Omnidirectional Printing of Soft Elastomer for Liquid-State Stretchable

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Organic Electronic Devices

Omnidirectional printing of soft elastomer for liquid-state stretchable electronics Jiachen Wang, Sennan Yang, Peitao Ding, Xiangyu Cao, Yue Zhang, Shitai Cao, Kuikui Zhang, Shixiao Kong, Yunlei Zhou, Xiaoliang Wang, Dongchan Li, and Desheng Kong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04730 • Publication Date (Web): 03 May 2019 Downloaded from http://pubs.acs.org on May 3, 2019

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ACS Applied Materials & Interfaces

Omnidirectional Printing of Soft Elastomer for Liquid-State Stretchable Electronics Jiachen Wang1, Sennan Yang2, Peitao Ding3, Xiangyu Cao1, Yue Zhang3, Shitai Cao1, Kuikui Zhang1, Shixiao Kong1, Yunlei Zhou1, Xiaoliang Wang3,*, Dongchan Li2,*, and Desheng Kong1,*

1College

of Engineering and Applied Sciences, National Laboratory of Solid State

Microstructure, Collaborative Innovation Center of Advanced Microstructures, and Jiangsu Key Laboratory of Artificial Functional Materials, Nanjing University, Nanjing 210093, China.

2College

of Chemical Engineering and Technology, Engineering Research Center of

Seawater Utilization Technology of Ministry of Education, State Key Laboratory of Reliability and Intelligence of Electrical Equipment, Hebei University of Technology, Tianjin 300130, China.

ACS Paragon Plus Environment

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3Key

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Laboratory of High Performance Polymer Materials and Technology of Ministry of

Education, Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China

KEYWORDS. stretchable electronics, three-dimensional printing, liquid-state device, skin-like electronics, liquid metal.

ABSTRACT. Stretchable electronics have emerged as a new class of electronic technology to expand the application of conventional electronics built on rigid wafers. Among various systems, liquid-state devices utilize electronically active liquids to achieve excellent stretchability and durability. The widespread adaption to such attractive form of devices is hindered by the lack of robust fabrication approach to precisely and efficiently assemble liquid-state materials into functional systems. In this study, an additive manufacturing platform for digital fabrication of three-dimensional


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elastomeric structures is reported. The shear-thinning ink is formulated to enable omnidirectional printing process. Various elastic features with complex architectures are generated without using sacrificial materials, which consist of overhanging parts, suspended structures, and embedded channels. Harnessing the unique printability allows facile creation of elastomeric sensors with strain and pressure sensing capabilities by simply filling the embedded microchannels with liquid metal. A smart glove to capture hand gestures is also demonstrated as a fully integrated electronic system with liquid-state components. Liquid-state stretchable electronics developed here may find potential applications in biomedical instruments, wearable devices, and soft robotics.

INTRODUCTION Current electronics are primarily driven by advanced microfabrication techniques to achieve faster and more efficient information processing. In spite of high performance, the applications of these wafer-based systems are often limited when flexibility is desired. With the rapid expansion of electronic technology, consumer devices in the


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form of smart wearables require deformable form factor for intimate integration with human body. The practical demand has fueled the development of stretchable electronics, which represents a paradigm shift towards a new class of electronic technology based on compliant materials.1-6 These electronic devices exhibit unique mechanical properties and complex sensing functionality to mimic the properties of human skin, which are able to be bent, twisted, stretched and interfacing with moving objects.7-10 They are attractive towards emerging application areas, such as soft robotics,11-13 advanced prosthetics,14-16 and health monitoring,17-19 Three major forms of stretchable electronic devices have been explored so far. As a straightforward approach, conventional devices are interconnected with stretchable conductors on elastomer substrates to create compliant electronic systems.19-24Another avenue is to construct devices using solid-state materials with intrinsic stretchability.25-28 Liquid-state electronics represent another attractive platform because liquid naturally exhibits ultimate deformability.29 A variety of sensors consisting of active liquid components embedded within elastomeric substrates have been demonstrated by using liquid metals,11, 30-33 ionic liquids,12, 29, 34, 35 and liquid-state composites.36


