A general strategy to impart mechanical stretchability to stretchable electronics involves engineering materials into special architectures to accommodate or eliminate the mechanical strain in nonstretchable electronic materials while stretched. We introduce an all solution–processed type of electronics and sensors that are rubbery and intrinsically stretchable as an outcome from all the elastomeric materials in percolated composite formats with P3HT-NFs [poly(3-hexylthiophene-2,5-diyl) nanofibrils] and AuNP-AgNW (Au nanoparticles with conformally coated silver nanowires) in PDMS (polydimethylsiloxane). The fabricated thin-film transistors retain their electrical performances by more than 55% upon 50% stretching and exhibit one of the highest P3HT-based field-effect mobilities of 1.4 cm 2 /V∙s, owing to crystallinity improvement. Rubbery sensors, which include strain, pressure, and temperature sensors, show reliable sensing capabilities and are exploited as smart skins that enable gesture translation for sign language alphabet and haptic sensing for robotics to illustrate one of the applications of the sensors.

Here, we report highly stretchable electronics and sensors that are made of intrinsically stretchable composite semiconductors and conductors. Specifically, we exploit P3HT nanofibril (P3HT-NF) percolated polydimethylsiloxane (PDMS) rubber composite as a stretchable semiconductor, Au nanoparticles with conformally coated silver nanowires (AuNP-AgNW) dispersed within PDMS as a stretchable conductor, and ion gel as a gate dielectric. In particular, instead of creating novel polymers for stretchable semiconductors, which heavily require sophisticated molecular design and synthesis, we use all commercially available materials as precursors to achieve highly stretchable semiconductors that can be manufactured in a repeatable and scalable manner and have stable performances. The P3HT-NF percolated PDMS rubber composite maintains semiconductor characteristics under 50% mechanical stretching along and perpendicular to the channel length directions. Simple solution processes are used to form thin films to construct stretchable devices, including TFTs and sensors, without any additional structural design to achieve large mechanical stretchability. Examples of intrinsically stretchable electronics that have been exploited include stretchable ion gel–gated TFTs and strain, pressure, and temperature sensors. The ion gel–gated TFTs exhibited a field-effect mobility (μ FE ) and an on/off ratio (I ON/OFF ) of 1.4 cm 2 /V∙s and 5.6 × 10 3 , respectively. While the mobility achieved a high value for the stretchable organic format of semiconductors, it only has a moderate decrease of less than 45% under 50% mechanical stretching. We demonstrate the application of these intrinsically stretchable electronics as multifunctional artificial robotic skins that can translate hand motions and gestures to provide haptic sensing capabilities.

The past decade has witnessed significant advancements in stretchable electronics. Owing to its superior mechanical characteristics (that is, soft, bendable, stretchable, and twistable), stretchable electronics hold promise in health monitors ( 1 , 2 ), medical implants ( 3 – 5 ), artificial skins ( 6 – 8 ), and human-machine interfaces ( 9 , 10 ). To date, most electronic materials, especially semiconductors, including inorganics (such as Si and GaAs) and organics [such as poly(3-hexylthiophene-2,5-diyl) (P3HT) and pentacene], are mechanically nonstretchable ( 11 , 12 ). To enable mechanical stretchability in electronic devices, special mechanical structures or architectures have generally been used to accommodate or eliminate mechanical strain in nonstretchable materials while stretched. Examples of these structures include out-of-plane wrinkles ( 13 ), in-plane serpentines ( 14 ), rigid islands with deformable interconnects ( 15 , 16 ), and kirigami architectures ( 17 – 19 ). An alternative route to eliminating the burden of constructing dedicated architectures and the associated sophisticated fabrication processes is to build stretchable electronics from intrinsically stretchable electronic materials, which have potential toward scalable manufacturing, high-density device integration, large strain tolerance, and low cost ( 11 , 20 ). Although conductors that are intrinsically stretchable have been reported extensively ( 21 ), semiconductors that are intrinsically stretchable have been a general challenge. Existing ways of improving stretchability in semiconductors include using conjugated polymer nanowire and nanofibril networks ( 22 – 24 ) and microcracked films ( 25 ). However, their mobilities are generally very low. Recently, stretchable polymer semiconductors with relatively high mobilities and the associated thin-film transistors (TFTs) have been successfully demonstrated from dedicated polymer and nanostructure designs ( 26 , 27 ).

