only did the P3D-eskin
show a more stable signal recording with lower signal variations than
PDMS-eskin but the temperature values recorded from the P3D-eskin
were also in good accordance with the standard temperature recorded
Nature Electronics
Conclusions
Bioelectronics that incorporate commercially available electronic
components (such as high-performance and inexpensive chips) with
stretchable printed circuits can provide high-quality and continuous
health monitoring and interventions. For long-term use, permeable
and stretchable 3D integrated electronic systems are needed. However, 3D integrated stretchable electronics that have high integration
density typically use impermeable, compact and thin-film materials
as the substrate, interlayer and encapsulation layers (Supplementary
Table 1), whereas systems with high permeability have low integration
density (Supplementary Table 2).
We have developed a P3D-eskin that enables rigid electronic
components to be integrated with stretchable fibrous substrates in
a 3D configuration. A stretchable hybrid LM solder was developed to
provide a reliable interface between the rigid components and the
soft LM circuit, forming a vertical electrical connection by LM VIA and
electrical insulation by fibre mat. Our integration strategy allows us to
fabricate multilayered soft and stretchable circuits using layer-by-layer
fabrication of in situ electrospun fibre mats and micropatterned LM
circuits connected by stretchable VIAs (Supplementary Fig. 24 and
Supplementary Video 7). Compared with eskins made with stretchable thin-film substrates, the P3D-eskin is lighter, thinner, softer and
more stretchable. It also exhibits long-term biocompatibility in an
on-skin test.
Methods
Materials
All the processing solvents were used as received. Dextran
(Sigma-Aldrich), LM, eutectic GaIn (LM, melting point, 15.7 °C;
Sigma-Aldrich), negative photoresist (NR9-1500P, Futurrex), developer
for NR9-1500P (DR6, Futurrex) and SBS (Kraton) were used as received.
Fabrication of P3D-eskins
The fabrication procedure of multilayered LM circuits combines
(1) the photopatterning, pattern transfer, selective wetting method and
(2) the stencil printing of LM, which took both advantages of photopatterning and stencil printing techniques (Supplementary Note 1 and
Supplementary Figs. 2 and 3). Here we patterned the permeable and
stretchable LM microelectrodes to create stretchable antennas, traces,
connections and contacts for the microcircuit. Specifically, a sacrificial
layer was prepared on the wafer by spin coating a dextran solution
Article
(10 wt% in water) at 4,000 r.p.m. for 40 s. After baking treatment (80 °C
for 1 min and 180 °C for 30 min), a negative photoresist (NR9-1500P)
was subsequently spin coated on the dextran-coated wafer, followed
by photolithography and developing process. Ag microcircuit (top
layer of the 3D circuit) was created using the lift-off treatment of the
deposited Ag film (300 nm thick) by thermal evaporation. A fibrous
SBS mat (40–100 μm thick, insulating layer) was directly electrospun
on the Ag microcircuit. The polymer solution was prepared by dissolving the SBS polymer with a weight ratio of 13 wt% in the mixed solvent
(tetrahydrofuran/dimethylformamide = 3:1). The voltage was set as
18 kV and the collecting distance was 15 cm. After dissolving the dextran
layer with deionized water, the Ag microcircuit was then transferred to
the SBS mat44. The Ag microcircuit layer was selectively wet with LM in
the glovebox, cut into a square-shaped piece and covered with a thin
electrospun SBS mat (15–30 μm thick, paste mask layer). The selective
wetting of LM lies in contrast between the LM lyophobic property of
the SBS mat and the LM lyophilic property of Ag. In the fabrication of
the LM microcircuit, EGaIn wets only the Ag-covered areas because of
reactive alloying, and dewets from the SBS surface because of the high
intrinsic surface tension of LM. When applying EGaIn on Ag, reactive
alloying between Ag and In forms AgIn alloys. Additional EGaIn will
subsequently wet the AgIn alloy layer and form the EGaIn/AgIn/Ag
trilayer (Supplementary Fig. 25).