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In terms of fabrication scheme, stretchable liquid-state devices are typically formed by filling soft template with functional liquids. Individual components are often separately prepared by soft lithography and then assembled into a complete unit with embedded microstructures.11, 29, 32 The designs are usually tailored to minimize potential risks of delamination. The overall fabrication process is often labor intensive and time consuming. 3D printing technology represents an alternative to traditional technologies by enabling the flexible creation of complex three-dimensional (3D) structures according to personal design.37, 38 The additive manufacturing approach enables facile preparation of functional objects without the need of extra molds, complicated post processing, or excessive material wastes.39 Among various approaches, direct ink writing (DIW) is a versatile 3D printing technique based on extruding shear-thinning ink into continuous filament under ambient conditions, which is compatible with a broad range of materials.40-42 In particular, elastomeric structures have been printed by using capillary suspension of polydimethysiloxane beads,43, heat or light curable liquid silicones,44-47 and concentrated solution of thermoplastic polyurethane.48 For DIW, ink deposition is typically confined to two-dimensional (2D) planes, thereby forming 3D structures by


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stacking materials in a layerwise sequence. In order to create embedded microstructures for liquid-state sensors, sacrificial inks should be employed that increases the complexity of printing process and post-processing.49-52 A facile technique suitable for printing complex elastomeric architecture is therefore highly desired.

In this study, we report an additive manufacturing platform for omnidirectional printing of silicone based elastomer to create soft and highly elastic 3D structures. The ink formulation exhibits unique rheological properties to create features of high fidelity and large aspect ratio. The printing process largely simplifies the challenges associated with the manufacturing of complex 3D architectures involving overhanging parts, suspended structure, and embedded channels. The ability to directly print soft microfluidic chips allows facile preparations of various stretchable electronic devices by filling the embedded microchannels with liquid metal. A smart glove capable of capturing hand gestures is further demonstrated as a fully integrated electronic system with liquid-state sensors. Liquid-state stretchable electronics based on digitally fabricated elastomers


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may find broad range applications in smart wearables, biomimetic devices, and soft robotic systems.

EXPERIMENTAL SECTION Materials. Silicone (Ecoflex 00-30), thickening agent (Thivex), and concentrated color pigments (Silc Pig) were purchased from Smooth-On, Inc. Fumed silica (AEROSIL 380) was acquired from Evonik Degussa GmbH and thoroughly dried in a vacuum oven before use. A cure retarder was prepared by mixing 1-ethynyl-1-cyclohexanol with tertiary butanol in a 1:38.75 weight ratio. Ecoflex 00-30 Part A, Ecoflex 00-30 Part B, Thivex, and cure retarder in a weight ratio of 100/100/4/0.05 were homogenized in a planetary mixer (JF-RVITV-150, Shenzhen Junfeng Technology Co., Ltd.) at 2000 rpm for 3 min. Subsequently, fume silica was added to the mixture at 5.4 wt% as a rheological modifier in the planetary mixer, followed by homogenization in a three-roll mill (S65, Shanghai Chile Industry Technology Co., Ltd.). The procedures yielded a viscous mixture as the ink for 3D printing. Galinstan, a liquid metal alloy, was prepared by melting mixed metal pieces of Ga (99.99%), In (99.99%), and Sn (99.999%) with a


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weight ratio of 68.5:21.5:10 at 180 °C for 2h in a glovebox. For coloring printed structures, Silc Pig (0.5 wt%) was homogenized with Ecoflex 00-30 Part A in a planetary mixer at 2000 rpm for 3 min prior to the ink preparations.

Rheology Measurements: The rheological properties of printable inks were characterized using a stress-controlled rheometer (Hakke RheoStress 600) equipped with a 35 mm parallel-plate testing fixture. The measurements were performed under ambient conditions with a fixed gap of 500 μm. Viscosity was measured with shear rate ranging from 0.01 to 200 s-1, whereas oscillation test were carried out at a frequency of 1 Hz with stress from 0.3 to 5000 Pa.

