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Mechanical and electrical numerical analysis of soft liquid-embedded deformation sensors analysis

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Soft sensors comprising a flexible matrix with embedded circuit elements can undergo large deformations while maintaining adequate performance. These devices have attracted considerable interest for their ability to be integrated with the human body and have enabled the design of skin-like health monitoring devices, sensing suits, and soft active orthotics. Numerical tools are needed to facilitate the development and optimization of these systems. In this letter, we introduce a 3D finite element-based numerical tool to simultaneously characterize the mechanical and electrical response of fluid-embedded soft sensors of arbitrary shape, subjected to any loading. First, we quantitatively verified the numerical approach by comparing simulation and experimental results of a dog-bone shaped sensor subjected to uniaxial stretch and local compression. Then, we demonstrate the power of the numerical tool by examining a number of different loading conditions. We expect this work will open the door for further design of complex and optimal soft sensors.
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Extreme Mechanics Letters 1 (2014) 42–46
Contents lists available at ScienceDirect
Extreme Mechanics Letters
journal homepage:
Mechanical and electrical numerical analysis of soft
liquid-embedded deformation sensors analysis
Johannes T.B. Overvelde a, Yiˇ
git Mengüç a,b,c, Panagiotis Polygerinos a,b,
Yunjie Wang a, Zheng Wang b,d, Conor J. Walsh a,b, Robert J. Wood a,b,
Katia Bertoldi a,
aSchool of Engineering and Applied Sciences (SEAS), Harvard University, Cambridge, MA, USA
bWyss Institute for Biologically Inspired Engineering, Harvard University, Boston, MA, USA
cSchool of Mechanical, Industrial and Manufacturing Engineering (MIME), Oregon State University, Corvallis, OR, USA
dDepartment of Mechanical Engineering, The University of Hong Kong, Hong Kong
a r t i c l e i n f o
Article history:
Received 23 October 2014
Received in revised form 14 November
Accepted 15 November 2014
Available online 4 December 2014
Finite element
Soft sensor
Liquid-embedded sensor
Large deformation
a b s t r a c t
Soft sensors comprising a flexible matrix with embedded circuit elements can undergo
large deformations while maintaining adequate performance. These devices have attracted
considerable interest for their ability to be integrated with the human body and have
enabled the design of skin-like health monitoring devices, sensing suits, and soft active
orthotics. Numerical tools are needed to facilitate the development and optimization of
these systems. In this letter, we introduce a 3D finite element-based numerical tool to
simultaneously characterize the mechanical and electrical response of fluid-embedded
soft sensors of arbitrary shape, subjected to any loading. First, we quantitatively verified
the numerical approach by comparing simulation and experimental results of a dog-bone
shaped sensor subjected to uniaxial stretch and local compression. Then, we demonstrate
the power of the numerical tool by examining a number of different loading conditions. We
expect this work will open the door for further design of complex and optimal soft sensors.
©2014 Elsevier Ltd. All rights reserved.
While engineering applications often use stiff and
rigid materials, soft materials like elastomers enable the
design of a new class of electronic devices that are flexi-
ble, stretchable, adaptive and can therefore be easily inte-
grated with the human body [1–6]. These include highly
conformable and extensible deformation sensors made
from flexible substrates with embedded circuit elements,
such as graphene sheets [7], nanotubes [8], interlocking
nanofibres [9], serpentine patterned nanomembranes of Si
[3,10,11], and conductive liquid micro channels [12–14].
A key feature of these sensors is their ability to re-
versibly stretch, bend, compress and twist to a great ex-
tent. Such deformations result in changes of the electrical
Corresponding author.
E-mail address: (K. Bertoldi).
resistance of the sensors, which are then used to deter-
mine the applied loading conditions. Therefore, the design
of the next generation of soft sensors requires the develop-
ment of numerical tools capable of predicting not only their
mechanical performances [15,16], but also their electrical
response. Such tools will enable the design of optimized
sensors that are sensitive to desired loading conditions and
also provide crucial insights into the working principles of
these soft devices.
In this letter, we propose a 3D finite element-based nu-
merical tool that predicts both the mechanical and electri-
cal response of arbitrary shaped soft sensors subjected to
any loading condition. In particular, we focus on sensors
comprising an elastomeric matrix embedded with a net-
work of channels filled with a conductive liquid [12–14],
but the numerical approach can be easily extended to other
types of soft sensors. The analyses are performed using the
commercial finite element (FE) code Abaqus, which is an
2352-4316/©2014 Elsevier Ltd. All rights reserved.