RESULTS

Intrinsically stretchable elastomeric composite conductors and semiconductors Figure 1A shows a schematic illustration of sensors (upper) and TFTs (lower), which were constructed from AuNP-AgNW/PDMS elastomeric conductor, P3HT-NF/PDMS elastomeric semiconductor, and ion gel dielectric. We used AuNP-AgNW/PDMS composites as stretchable conductors to form an ohmic contact with P3HT-NF/PDMS because an energy barrier would exist for AgNW-based electrodes (28). We coated the AgNWs with AuNPs using a galvanic replacement process to form AuNP-AgNW (29). Comparison studies in transistor characteristics are described to further emphasize the necessity of developing these conductors. The detailed electrode preparation process is presented in Materials and Methods. Figure 1B shows the schematic illustrations of the AgNW before (upper left) and after (upper right) AuNP conformal coating. Note that only the exposed AgNWs will be coated with AuNPs. Scanning electron microscopy (SEM) images of AgNWs before (bottom left) and after (bottom right) conformal AuNP coating indicate the success of the galvanic replacement process (fig. S1). We performed stretching tests to examine the electrical characteristic change associated with the applied mechanical strain. Specifically, the AuNP-AgNW/PDMS electrode can be elastically stretched, and its sheet resistance increases from 2 to 5 ohms/□ upon 50% uniaxial stretching, as shown in Fig. 1C (upper). The SEM images in Fig. 1C (lower) revealed that no obvious crack was observed. Fig. 1 Intrinsically stretchable electronic materials. (A) Schematic illustrations of a sensor (upper) and a TFT (lower), consisting of AuNP-AgNW conductors, P3HT-NF/PDMS semiconductor composite, and ion gel dielectric vertically stacked on a PDMS substrate. (B) Schematic illustrations (upper) and SEM images (lower) of the AgNWs before (left) and after (right) the galvanic replacement process to conformally coat AuNPs on exposed AgNWs. (C) Sheet resistance (ohms/□) of the AuNP-AgNW/PDMS conductor under different levels of mechanical strain. The SEM images of the stretched conductor are shown at the bottom. (D) AFM surface topography (left) and phase mode (right) images of the P3HT-NF/PDMS film. (E) AFM phase mode images of P3HT-NF/PDMS coated on a PDMS substrate upon uniaxial stretching. Yellow arrows indicate P3HT-NF rupture. (F) Photographs of the P3HT-NF/PDMS film on a thin PDMS substrate under various forms of mechanical deformation. (G) Photographs of free-standing ion gel dielectric before and after stretching. The stretchable elastomeric composite semiconductor is prepared by two major sequential steps: (i) forming the one-dimensional self-assembled π-conjugated P3HT-NFs in the solvent under a cooling process and (ii) blending the resulting P3HT-NFs with PDMS to yield percolated nanofibrils in a rubber matrix. This process not only forms percolated pathways for the carriers to transport but also results in P3HT-NFs that significantly improve crystallinity and therefore enhance the carrier mobility, as detailed in previous studies (22, 23, 30). Specifically, P3HT was first dissolved in the m-xylene solvent at 60°C (fig. S2A, bright orange) and then cooled down and kept for 10 min at −20°C to form P3HT-NF (fig. S2A, dark purple). Transmission electron microscopy (TEM) and atomic force microscopy (AFM) images of the prepared P3HT-NF indicate that the diameter and length of P3HT-NF are 40 nm and 1.5 μm, respectively (fig. S2). The P3HT-NF solution was blended with a solvent (m-xylene)–diluted PDMS [10:1 (w/w) prepolymer/curing agent] to allow thorough mixing. A thin-film P3HT-NF/PDMS stretchable semiconductor is achieved by spin-coating, followed by solidifying at 60°C. We obtained the optimized weight composition of P3HT through systematic studies of the percolation behaviors, morphologies, and their electrical characteristics (fig. S3). The P3HT/PDMS weight ratio of 2:8 was chosen owing to the absence of microcracks and a moderate decrease in mobilities, as shown in fig. S4. We carried out AFM scanning to examine the microstructures of the nanofibrils and the detailed surface morphology of the P3HT-NF/PDMS thin film spin-coated on a PDMS substrate. The left AFM image in Fig. 1D with a scanning area of 1 μm × 1 μm shows the surface topography of the film. The right AFM image in Fig. 1D under phase mode illustrates the stiffness contrast between P3HT-NFs (~0.3 GPa) (31) and PDMS (~1 MPa) (32). The schematic illustration in fig. S2E shows how the P3HT-NFs behave upon stretching at different extent. The P3HT-NFs first straighten and then eventually start to rupture, which can be seen from the AFM images in Fig. 1E. As the yellow arrows indicate, some P3HT-NFs rupture when stretched up to or beyond 50%. Nevertheless, majorities of the P3HT-NFs remain connected at 50%. Owing to the composite merits of elastomeric mechanical properties from the elastomer PDMS and the electrical conductance from the percolated P3HT-NFs, the P3HT-NF/PDMS can be stretched or twisted and return to its original state without any cracks, as shown in Fig. 1F. The stretchable ion gel dielectric was prepared by the solution-casting method. The detailed preparation can be found in Materials and Methods. The composite shows relatively stable electrical conductivity even after exposure in the environment (25°C, 75% relative humidity) for 600 hours (fig. S5). No obvious crack of the membrane was observed when it was stretched by 50%, as shown in Fig. 1G.