Three-Dimensional Printing: All structures were produced by a 3D printer (PTC RD441, Shanghai Pioneer-elec Tech. Co.) equipped with a pneumatic syringe dispenser. As prepared ink was loaded into the syringe barrel (10 cc or 30 cc), followed by deaeration in a planetary mixer to thoroughly remove the trapped bubbles. Musashi precision needle (HN-0.2ND) with an inner diameter of 200 μm was employed for high quality printing. All structures were printed on octadecyltrichlorosilane modified glass to


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facilitate the release. Printing conditions were optimized with a translation speed of 7.5 to 8.5 mm/s and an applied pressure of 530 to 570 kPa. All printing paths were generated by converting 3D models into compatible G-code using an open source slicing software (Slic3r 1.3.0-dev, Prusa). The printed structures were cured either at room temperature over extended period or at 120 ℃ in an oven for an hour.

Material Characterizations: Images and 3D reconstructions were acquired by Keyence VHX-6000 Digital Microscope. The samples were cut off by a knife for cross sectional imaging. Digital images and videos were acquired by a Fujifilm X-T10 camera. A Shimadzu AGS-X universal testing machine equipped with a 50 N load cell was used to measure the mechanical properties. Tensile specimens in rectangular shape (120 mm×6 mm×1.5 mm) were printed with the printing path either in parallel or vertical to the stretching direction, respectively. Control samples were obtained by cutting 1.5 mmthick cured sheet of Ecoflex 0030 into rectangular bars. All measurements were performed with an extension rate of 20 mm/min. Tensile modulus and elongation at break were obtained as the averaged values from three samples. The resistance of


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embedded liquid metal was measured using a Keithley 2110 digital multimeter with fourpoint probe configuration to eliminate contact resistance. The resistance change upon stretching was evaluated on home-made motorized linear translation stage.

Sensor fabrications: Elastomeric structures with embedded microchannels were created by 3D printing. Galinstan was then manually injected into microchannels using syringes. Tinned copper wires are inserted into the inlets and outlets of microchannels for interfacing with measurement equipment. The microchannels are subsequently sealed with silicone sealant (Dow Corning 734) to complete the fabrication.

Integrated strain sensing glove fabrication: A four-layer hand-shaped elastomeric structure with five embedded strain sensor was created by 3D printing. A concave slot is carefully designed to guide the placement of flexible connector. At the same time, A 100 μm-thick water soluble film was formed by casted aqueous solution of poly(vinyl alcohol) (PVA, MW=67000, Shanghai Aladdin Bio-Chem Technology Co., Ltd.) into a stainless steel dish, followed by natural evaporation under ambient conditions. The PVA film was cut into hand shape by using a CO2 laser engraving machine (4030-50W, Jingwei Laser


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Equipment Co., Ltd.). Silicone film of 1mm in thickness was prepared by casting mixed liquid precursors (Ecoflex 0030) onto OTS-modified glass (20 cm × 20 cm) using doctor blade process, followed by thermal curing at 120 ℃ for an hour. After transferring the hand-shaped PVA film onto the surface of the silicone, another layer of liquid precursors was casted by doctor blade process and thermally cured. The laminate was cut along the border of PVA film with the only bottom side exposed. Dissolving the sacrificial PVA film in hot water (~80 ℃) allows the separation of internal silicone films to form the elastic glove. The silicone glove was then bonded with hand-shaped sensor system with a thin layer of silicone sealant (Dow Corning 734) to complete the fabrication of the total glove. After that, the flexible connector consisting of an array of metal lines was formed by photolithography and wet-etching on copper-clad polyimide. The ends of metal lines were coated with a thin layer of Galinstan to facilitate the connection with liquid metal based sensors. The connector was dipping into liquid metal in the presence of hydrogen chloride solution (HCl, 3 wt％), in order to harness the selective wetting behavior of nonoxidized liquid metal onto copper. Extra Galinstan was removed by spinning the connector on a spin coater at 3000 rpm for 40 s. A thin layer of primer (1200 OS, Dow


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Corning) was brush coated onto the back side of the flexible connector as the adhesion promoter. The connector was then placed at the designed slot and effectively bonded to silicone after curing. Galinstan was injected into the microchannels of strain sensors, in which naturally overflowed liquid metal at the inlets and outlets of microchannels forms the connections with the flexible connector. The contact region was then encapsulated with silicone sealant.