J.T.B. Overvelde et al. / Extreme Mechanics Letters 1 (2014) 42–46 43
Fig. 1. The proposed FE-based numerical tool consists of three steps. (A) The model is created using CAD software and then meshed. (B) The deformation
of the sensor is determined by using non-linear FE analysis, in which the contours show the normalized Von Mises stress σvm. (C) The resistance at different
levels of deformation is obtained by performing a steady-state linear electrical conductivity analysis. The contours show the potential across the channel.
Fig. 2. Uniaxial extension of a dog-bone shaped soft sensor. (A) Experimental images of the undeformed (ux/L=0) and deformed (ux/L=0.5) sensor. (B)
Numerical images of the undeformed (ux/L=0) and deformed (ux/L=0.5) sensor. The contours in the snapshot show the distribution of the electrical
potential. (C) Cross-sectional profile measured with a laser interferometer (green) and cross-sections used in simulations. (D) Reaction force obtained in
experiments and simulations as a function of the applied strain. (E) Cross-sections of the undeformed and deformed (ux/L=0.5) channels as predicted by
the FE analysis. (F) Electrical resistance measured in experiments and simulations as a function of the applied strain.
attractive platform because it is well-known, widely avail-
able and particularly suitable for analyses involving large
deformations. By making our code available online, we ex-
pect the proposed tool to be widely used and expanded to
design more complex soft sensors with new and improved
Our FE-based numerical tool consists of three steps, as
indicated in Fig. 1 (the Abaqus script files and Matlab files
used for our analysis are available online as Supporting
Step A: Creating the model. A 3D model comprising both
the flexible matrix and the circuit elements is first created
using CAD software and then meshed. In particular, for
the case of liquid-embedded soft sensors considered in
this letter, the elastomeric matrix is meshed using linear
tetrahedral elements (Abaqus element type C3D4), while
a solid mesh of the channels is not created. In fact, only
the surface mesh of the channel will be used to apply the
pressure exerted by the fluid to the elastomer.
Step B: Determining the deformation. To determine the de-
formation of the sensor under specific loading conditions,
a non-linear FE analysis is performed using the commer-
cial package Abaqus/Explicit (v6.12). In the simulations, we
fully account for contact between all faces of the model
and ensure quasi-static conditions by monitoring the ki-
netic energy and introducing a small damping factor. For
the case of liquid-embedded sensors investigated here, the
response of the flexible matrix is captured using a nearly
incompressible neo-Hookean material characterized by an
initial shear modulus µ. Moreover, we consider the chan-
nels to be completely filled with a nearly incompressible
fluid with bulk modulus K=100µand use the surface-
based fluid cavity capability in Abaqus, so that the pressure
applied by the fluid to the surface of the channel is deter-
mined from the cavity volume.
Step C: Analyzing the electrical resistance. To determine the
electrical resistance of the deformed sensor, an isothermal
steady-state linear electrical conductivity analysis (Abaqus
step Coupled thermal–electrical) is performed on the de-
formed solid mesh of the circuit elements. Assuming the
circuit elements are made of a material with electrical re-
sistivity ρ, an electrical potential difference Uis applied
between the two ends of the deformed circuit mesh and
the dissipated work (W) over a time period of tis cal-
culated. The electrical resistance of the channel (R) is then
obtained as
44 J.T.B. Overvelde et al. / Extreme Mechanics Letters 1 (2014) 42–46
Fig. 3. Compression of a dog-bone shaped soft sensor using a circular punch. (A) Experimental set-up. (B) Cross-sections of the undeformed and deformed
(uz/L= −0.11) channels as predicted by the FE analysis. (C) Numerical images of the undeformed (uz/L=0) and deformed (uz/L= −0.3) sensor. The
contours in the snapshot show the distribution of the electrical potential. (D) Reaction force obtained in experiments and simulations as a function of the
applied compressive strain. (E) Electrical resistance measured in experiments and simulations as a function of the applied compressive strain.
For the sake of convenience, in all our simulations we use
U=1V and t=1s, so that R=1/W. Note that for
the specific case of liquid-embedded soft sensors, an ad-
ditional meshing step is required, in which the deformed
solid tetrahedral mesh of the channel is created starting
from the deformed surface mesh obtained in Step B.