Intrinsically stretchable rubbery transistors We constructed stretchable ion gel–gated TFTs by using these intrinsically stretchable electronic materials based on solution processes, including spin casting and stencil printing. Figure 2A shows the schematic structure of an ion gel–gated TFT. Specifically, the source and drain AuNP-AgNW/PDMS electrodes and P3HT-NF/PDMS composite semiconductor layer were patterned with a thin Kapton film–based stencil masks, which were generated by a cutting machine (Silhouette Cameo). A 140-μm-thick ion gel film was aligned and laminated on the source and drain electrodes to serve as ion-gated dielectric. The detailed fabrication process is described in Materials and Methods and fig. S6. Figure 2 (B and C) shows a photograph of fabricated TFT and an SEM image showing a channel length of 50 μm, respectively. The width of the channel was 5 mm. The thicknesses of the P3HT-NF/PDMS composite semiconductor and TFT were measured to be ~280 nm and ~430 μm, respectively (fig. S7). Fig. 2 Intrinsically stretchable rubbery transistors. (A) Exploded schematic illustration of the rubbery TFT. (B) Photograph of the TFT. (C) SEM image of the source, drain electrodes, and channel of the TFT. (D and E) Transfer and output curves of the TFT without any mechanical strain. (F and G) Transfer curves of the TFTs under different levels of mechanical strain along and perpendicular to the channel length directions. (H and I) μ FE and V th of the TFTs under different levels of mechanical strain along and perpendicular to the channel length directions. To verify the necessity and feasibility of AuNP-AgNW/PDMS electrodes for the intrinsically stretchable rubbery TFT, we tested several different electrodes and carefully examined their corresponding characteristics. Specifically, we constructed TFTs and tested electron beam–evaporated Au and Ag electrodes and AgNW/PDMS– and AuNP-AgNW/PDMS–based electrodes. Only Au electrodes and AuNP-AgNW/PDMS–based TFTs showed transistor characteristics. The comparison results are presented in fig. S8. The results suggest that only Au or AuNP-AgNW can form an ohmic contact with the P3HT-NF/PDMS, and the Ag film or AgNW cannot (28). Figure 2D shows the transfer I ds ~ V G and corresponding | I ds | 1 2 ∼ V G curves of the stretchable ion-gated TFT without any mechanical stretching. The drain current was obtained while the gate voltage (V G ) was swept from 0 to −4 V, with the drain voltage kept at −0.5 V. The I ON/OFF was calculated to be ~5.6 × 103, which is comparable to other P3HT-NF–based stretchable TFTs (23). Figure 2E shows the source-drain I DS -V DS characteristics of the TFT with a gate voltage decrease from −1.5 to −4 V at a voltage step of −0.5 V. The drain voltage was decreased from 0 to −0.5 V. The drain current was as high as ~0.35 mA at V G = −4 V and V DS = −0.5 V. To verify that the TFTs from P3HT-NF/PDMS composite semiconductor can offer enhanced device performance, we construct TFTs with a semiconductor from pristine P3HT and PDMS (p-P3HT/PDMS) composite on Au electrodes as a comparison. The TFTs from p-P3HT/PDMS and P3HT-NF/PDMS composite showed μ FE of 4.5 × 10−3 and 1.3 cm2/V∙s and I ON/OFF of 1.68 × 102 and 3.82 × 102, respectively. Details are illustrated in fig. S9. Compared to p-P3HT, P3HT-NFs obtained through π conjugation of the thiophene rings yielded more efficient carrier transport and better device performance. These results are also consistent with the reported P3HT-NF–based devices (33). To examine the transistor performances under mechanical strain, the characteristics of the fabricated TFT were measured while different levels of mechanical strain were applied using a custom-made stretcher. The strain was applied both along and perpendicular to the channel length directions. Because the TFT was constructed fully with elastomeric materials whose moduli are comparable and thus with no significant strain isolation effect (34, 35), the stretching strain applied is assumed to be evenly distributed across the whole device. Figure 2 (F and G) shows the transfer curves of the TFTs under 10, 30, and 50% mechanical strain along and perpendicular to the channel length directions, respectively. More detailed results are shown in fig. S10. On the basis of the linear regime of these transfer curves, the μ and threshold voltage (V th ) of the TFTs under different levels of mechanical strain were calculated (see the Supplementary Materials for details). On one hand, once the TFT was stretched by up to 50% along the channel length direction, a moderate decrease in μ from 1.4 to 0.8 cm2/V∙s and a slight increase in V th from −2.56 to −2.45 V were obtained (Fig. 2H). On the other hand, once it was stretched by up to 50% perpendicular to the channel length direction, a relatively larger decrease in μ from 1.4 to 0.4 cm2/V∙s and a slight decrease in V th from −2.56 to −2.61 V were obtained (Fig. 2I). Note that owing to the gating effect, the TFT experiences less decrease in the channel current upon mechanical stretching compared with the devices without a gate, such as strain sensors, as described below. A similar phenomenon has been reported elsewhere (36). All these results suggest that the intrinsically stretchable rubbery transistors can maintain normal operation and relatively stable device performances while undergoing large mechanical stretching.