Device Measurements: The resistance of strain sensors was measured by a Keithley 2110 digital multimeter. The response to tensile strain was carried out on a motorized linear translation stage at a fixed stretching speed of 4.5 mm/s. In order to acquire the temperature dependency, the strain sensor was stretched by a manual linear translation stage inside an ultra-high precision temperature bath (XOGDH-2010, ±0.01 ℃, Nanjing ATPIO Technology Co., Ltd.) to modulate the operation temperature. The pressure sensor was measured by applying 40 mV across the input nodes of the bridge using a Keithley 2634B sourcemeter. The voltage across the output nodes was amplified by 260× with an AD8422 preamplifier (Analog Devices, Inc.) and recorded by the


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sourcemeter. The load was applied at a speed of 0.02 N/s by a universal testing machine (Shimadzu AGS-X) with a customized cylindrical probe (22 mm diameter). Two dummy frogs in different sizes (1.78 g and 2.97 g, respectively) were produced by a UV LCD 3D printer (X-Cube, Foshan Stek Technology Co., Ltd.) for evaluating the response of the pressure sensors. A precision hot plate (KJ-18, ±0.5 ℃, Shenyang Sile Machine Manufacturing Factory) was mounted on the universal testing machine to acquire the temperature dependence of the pressure sensor. For integrated strain sensing glove, an additional flexible flat cable (FFC) is introduced to interface with external recording equipment. FFC was bonded with the flexible connector using anisotropic conductive film (ACF, AC-9865AY-35, Hitachi) by applying heat (200 ℃) and pressure (0.4 MPa) for 20 s on a pulse heat machine (OL D16, ShenZhen Olian Automatic Equipment Co.) A drive current of 40 mA was applied to the five strain sensors connected in series using a Keithley 2634B sourcemeter. The voltage outputs of all strain sensors were simultaneously recorded with a commercial data acquisition card (USB3202, Beijing Art Technology Development Co., Ltd.) to reflect the change of hand gestures.


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RESULTS AND DISCUSSIONS The printing platform is schematically illustrated in Figure 1a. A syringe filled with printable ink is mounted on a three-axis, motorized linear stage. The ink is pneumatically extruded from a micronozzle in filamentary form with the dimension comparable to the inner diameter of the needle, consequently forming 3D features with coupled motions of the three-axis stage. In terms of the ink formulation, the base elastomer is a commercial silicone (Ecoflex 0030) widely used in stretchable electronics and soft robotics. A cure retarder based on 1-ethynyl-1-cyclohexanol is introduced to slow down the chemical crosslinking process,53 which effectively extends the printing window to ~2 days under the ambient conditions. The additive is essential to achieve consistent printing outcome by decoupling the printing process with subsequent curing step. Three inks with different viscosity are termed as liquid, intermediate, and viscous inks, respectively (see details in Experimental Sections). The rheological behaviors of the inks are measured by using a Hakke RheoStress 600 rheometer, as shown in figure 1b and 1c. Liquid ink exhibits low viscosity (η) in the range from 3.0 Pa·s to 11.8 Pa·s, which is weakly dependent on the shear rate. The storage modulus (G’) is lower than its


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loss modulus (G”) throughout the entire range of shear stress, revealing its liquid-like property. The ink is highly flowable without the ability to hold its structure. For intermediate ink, the addition of thickening agent effectively increases the viscosity under quasi-static condition, which exhibits η of ~3.9 × 103 Pa·s at 0.01 s-1. The viscosity drops by three orders of magnitude with the shear rate changing from 0.01 s-1 to 200 s-1 due to its strong shear thinning behavior, showing low viscosity comparable to liquid ink. The ink also exhibits a plateau value of G’∼5×103 Pa that is over twice the value of G”, suggesting a solid-like behavior in stationary conditions. The moduli curves have a crossover point at ~50 Pa defined as the shear yield stress (τy), above which the ink transforms into a liquid-like fluid. The yield flow behavior is in fact the basic requirement for DIW. Viscous ink is formulated by further incorporating fumed silica into intermediate ink,54, 55 which increases the viscosity by more than an order of magnitude under quasi static conditions. The rheological modifiers also largely increase quasi static G’ to ∼6×104 Pa and τy to ~3×103 Pa. The ink behaves as a shear thinning fluid with η of 105 Pa·s at 0.01 s-1 and of 5.5 Pa·s at 200 s-1, respectively. The high stiffness,