To validate our simulations, we focus on a dog-bone
shaped soft sensor with a serpentine channel aligned along
its length (see Fig. 2A), and compare the numerical pre-
dictions to experimental results for two different loading
conditions: uniaxial tension and local compression. The
sample is fabricated using silicone rubber (EcoFlex 0030,
Smooth-On, Easton, PA, USA) and the channel is filled with
the conductive liquid eutectic Gallium Indium (eGaIn) (Alfa
Aesar, MA, USA) with resistivity ρ=29.4·108m [17].
Details on the fabrication of the sample are reported in pre-
vious papers [12,18,19]. All experiments were conducted
on a uniaxial materials testing machine (model 5544A, In-
stron Inc., MA, USA). To determine the electrical resistance
Rof the sensor, the sample was connected in series with
a resistor with resistance Rref . A potential difference Uref
was applied to the entire system by a power supply, so that
Uref U,
where Uis the potential difference between the two ends
of the sensor’s channel measured using a data acquisition
card (BNC-2111, National Instruments Corp., TX, USA).
The dog-bone sample considered in this study has a
central slender section of length L=50 mm and a rect-
angular cross-section of 8 mm by 2 mm. Despite the fact
that the sensor is designed to have an embedded channel
with an overall length of 360 mm and a rectangular cross-
section of 0.25 mm by 0.15 mm, we find that the actual
cross-sectional shape of the channel is closer to that of a
circular segment (see Fig. 2C). Therefore, in our simulations
we consider two different cross-sectional shapes for the
channel: (i) a rectangle of 0.3 mm by 0.172 mm and (ii) a
circular segment with an angle of 111 degrees and a radius
of 232 mm. Note that in both cases the area of the channel is
equal to the experimentally measured value of 0.052 mm2.
First, we load the sample uniaxially, as shown in
Fig. 2A and B and Movies S1 and S2. From the mechanical
response of the sensor, we find that the experimental mea-
sured stress–strain behavior can be fully captured mod-
eling the elastomeric matrix as a neo-Hookean material
with µ=0.0221 MPa (see Fig. 2D). As expected, we find
that both the rectangular and circular segment channels
deform in a similar manner and contract isotropically as
the deformation increases (see Fig. 2E). The decrease in
cross-sectional area of the channels results in an increase of
the electrical resistance, which is captured in both experi-
ments and simulations, as indicated in Fig. 2F. Remarkably,
the numerical simulations, which do not require any fitting
parameter, capture not only qualitatively but also quan-
titatively the evolution of Ras a function of the applied
deformation. Note that for this specific loading case our
simulations indicate that both the electrical and mechani-
cal response of the sensor are not affected by the shape of
the channels, but only by their area (see Fig. 2F).
Next, we locally compress the sample in its center with
a circular flat punch of radius 5 mm (see Fig. 3A and C
and Movies S3, S4 and S5). For this loading condition the
electrical response of the sensor is characterized by two
distinct regimes (see Fig. 3E) [13,14,19]. For low values of
J.T.B. Overvelde et al. / Extreme Mechanics Letters 1 (2014) 42–46 45
Fig. 4. Sensitivity of a dog-bone shaped soft sensor to various load cases. (A) Undeformed configuration. (B) Twist. (C) Roll. (D) Uniaxial compression in
longitudinal direction. (E) Shear. The contours in the snapshot show the distribution of the electrical potential.
applied deformation (uz/H<0.15), the resistance R
of the sensor is constant and not affected by the applied
deformation. However, when the applied deformation is
large enough to locally close the channel (see Fig. 3B), R
increases rapidly. Remarkably, by using the same mate-
rial parameters determined in our previous analysis, the
numerical simulations exactly capture the experimentally
measured mechanical response (see Fig. 3D). Furthermore,
as shown in Fig. 3E, the simulations also correctly capture
the electrical response of the sensors and reveal that the
level of applied deformation at which Rstarts to rapidly
increase is highly sensitive to the cross-sectional shape of
the channel. Such sensitivity has been previously shown
experimentally [12,20] and is dictated by the fact that the
load required to locally close the channel is highly affected
by its shape. In particular, we find that the circular seg-
ment cross-section completely closes for smaller values of
applied strain (Fig. 3B), resulting in a sensor that is more
sensitive to local compression.