Strain, pressure, and temperature sensors Various sensors, such as strain, pressure, and temperature sensors, were exploited by using intrinsically stretchable electronic materials. Figure 3A shows a schematic illustration of a strain sensor with a two-terminal configuration in which a P3HT-NF/PDMS stretchable semiconductor is connected at both ends with AuNP-AgNW/PDMS electrodes. The length and width of the channel were 50 μm and 5 mm, respectively. The thickness of the P3HT-NF/PDMS stretchable semiconductor was ~100 nm. The device was fabricated in a way similar to that of the transistor. Figure 3B presents a series of images of a device stretched from 0 to 50%. Upon stretching, the electrical resistance increases. Figure 3C shows the plot of the measured electrical resistance versus the mechanical strain from 0 to 50% for both directions. When the applied strain was along the channel length direction, the resistance increased from 0.3 to 4.6 gigohms, and an approximately linear increase of resistance was obtained upon mechanical stretching. Note that the increase of resistance was mainly attributed to the semiconductor rather than to the conductor. The conductor contributes negligibly, as exhibited in Fig. 1C, compared with the overall resistance increase. Fig. 3 Rubbery strain, pressure, and temperature sensors. (A) Exploded schematic illustration of the strain sensor. (B) Photographs of the sensors under different levels of mechanical strain. (C) Measured electrical resistance of the strain sensor under different levels of mechanical strain along the channel length direction (black) and perpendicular to the channel length direction (blue). (D) Relative change of the resistance (ΔR/R o ) under cyclic stretching and releasing. (E) GF of the strain sensor with respect to the different strain. (F) Relative electrical resistance (R/R o ) change of the pressure sensor with respect to time under different levels of pressure. (G) Relative electrical resistance change of the pressure sensor under a loading (red) and unloading (blue) cycle. (H) Relative electrical resistance change of the temperature sensor with respect to the different temperature. When the applied strain was perpendicular to the channel length direction, a much smaller increase in resistance from 0.3 to 0.64 gigohms was observed. Cyclic mechanical stretching and releasing of the strain sensor under a cyclic mechanical tester (CK-700FET, CKSI Co. Ltd.) were performed to evaluate the stability of the sensor. The stretching increased from 0 to 50%, with a step increase of 10% along the channel length direction. Figure 3D shows the normalized electrical resistance during cyclic stretching and releasing at a rate of 1 Hz. These results agree with the measured resistance change, as shown in Fig. 3C. The inset in Fig. 3D shows a magnified resistance change of the sensor at 30% cyclic stretching, and stable and repetitive response was achieved. The calculated gauge factor (GF), defined as the ratio between the relative resistance change (ΔR/R o ) and the extent of mechanical stretching, is shown in Fig. 3E. A GF of 33 is achieved for the 50% stretching. Note that this GF value is the highest, to our best knowledge, compared with GF values of other organic material– and inorganic material–based strain sensors that can be directly stretched to a similar extent, such as 50% (37, 38). A similar two-terminal device configuration was adopted for pressure sensors because the applied pressure induces semiconductor deforming, which results in resistance change. Specifically, relative electrical resistance (R/R o ) was consecutively measured by applying different Hertzian contact pressures (see the Supplementary Materials for details). As shown in Fig. 3F, the applied pressure ranges from 0.66 to 1.2 MPa, and the resistance changes accordingly. With an increase of the applied Hertzian pressure from 0.66 to 1.2 MPa, the R/R o increases from ~0.98 to ~3.3. The change of R/R o corresponding to different applied pressures is plotted in Fig. 3G. The near hysteresis-less characteristic for the loading/unloading cycle suggests that this device is a viable pressure sensor. Temperature sensors were also exploited with the similar two-terminal device structure. The resistance changes of R/R o with respect to different temperatures were carefully examined. As shown in Fig. 3H, the R/R o decreased from 1.0 to 0.67 with an increase of temperature from 30° to 50°C. The experienced resistivity decrease is a major result of increased carrier concentration, which agrees with the reported work on P3HT from others (39–41). The measured temperature coefficients of resistance at 303 K (30°C), 313 K (40°C), and 323 K (50°C) are −0.028/K, −0.023/K, and −0.016/K, respectively (see the Supplementary Materials for details).