We then flipped over this top circuit layer and stencil printed
the LM traces of the base circuit layer. After electrospinning another
SBS mat (40–100 μm thick) as the substrate, we engineered the VIAs
between the two layers of the 3D LM circuits by the laser-cutting
method (LPKF ProtoLaser U4) and filled these VIAs with LM. We flipped
over the circuit board again and stencil printed the oLM ink. The oLM
ink was prepared by heating pristine LM in air at a set temperature of
80 °C for 16 h. The oLM was printed onto the paste mask layer via a
customized mask, serving as contact pads for the electronic components. After placing the components on the paste mask layer, additional
pristine LM paste was applied at the pin/oLM interfaces to form the
ultrastretchable hLM solders. Here the weight ratio between the oLM
pad and LM paste was 1:2.
A detailed circuit diagram design and printed circuit board design
are provided in Supplementary Fig. 1. The components are listed in
Supplementary Table 3. Key components in each layer included an
MCU, oscillator, MUX, current mirror, DAC, OP-AMP, HV (20.0 V) and
LDO (3.3 V). For the wireless communication, the P3D-eskin system was
equipped with a BLE 5.1 built-in MCU (CC2640, Texas Instruments) and a
matched 2.4 GHz LM BLE antenna (planar inverted F-shaped antenna) to
achieve data acquisition, transmission and functional control by simply
using a smartphone with a mobile app. Code Composer Studio was used
for MCU programming. The Android application used for communication by mobile devices was developed by Android Studio. The power of
P3D-eskin was supplied by a lithium-ion battery, and the voltage was
regulated by the LDO (Supplementary Fig. 26a,b). Finally, the whole
permeable stretchable circuit board was conformally encapsulated
with a permeable but waterproof SBS mat to ensure stable operations.
Fabrication of PDMS-eskins
As a reference, we also fabricated 3D eskins with the same device design
and configuration using thin PDMS as the substrate, interlayer and
encapsulation material. First, a layer of PDMS (SYLGARD 184, 10:1)
was spin coated (500 r.p.m., 30 s) onto the clean and dry glass sheet,
and cured in the oven (80 °C, 30 min). Meanwhile, two Cu/polyimide
films (18.0/12.5 μm) were laser cut (LPKF ProtoLaser U4) into patterns
on the top and bottom layers of the circuit, respectively. Picked up
by water-soluble tapes, their polyimide side was deposited with Ti/
SiO2 layers (5/100 nm) by electron-beam evaporation as the adhesive
interface between the circuits and PDMS substrate. After treating
both surfaces with ultraviolet–ozone for 5 min, the base layer of the
circuit pattern was transferred onto the PDMS substrate with strong
Nature Electronics
https://doi.org/10.1038/s41928-024-01189-x
bonding. Rinsing in water removed the water-soluble tape. Another
layer of PDMS was spin coated in the same way on top of the bottom
circuit layer and cured, which functioned as the intermediate insulating layer. The laser-cutting method was used to fabricate VIAs on the
PDMS layer. The top Cu circuit layer was then transferred and printed
onto the insulating layer in the same way after ultraviolet–ozone treatment and aligning with the base layer. Then, the VIAs were filled with
commercial soldering paste, and the electronic components were
placed on paste-applied pads. The components were soldered onto the
multilayered circuit with a hot-wind blower. Finally, the circuit board
was fully encapsulated by casting the PDMS solution and curing it in
the oven (80 °C, 15 min).