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yield flow behavior, and strong shear thinning response render the ink as an ideal candidate for DIW.

The performances of the inks are evaluated by printing interconnected line segments. Fully focused optical images and corresponding height profiles of as printed features are presented in Figure 1d. Liquid ink allows smooth extrusion by the micronozzle with low pressure of ~120 kPa as a result of its low viscosity. Printed filaments show significant lateral spreading into completely distorted morphology. The liquid-like nature renders the ink incompatible with 3D printing. Intermediate ink allows stable extrusion at ~230 kPa, thanks to its shear-thinning property. The height profile of the printed feature reveals partially collapsed filaments to form interconnected region, which exhibits certain resemblance to its original model. The solid-like behavior of the ink allows improved structural retention under static conditions. The intermediate ink is therefore suitable for DIW of 3D design without delicate microstructures. Viscous ink is a shearthinning fluid to enable stable printing under ~550 kPa without clogging. The ink transforms into solid-state structures after deposition in the absence of additional shear,


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thereby generating line segments with high fidelity as revealed by the height profile. The ink is therefore an attractive candidate material for DIW.

In terms the ability to create complex 3D architecture, DIW techniques typically utilize additional support structures or fugitive inks to suppress material drifts and structural collapse.36, 56-58 The downsides of these approaches include complicated model designs, additional material wastes, and extra post-processing procedures. In contrast, it is feasible to directly extrude in air across unsupported regions with viscous ink. Figure 2a presents a schematic illustration to create an array of non-coplanar filaments that bridge two surfaces with large height differences. As printed feature consists of filamentary segments inclined at 60° with respect horizontal plane, which effectively demonstrate the ability to print vertically with steep angles (see Figure 2b). Corresponding video S1 in Supporting Information reveals the dynamic process to extrude along 3D paths. In addition, it is also possible to generate spanning structures across large horizontal gaps as schematically shown in Figure 2c. A membrane is successfully produced to bridge elastomeric pillars, thereby confirming the robust nature


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of printing process irrespective of the lack of underlying supports up to 20 mm wide (see Figure 2d). Notice that the filaments slightly bend under gravity as the unsupported region extends. These results demonstrate the essential characteristics of omnidirectional printing which allows flexible design of printing paths beyond conventional techniques.49, 59, 60 The unique rheological behavior of viscous ink is the enabler of freeform fabrication along 3d printing paths. The strong shear thinning response allows the ink to flow through the nozzle under pneumatic extrusion. Subsequently, the ink immediately solidifies and thereby forms self-supporting structures. The solid state filaments serve as the building blocks for these unconventional structures without direct support.

Omnidirectional printing also largely extends the compatible 3D models by using conventionally layer-by-layer build sequence. Various 3D models are converted into layerwise printing paths by using open source slicing software. Figure 3a presents the printing process to generate a T-shaped structure, involving suspended regions of the horizontal bar at both sides. Extruded filaments behave like solid material attached to


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already printed parts, thereby enabling the formation of overhanging parts without collapse. A miniaturized bridge is also created in a layerwise sequence as shown in Figure 3b. The filaments are able to span across the gaps between the two piers, which allows the formation of the deck to complete the structure. In addition, a soft microfluidic chip is produced by sequentially printing patterned grooves and a flat cover layer (see Figure 3c). The microchannels of cured microfluidic chip are filled with blue aqueous solution to facilitate visualization. As compared with conventional fabrication technique via soft lithography and manual assembly, the fully automatic printing process represent an efficient and precise manufacturing route.61 These results demonstrate the unique capability of the printing platform to create complicated 3D structures with layerwise build sequence. It provides the basis to employ regular slicing software for simplified design of printing paths.