Now that we have verified the robustness of our nu-
merical tool, we use it to characterize the sensitivity of
our sensor to different loading conditions. In particular, we
consider four different loading cases: (i) we twist the sen-
sor by rotating one side around the x-axis by a 6πangle
as shown in Fig. 4B and Movies S6; (ii) we roll the sensor
by rotating one of its ends around the y-axis by a 4πan-
gle (see Fig. 4C and Movie S7); (iii) we compress the sen-
sor uniaxially along the x-axis by ux/L= −0.5 (Fig. 4D and
Movie S8); (iv) we shear the sensor by displacing one of the
ends along the y-axis by uy/L=0.5 (Fig. 4E and Movie S9).
Note that for the loading cases (i), (ii) and (iv) the sensor is
free to move in the z-direction, so that it does not stretch
uniaxially. Surprisingly, although in all these simulations
the channels are drastically deformed, the electrical resis-
tance of the sensor remains nearly unaltered, i.e. R/R01.
Therefore, our simulations indicate that this dog-bone
sensor is extremely well suited for applications in which
uniaxial tensile strain applied along the longitudinal direc-
tion needs to be monitored. In fact, much higher forces are
required to make the sensor sensitive to local compression
in the z-direction, i.e. 0.4N and 2N need to be applied to in-
crease the electrical resistance by 100% under uniaxial ex-
tension and local compression, respectively. Moreover, all
other simulated loading conditions are found to leave the
electrical resistance unchanged even for extreme values of
applied deformation.
In summary, we introduced an effective finite element
procedure to characterize simultaneously the mechanical
and electrical response of soft sensors of any shape sub-
jected to arbitrarily loading conditions. The accuracy and
robustness of the proposed numerical method was verified
by comparing the numerical and experimental results for
the case of a dog-bone shaped soft sensor with a serpentine
channel aligned along its length. The simulations not only
46 J.T.B. Overvelde et al. / Extreme Mechanics Letters 1 (2014) 42–46
quantitatively capture both the mechanical and electrical
response of the sensor, but also enable us to determine the
role played by the shape of the cross-section and different
loading conditions, facilitating the design of sensors which
are only sensitive to specific loads.
This work opens the door for further simulation-based
studies of soft sensors with complex microstructures,
which would otherwise be intractable to address analyt-
ically or experimentally. In particular, the proposed nu-
merical tool may serve as a platform to accelerate the
design of devices such as embedded 3D printed circuits
[21], liquid metal pumps [22], and health-monitoring de-
vices [3,8,19,23].
This work was supported by the Materials Research
Science and Engineering Center under NSF Award No.
DMR-0820484. K.B. also acknowledges support from the
National Science Foundation (CMMI-1149456-CAREER)
and the Wyss institute through the Seed Grant Program.
Appendix A. Supplementary data
Supplementary material related to this article can be
found online at
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... To the best knowledge of the authors, there are only a few studies that have numerically investigated these types of sensors. For example, Overvelde et al. [25] developed a finite element-based model to simulate the mechanical and electrical performance of a liquid metal flexible sensor. They assumed a neo-Hookean material model for EcoFlex substrate with rectangular and semi-circular crosssectional micro-channels and focused on different loading conditions. ...
... Fig. 11 compares three common sensor patterns widely reported in the literature. These are spiral (S) [19], [26], vertical serpentine (V) [19], [25], [34], and horizontal serpentine (H) [19] patterns. Results suggest that the sensitivity of the sensor may exhibit positive or negative slopes for different patterns. ...
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... under the assumption that the AC impedance from the inductor and the capacitor is negligible. The output voltage of the circuit reflects the changes in the resistance of the EGaIn channel caused by the contact force applied to the sensor 56,57 . The relationship between the output voltage and the resistance change of the sensor module can be calculated as Eqs. ...
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The modeling of soft structures, actuators, and sensors is challenging, primarily due to the high nonlinearities involved in such soft robotic systems. Finite element modeling (FEM) is an effective technique to represent soft and deformable robotic systems containing geometric nonlinearities due to large mechanical deformations, material nonlinearities due to the inherent nonlinear behavior of the materials (i.e., stress-strain behavior) involved in such systems, and contact nonlinearities due to the surfaces that come into contact upon deformation. Prior to the fabrication of such soft robotic systems, FEM can be used to predict their behavior efficiently and accurately under various inputs and optimize their performance and topology to meet certain design and performance requirements. In this article, we present the implementation of FEM in the design process of directly three-dimensional (3D) printed pneumatic soft actuators and sensors to accurately predict their behavior and optimize their performance and topology. We present numerical and experimental results to show that this approach is very effective to rapidly and efficiently design the soft actuators and sensors to meet certain design requirements and to save time, modeling, design, and fabrication resources.