Characterizations
The morphology of the LM 3D circuits and surface oxidation states
of the oLM were explored using scanning electron microscopy (SEM;
TESCAN VEGA3) and X-ray photoelectron spectroscopy (Thermo Fisher
Scientific Xesa), respectively. Both air permeability and moisture permeability tests were performed at constant temperature (22 °C) and
humidity (63%). The air permeability tests were conducted according to
the ASTM D737-08 standard using a MO21S air permeability tester (SDL
America) with an airflow pressure of 100 kPa. Moisture permeability
tests were performed according to the E96/E96M-13 standard with the
cup method. The testing duration was 72 h. The waterproofness of the
P3D-eskin system was characterized by a standard rain test according to
AATCC Test Method 35–2006. The sample size was set as 20 cm × 20 cm,
and the water-spraying duration was 2 min. The sweat-resistance tests
of the P3D-eskin system were performed by immersing the P3D-eskin
system in water and artificial sweat (pH, 4.7 ± 0.1; ZW-HY-1000, Zhongwei Equipment) with a stirring rate of 300 r.p.m. The luminance stability
of the embedded LEDs inside the P3D-eskin system indicated the sweat
resistance of the system. The statistic values (mean and s.d.) were
obtained with three to six parallel samples. Each sample was tested
for at least three times. The mechanical properties of the materials
were characterized using a universal testing machine (Instron 5566).
The electrical resistance of the resistors connected with the LM, oLM
and hLM under different strains was measured by a four-terminal
method with a source meter (Keithley 2400) coupled with a customized
stretching machine (Zolix). The output and transfer characteristics of
the stretchable MOSFETs and multilayer stretchable switch array were
characterized using a semiconductor analyser (Keithley 4200A-SCS
parameter analyser) connected with a probe station (Micromanipulator) and a customized stretching setup. The stretchable logic circuits
were characterized using a digital oscilloscope (Rigol).
FEA
Static structural mechanics of the systems were analysed using FEA.
Material mechanical parameters are summarized in Supplementary
Table 4. The modulus of LM and oLM was measured from the stress–
strain curves. Since LM or oLM was neither a self-supporting material
nor pure liquid (with the inevitable existence of an oxide layer), here
we tested Young’s modulus of LM or oLM supported with the substrate
(SBS fibre mat). Accordingly, Young’s modulus values of the LM traces
(LM on the SBS fibre mats) were adopted for FEA of the stress distribution of the electrical interfaces. We conducted a sensitivity test by
varying the modulus of LM ranging from 0.1 Pa (fluid-like substance) to
1011 Pa (near the modulus of Cu) and observed the change in stress concentration factor (the ratio of maximum stress to average stress). The
modulus of LM showed little influence on the stress concentration factor within the modulus range of 0.1 Pa to 1 MPa, of which all these stress
concentration factors were much smaller than those in the controlled
groups of simulations (for example, Al2O3 for chip and Cu antenna;
Supplementary Fig. 27). Four systems were investigated, namely, serpentine Cu antenna, intrinsically stretchable LM antenna, a chip with
LM or oLM solder and a chip with hLM solder. These systems were
Article
https://doi.org/10.1038/s41928-024-01189-x
subjected to mechanical tension of 50% strain. The stress responses
were collected. The ratio of maximum stress to average stress (σmax/σavg)
was used as the stress concentration factor. The input parameters and
output results of the FEA are summarized in Supplementary Table 4.
Reporting summary
Animal experiments
Source data are provided with this paper. Other data that support the
findings of this study are available from the corresponding authors
upon request.
The animal experiments followed the Ethical Review of Research Experiments Involving Animal Subjects (A-0664) approved by the Research
Committee (Animal Research Ethics Sub-Committee) of the City University of Hong Kong. Before transcutaneous electrical stimulations
and electrophysiological signal recording, a healthy male Sprague
Dawley rat (aged 4–5 weeks, ~200 g) was utilized for bicep femoris muscle stimulation. The rat was first treated with gaseous light anaesthesia
(isoflurane, 3%), followed by deep anaesthesia by an intraperitoneal
injection of a mixed solution of ketamine (100 mg kg−1) and xylazine
(10 mg kg−1). The hair on the skin of both legs was shaved for attaching
the P3D-eskin patch.
Wireless transcutaneous electrostimulations and recording
the corresponding EMG signals
All the procedures involving the on-skin attachment of P3D-eskins
and PDMS-eskins on the human body followed ethical guidelines,
which were approved by The Hong Kong Polytechnic University
(HSEARS20230101001). Before the on-skin attachment, we adopted
a biocompatible, soft and wet-adhesion adhesive55 to adhere the
edges of the eskins (PDMS-eskin and P3D-eskin) onto the skin.