As printed structures are solid-state in nature due to their rheological behaviors, which enables the fabrication of delicate structures without instantaneous curing. Figure 3d shows a miniaturized vase produced by 3D printing. As-printed structure is extremely


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stable in time which allows natural curing under ambient conditions. Negligible changes in the texture and morphology are identifiable after three weeks. Accordingly, the printing process is effectively decoupled from subsequent curing step, which ensures robust manufacturing capability with high consistency. Notice that the cure time can also be largely shortened by chemical crosslinking under elevated temperatures (see Experimental section). Additional digital models and corresponding printed structures are presented in Figure S1 in Supplementary Information. The strong resemblance confirms the ability of the manufacturing platform to produce elastomeric features of high fidelity.

The elasticity of printed structures determines their potentials towards cutting edge applications. A Shimadzu AGS-X universal testing machine was employed to measure their mechanical responses with respect to tensile strains. We have prepared tensile specimens with printing paths either along or perpendicular to the stretching direction. As shown in Figure 4a, longitudinal specimen exhibits Young's modulus (E) of 190 ± 11 kPa and fracture strain (εf) of 646 ± 3 %, whereas the transverse specimen has Young's


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modulus of 181 ± 10 kPa and fracture strain of 621 ± 2%. As printed elastomers exhibit isotropic mechanical properties almost independent of build directions. It provides the basis for flexible designs of printing paths optimized for feature geometry, which for creating delicate features with smooth edges (see Figure S2-S4). As compared with the base silicone Ecoflex 0030, the elastomer exhibits comparable or better properties in terms of Young's modulus, rupture strain, and rupture strength (see Figure 4a and Table S1). The results suggest the ink formulation and 3D print process do not substantially increase defects to compromise the mechanical properties, which allows the printed structure to fully benefit from the compliant nature of silicone based elastomer. In Figure 4b, a miniaturized cup is bent under the weight of 10 mm stainless steel ball and fully recovered after release, reflecting the excellent elasticity of as-prepared elastomeric structures. In addition, a balloon is pneumatically inflated to 5.9 times of its original volume without rupture (see Figure 4c), which essentially confirms the strong interfacial bonding between printed filaments as a result of chemical crosslinking process. These examples demonstrate the printable elastomer as a compliant material with desired mechanical properties for practical applications.


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We further employ omnidirectional printing of soft elastomer to fabricate stretchable electronic devices. A soft microfluidic chip with embedded microchannel of four point probe pattern is created by 3D printing (see Figure 5a). The microchannel is subsequently filled with Galinstan and sealed with silicone to form a functional liquidstate device (see Experimental Section).11 Galinstan is a non-toxic liquid metal alloy with high electrical conductivity (3.46×104 S cm-1), low melting point (-19 ℃), and negligible vapor pressure, which is ideal for device applications.11, 31-33, 62 As prepared liquid-state device is visually identifiable by shiny silver color. Figure 5b is a cross sectional optical microscopy image of the microchannel revealing its rectangular shape. The microchannel further defines the structure of the liquid metal device. The lateral printing resolution is ~ 100 μm as controlled by the positioning precision of the 3D printer (Figure S5). The resistance of liquid-state device with 200 μm by 200 μm cross section scales –linearly with the channel length as shown in Figure 5c. The resistance values are consistent with the theoretical predictions based on known conductivity of Galinstan. The results in fact confirm the uniformity of the microchannels, which allows facile prediction of device property according to its geometric shape. The stretchabiltiy


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of the liquid-state device is evaluated by measuring the resistance in response to uniaxial strain (see Figure 5d). The resistance rises with tensile strain as a result of the deformation of microchannel. Liquid is able to flow and reconfigurate within elastomeric encapsulation, thereby giving rise to its exceptional mechanical properties.63 These results demonstrate the unique features of liquid-state devices by harnessing the excellent conductivity and deformability of metallic alloy in liquid phase.