... However, the responses are the result of the mechanisms under which the lines deform, which are dictated by the disparate mechanical properties of the two types of conductor, as schematized in Fig. 3b. Under increasing pressure, the channels locally close in a controlled fashion, thus increasing their resistance 53 . In the solid wire-elastomer structure, however, only the spacings between the centre and shield conductors vary, inducing a change in capacitance. ...
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Mechanical sensing is a key functionality in soft electronics intended for applications in health monitoring, human–machine interactions and soft robotics. Current methods typically use intricate networks of sensors specific to one type of deformation and one point in space, which limits their sensing capabilities. An alternative approach to distributed sensing is electrical reflectometry, but it is challenging to build the necessary transmission lines out of soft materials. Here, we report the scalable fabrication of microstructured elastomeric fibres that integrate tens of liquid metal conductors and have the length and cross-sectional integrity necessary to successfully apply time-domain reflectometry. Our soft transmission lines allow the detection of the mode, magnitude and position of multiple simultaneous pressing and stretching events. Furthermore, as a result of the dynamically responsive conductors, the pressure sensitivity is improved by a factor of 200 compared to rigid line probes. By integrating a single soft transmission line with a single interface port into a larger fabric, our technique can be used to create an electronic textile that can decipher convoluted mechanical stimulation.
Ultrathin devices are rapidly developing for skin-compatible medical applications and wearable electronics. Powering skin-interfaced electronics requires thin and lightweight energy storage devices, where solution-processing enables scalable fabrication. To attain such devices, a sequential deposition is employed to achieve all spray-coated symmetric microsupercapacitors (μSCs) on ultrathin parylene C substrates, where both electrode and gel electrolyte are based on the cheap and abundant biopolymer, cellulose. The optimized spraying procedure allows an overall device thickness of ≈11 µm to be obtained with a 40% active material volume fraction and a resulting volumetric capacitance of 7 F cm⁻³. Long-term operation capability (90% of capacitance retention after 10⁴ cycles) and mechanical robustness are achieved (1000 cycles, capacitance retention of 98%) under extreme bending (rolling) conditions. Finite element analysis is utilized to simulate stresses and strains in real-sized μSCs under different bending conditions. Moreover, an organic electrochromic display is printed and powered with two serially connected μ-SCs as an example of a wearable, skin-integrated, fully organic electronic application.
Cephalopods could simultaneously achieve both accurate positioning and agile bodily maneuvers by coordinating the mantle and the funnel, which is ideal for underwater robotic applications toward a compact propulsor with combined thrust vectoring and regulation. For a wide range of underwater applications from videography to manipulation, this novel approach would offer a compact and integrated alternative to the state-of-the-art with multiple vectoring thrusters. This article presents a biomimetic soft-robotic siphon (BSRS) as the propulsor unit, consisting of a novel central flow-regulative duct (CFRD) encircled by three circumferential siphon actuation muscles (SAMs). Hydraulic pressurization of the SAMs could enable both thrust vectoring by deflecting the BSRS and flow regulation by proportionally alternating the orifice of the CFRD. The design, modeling, and fabrication of the BSRS are presented in detail. Experiments using a prototype BSRS were conducted for validating the performances of deflection deformation and flow regulation, showing bending range of over 180° and flow-restricting capability of up to 100%. A burst effect was achieved with the ability of exceeding the constant flow rate by up to 50%, enabling tremendous thrust increase in very short time. This work proves the feasibility of combining omnidirectional deflection with flow regulation within a soft-robotic mechanism, paving the way to compact water-jetting propulsion for underwater robots.
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Octopus, squid, cuttlefish, and other cephalopods exhibit exceptional capabilities for visually adapting to or differentiating from the coloration and texture of their surroundings, for the purpose of concealment, communication, predation, and reproduction. Long-standing interest in and emerging understanding of the underlying ultrastructure, physiological control, and photonic interactions has recently led to efforts in the construction of artificial systems that have key attributes found in the skins of these organisms. Despite several promising options in active materials for mimicking biological color tuning, existing routes to integrated systems do not include critical capabilities in distributed sensing and actuation. Research described here represents progress in this direction, demonstrated through the construction, experimental study, and computational modeling of materials, device elements, and integration schemes for cephalopod-inspired flexible sheets that can autonomously sense and adapt to the coloration of their surroundings. These systems combine high-performance, multiplexed arrays of actuators and photodetectors in laminated, multilayer configurations on flexible substrates, with overlaid arrangements of pixelated, color-changing elements. The concepts provide realistic routes to thin sheets that can be conformally wrapped onto solid objects to modulate their visual appearance, with potential relevance to consumer, industrial, and military applications.