The electrostimulation process was wirelessly controlled via a
mobile-Android-system-based mobile phone. The anode and common
cathode were applied to the animal’s skin to form a closed circuit. The
current control module was connected between the ground (GND) of
the circuit and the common cathode, providing a virtual ground potential that changed according to the permitted current. The permitted
current intensity was controlled by a simple current mirror circuit. The
current intensity in the second transistor was identical to the reference
current through the first transistor by sharing the same gate (G) and
source (S). By sending predefined serial commands to the DAC, the
voltage was precisely controlled in the range of ~0–3.3 V. This voltage
was then applied to the drain (D) and G of the transistor to define the
reference current intensity. A fixed resistor (50 Ω) was connected in
serial to the common D and GND, where the voltage on D reflected the
total current. This voltage could be read by a 14-bit analogue–digital
converter on the MCU. To prevent overload voltage that can cause
damage to the MCU, an OP-AMP that functioned as a voltage follower
(providing identical voltage) was added between D and MCU. Next,
the output voltage and current data were measured by a data acquisition multimeter system (Keithley DAQ6510) at a sampling frequency
of 10 kHz. The EMG signals were measured using a high-precision
data acquisition system (PowerLab 16/35, AD Instruments) and with
a biological signal amplifier (BioAmp FE132, AD Instruments) with a
sampling rate of 10 kHz. The raw signal data were digitally filtered by
two notch filters at 50 and 100 Hz to obtain the representative EMG
waveforms avoiding the baseline noises.
Characterizations of the NFC P3D-eskin system
To develop a battery-free P3D-eskin, we adopted an NFC-embedded
MCU (RF430FRL152H, Texas Instruments), and modified
the temperature-sensing circuit from the reference design
(TIDM-RF430-TEMPSENSE). The temperature data can be acquired
and read by the graphical user interface of a mobile Android app for
temperature sensing (RF430FRL152H Demo, Texas Instruments; Supplementary Fig. 21) or an NFC reader (MSP-EXP430G2ET with TRF7970A
NFC/RFID booster pack; Supplementary Fig. 26c). Characteristics of
the LM inductive antenna including inductance, Q factor, impedance
and phase were characterized using an impedance analyser (E4991B,
Keysight Technologies).
Nature Electronics
Further information on research design is available in the Nature
Portfolio Reporting Summary linked to this article.
Data availability
Code availability
The code supporting the findings of this study is available from the
corresponding authors upon request.
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Acknowledgements
We acknowledge financial support from the RGC Senior Research
Fellow Scheme (SRFS2122-5S04), The Hong Kong Polytechnic
University (1-ZVQM, 1-BBXR and 1-CD44), Research Grants Council
of the Hong Kong Special Administrative Region (grant nos.
RFS2324-1S03, 15304823, 11213721, 11215722, 11211523),
City University of Hong Kong (grant nos. 9667221 and 9678274)
and National Natural Science Foundation of China (NSFC)
(grant nos. 61421002 and 62122002), as well as in part by InnoHK
Project on Project 2.2—AI-based 3D ultrasound imaging algorithm
at the Hong Kong Centre for Cerebro-Cardiovascular Health
Engineering (COCHE).
Author contributions
Q.Z. and Z.Z. initiated the idea and proposed the project. K.Y. designed
the circuits of the electronic systems. Q.Z. and K.Y. characterized
the overall systems. Z.Z., X.Y., Q.Z. and K.Y. wrote the paper. C.Z.
conducted the FEA. X.S. and Y. Zhou facilitated the design of logic
circuits. J.Z. facilitated the debugging process. Y. Zhang and Q.H. gave
comments on the organization of figures.
Competing interests
The authors declare no competing interests.
Additional information
Extended data is available for this paper at https://doi.org/10.1038/
s41928-024-01189-x.
Supplementary information The online version contains
supplementary material available at https://doi.org/10.1038/s41928024-01189-x.