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We report a new method, embedded-3D printing (e-3DP), for fabricating strain sensors within highly conformal and extensible elastomeric matrices. e-3DP allows soft sensors to be created in nearly arbitrary planar and 3D motifs in a highly programmable and seamless manner. Several embodiments are demonstrated and sensor performance is characterized.
Wearable robots based on soft materials will augment mobility and performance of the host without restricting natural kinematics. Such wearable robots will need soft sensors to monitor the movement of the wearer and robot outside the lab. Until now wearable soft sensors have not demonstrated significant mechanical robustness nor been systematically characterized for human motion studies of walking and running. Here, we present the design and systematic characterization of a soft sensing suit for monitoring hip, knee, and ankle sagittal plane joint angles. We used hyper-elastic strain sensors based on microchannels of liquid metal embedded within elastomer, but refined their design with the use of discretized stiffness gradients to improve mechanical durability. We found that these robust sensors could stretch up to 396% of their original lengths, would restrict the wearer by less than 0.17% of any given joint's torque, had gauge factor sensitivities of greater than 2.2, and exhibited less than 2% change in electromechanical specifications through 1500 cycles of loading-unloading. We also evaluated the accuracy and variability of the soft sensing suit by comparing it with joint angle data obtained through optical motion capture. The sensing suit had root mean square (RMS) errors of less than 5 degrees for a walking speed of 0.89m/s and reached a maximum RMS error of 15 degrees for a running speed of 2.7m/s. Despite the deviation of absolute measure, the relative repeatability of the sensing suit's joint angle measurements were statistically equivalent to that of optical motion capture at all speeds. We anticipate that wearable soft sensing will also have applications beyond wearable robotics, such as in medical diagnostics and in human-computer interaction.
Stretchable electronics that require functional components with high areal coverages, antennas with small sizes and/or electrodes with invisibility under magnetic resonance imaging can benefit from the use of electrical wiring constructs that adopt fractal inspired layouts. Due to the complex and diverse microstructures inherent in high order interconnects/electrodes/antennas with such designs, traditional non-linear postbuckling analyses based on conventional finite element analyses (FEA) can be cumbersome and time-consuming. Here, we introduce a hierarchical computational model (HCM) based on the mechanism of ordered unraveling for postbuckling analysis of fractal inspired interconnects, in designs previously referred to as ‘self-similar’, under stretching. The model reduces the computational efforts of traditional approaches by many orders of magnitude, but with accurate predictions, as validated by experiments and FEA. As the fractal order increases from 1 to 4, the elastic stretchability can be enhanced by∼200 times, clearly illustrating the advantage of simple concepts in fractal design. These results, and the model in general, can be exploited in the development of optimal designs in wide ranging classes of stretchable electronics systems.
Conference Paper
Motion sensing has played an important role in the study of human biomechanics as well as the entertainment industry. Although existing technologies, such as optical or inertial based motion capture systems, have relatively high accuracy in detecting body motions, they still have inherent limitations with regards to mobility and wearability. In this paper, we present a soft motion sensing suit for measuring lower extremity joint motion. The sensing suit prototype includes a pair of elastic tights and three hyperelastic strain sensors. The strain sensors are made of silicone elastomer with embedded microchannels filled with conductive liquid. To form a sensing suit, these sensors are attached at the hip, knee, and ankle areas to measure the joint angles in the sagittal plane. The prototype motion sensing suit has significant potential as an autonomous system that can be worn by individuals during many activities outside the laboratory, from running to rock climbing. In this study we characterize the hyperelastic sensors in isolation to determine their mechanical and electrical responses to strain, and then demonstrate the sensing capability of the integrated suit in comparison with a ground truth optical motion capture system. Using simple calibration techniques, we can accurately track joint angles and gait phase. Our efforts result in a calculated trade off: with a maximum error less than 8%, the sensing suit does not track joints as accurately as optical motion capture, but its wearability means that it is not constrained to use only in a lab.
Wearable systems that monitor muscle activity, store data and deliver feedback therapy are the next frontier in personalized medicine and healthcare. However, technical challenges, such as the fabrication of high-performance, energy-efficient sensors and memory modules that are in intimate mechanical contact with soft tissues, in conjunction with controlled delivery of therapeutic agents, limit the wide-scale adoption of such systems. Here, we describe materials, mechanics and designs for multifunctional, wearable-on-the-skin systems that address these challenges via monolithic integration of nanomembranes fabricated with a top-down approach, nanoparticles assembled by bottom-up methods, and stretchable electronics on a tissue-like polymeric substrate. Representative examples of such systems include physiological sensors, non-volatile memory and drug-release actuators. Quantitative analyses of the electronics, mechanics, heat-transfer and drug-diffusion characteristics validate the operation of individual components, thereby enabling system-level multifunctionalities.
Thin, highly compliant sensing skins could provide valuable information for a host of grasping and locomotion tasks with minimal impact on the host system. We describe the design, fabrication, and characterization of a novel soft multi-axis force sensor made of highly deformable materials. The sensor is capable of measuring normal and in-plane shear forces. This soft sensor is composed of an elastomer (modulus: 69 kPa) with embedded microchannels filled with a conductive liquid. Depending on the magnitude and the direction of an applied force, all or part of the microchannels will be compressed, changing their electrical resistance. The two designs presented in this paper differ in their flexibility and channel configurations. The channel dimensions are approximately 200 × 200 μm and 300 × 700 μm for the two prototypes, respectively. The overall size of each sensor is 50 × 60 × 7 mm. The first prototype demonstrated force sensitivities along the two principal in-plane axes of 37.0 and -28.6 mV/N. The second prototype demonstrated the capability to detecting and differentiating normal and in-plane forces. In addition, this paper presents the results of a parameter study for different design configurations.
Means for high-density multiparametric physiological mapping and stimulation are critically important in both basic and clinical cardiology. Current conformal electronic systems are essentially 2D sheets, which cannot cover the full epicardial surface or maintain reliable contact for chronic use without sutures or adhesives. Here we create 3D elastic membranes shaped precisely to match the epicardium of the heart via the use of 3D printing, as a platform for deformable arrays of multifunctional sensors, electronic and optoelectronic components. Such integumentary devices completely envelop the heart, in a form-fitting manner, and possess inherent elasticity, providing a mechanically stable biotic/abiotic interface during normal cardiac cycles. Component examples range from actuators for electrical, thermal and optical stimulation, to sensors for pH, temperature and mechanical strain. The semiconductor materials include silicon, gallium arsenide and gallium nitride, co-integrated with metals, metal oxides and polymers, to provide these and other operational capabilities. Ex vivo physiological experiments demonstrate various functions and methodological possibilities for cardiac research and therapy.
Small-scale pumps will be the heartbeat of many future micro/nanoscale platforms. However, the integration of small-scale pumps is presently hampered by limited flow rate with respect to the input power, and their rather complicated fabrication processes. These issues arise as many conventional pumping effects require intricate moving elements. Here, we demonstrate a system that we call the liquid metal enabled pump, for driving a range of liquids without mechanical moving parts, upon the application of modest electric field. This pump incorporates a droplet of liquid metal, which induces liquid flow at high flow rates, yet with exceptionally low power consumption by electrowetting/deelectrowetting at the metal surface. We present theory explaining this pumping mechanism and show that the operation is fundamentally different from other existing pumps. The presented liquid metal enabled pump is both efficient and simple, and thus has the potential to fundamentally advance the field of microfluidics.
Cross-sectional geometry influences the pressure-controlled conductivity of liquid-phase metal channels embedded in an elastomer film. These soft microfluidic films may function as hyperelastic electric wiring or sensors that register the intensity of surface pressure. As pressure is applied to the elastomer, the cross-section of the embedded channel deforms, and the electrical resistance of the channel increases. In an effort to improve sensitivity and reduce sensor nonlinearity and hysteresis, we compare the electrical response of 0.25 mm2 channels with different cross-sectional geometries. We demonstrate that channels with a triangular or concave cross-section exhibit the least nonlinearity and hysteresis over pressures ranging from 0 to 70 kPa. These experimental results are in reasonable agreement with predictions made by theoretical calculations that we derive from elasticity and Ohm's Law.