Correspondence and requests for materials should be addressed to
Xinge Yu or Zijian Zheng.
Article
Peer review information Nature Electronics thanks John Ho, Yanchao
Mao and the other, anonymous, reviewer(s) for their contribution to
the peer review of this work.
https://doi.org/10.1038/s41928-024-01189-x
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2024
Nature Electronics
Article
https://doi.org/10.1038/s41928-024-01189-x
Extended Data Fig. 1 | Layer-by-layer fabrication of wireless P3D-eskins. a, Schematic illustration showing the detailed processing flow of layer-by-layer fabrication
of P3D-eskins. b, Digital images showing the structure and functions of each layer in P3D-eskins.
Nature Electronics
Article
Extended Data Fig. 2 | Characterizations of the oLM on the SBS fibre mats
(oLM/SBS) at different oxidation conditions. a, SEM images of LM/SBS with
various heating durations before and after stretch. The oLM was prepared by
oxidizing LM in the air. After heating LM with increasing duration ranging from
0 to 24 h, the size of the gallium (Ga) oxide enhanced from several μm to several
hundred μm. After being pre-stretched under 1500% strain for 12 cycles, the
continuous thin film (heating duration less than 16 h) can self-organize into
a laterally mesh-like and vertically buckled structure, with the formation of
nodes from the strong oxide layer. b, XPS results of oLM after various heating
durations. Ga 2p (3/2) spectrum shows a predominant peak with a binding
energy of 1118.8 eV from Ga2O3, with the presence of Ga metal (1116.5 eV) and
Ga2O (1118.2 eV). c, Schematic illustration of the formation of the gallium oxides
(Ga2O3, and Ga2O) during the heating of LM. With increasing heating duration,
Nature Electronics
https://doi.org/10.1038/s41928-024-01189-x
the signals of Ga2O3 and Ga2O became stronger. d and e, Young’s modulus and
electrical conductivity of oLM/SBS after various heating durations. Data are
presented as (dots) mean values with (error bars) SD; n = 6 independent tests.
Due to the strong oxidation of LM, the average modulus of oLM/SBS enhanced
from ~0.1 MPa (0.09841 MPa) with the heating duration of 0 h (that is, LM/SBS)
to ~0.31314 MPa with the heating duration of 24 h. Accordingly, the stiffness was
also enhanced by around 2 folds when the thickness was unchanged. The oLM/
SBS with a heating duration of 16 h maintained a high electrical conductivity of
over 28,300 S/cm. The error bar in the figure stands for SD, and the scatter value
represents mean value. f, Resistance changes of hybrid LMs (weight ratio of oLM
and LM and = 1: 2, oLM with various heating durations) on the SBS fibre mats as
the function of tensile strain.
Article
Extended Data Fig. 3 | See next page for caption.
Nature Electronics
https://doi.org/10.1038/s41928-024-01189-x
Article
Extended Data Fig. 3 | Characterizations of the electrical stability of 3D
integrated interfaces of various stretchable logic circuits. a, Design of
the permeable stretchable logic circuits including inverse gate, NOR gate,
and clock-controlled switch. b, Outputs of the logics validated with the rigid
printed circuit boards. c and d, Digital images of the inverse gate and NOR gate,
respectively. e and f, Logic outputs of the inverse gate, and NOR gate respectively.
g, Schematic illustration of the permeable 3D integrated stretchable switch
array. h, Threshold driving voltage of the switch array at a strain of 100%. i,
Nature Electronics
https://doi.org/10.1038/s41928-024-01189-x
Statistic analysis of the transconductance of the 64-channel switch array. j,
Digital images of the permeable 3D integrated stretchable switch array at 100%
strain. The switches were used for controlling loads and in complementary
metal-oxide semiconductor (CMOS) digital circuits as they operated between
their cut-off and saturation regions. The multi-channel switch array showed a
uniform threshold driving voltage (Vg) of ~1.75 V at a strain of 50%, and an average
transconductance of ~100 mS.
Last updated by author(s): Apr 26, 2024
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