Functional skin-like sensors are created using proper channel design. Strain sensor consists of an array of interconnected parallel microchannels (Figure 6a). Figure 6b shows the resistance change as a function of tensile strain. Representative images of stain sensors at different stretched states are displayed as the inset. The varying resistance is associated with increased channel length and reduced cross sectional area. The resistance increases almost linearly with the tensile strain up to ~50%, which exhibits an approximate gauge factor of 2.1 within the typical range of strain sensors based on active liquid components.36, 64-67 The strain sensor exhibits excellent dynamic response and minor drifts with operation temperature and strain rate (see Figure S6,


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Figure S7 and Figure S8). Notice that the sensors are able to operate in applications requiring much higher extensibility in spite of the loss of linearity at large strains (see Figure S9). Alternatively, a pressure sensor is prepared consisting of four sensing components forming an equivalent Wheatstone bridge (see Figure 7a). The change in output voltage under external pressure is due to compressed inner sensing channels and expanded of outer sensing channels (see Figure S10).32 Thanks to the additive manufacturing platform, cavities can be conveniently introduced and integrated with the microchannels that concentrate external pressure to inner sensing components (Figure 7b). A universal testing machine with a cylindrical probe is employed to apply uniform pressure onto the sensor (see Figure 7c). A schematic illustration of the mechanical deformation of the pressure sensor under external pressure is displayed in Figure 7d. The change of output voltage as a function of pressure is shown in Figure 7e. The sensor exhibits an averaged sensitivity of 0.11 kPa-1 and certain hysteresis. The hysteresis is associated with the viscoelastic behavior of silicone based elastomer.68 The cavity effectively improves the linearity and sensitivity (0.29 kPa-1) of the sensor. In addition, the hysteresis is also largely suppressed which generates reproducible


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responses irrespective of the load history. We have also verified the operation temperature exhibits weak influence on the sensing capability (see Figure S11). The real-time operation of the pressure sensor is shown in Fig. 7f. The sensor is able to detect small weight of a dummy frog that corresponds to a pressure of 46 Pa. The pressure sensor exhibits excellent dynamic properties with a fast response/recovery time of ~0.2 s (Figure S12).

Encouraged by the above results, we utilize the digital fabrication to prepare a smart glove with integrated sensory system. The architecture of the smart glove is schematically illustrated in Figure 8a. The skin-like electronic device is designed as an integrated 3D model consisting of embedded microchannels in the form of sensors and interconnects. Individual strain sensor with tailored dimension is assigned to each finger that reflects the bending induced stretch (see Figure S13). Wide microchannels serve as the interconnects to join the sensors as a functional circuit. A concaved slot in the elastomer is designed for the placement of the flexible connector to interface with external recording equipment. Sequential images of the printing process flow are


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provided in Figure S14. As printed elastomeric structure is attached to a silicone glove and filled with liquid metal to complete the fabrication. Figure 8b shows the smart glove that fits on a user’s hand. The glove is capable of capturing various hand gestures by recording potentials corresponding to finger movements. The real-time response of the smart glove as the user continuously making different hand gestures is shown in Figure 8c. The demonstration show omnidirectional printing of soft elastomer as a versatile approach to design and produce various skin-like electronic systems.

CONCLUSIONS In this study, we have developed an omnidirectional printing technique to create complicated 3D structures based on soft elastomers. Elastomeric ink is formulated to exhibit strong shear thinning behavior as the enabler for the printing process. The rapid solidification of printed ink allows the formation of self-supporting features without instantaneous curing. The unique printability is demonstrated by 3D extruded forms and suspended structures. Various stretchable liquid-state devices are formed by filling the embedded microchannels of printed microfluidic chips with liquid metal Galinstan. We


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further harness the digital fabrication to create a smart glove with fully integrated sensor system. The printing process reported in this study represents an attractive manufacturing approach to generate delicate and complex architectures of compliant materials, which paves the way for potential applications in wearable electronics, advanced prosthetics, human-machine interfaces, soft robotics, and microfluidics.

Figure 1. Influence of ink rheology on the printing quality. (a) Schematic of the printing platform. (b) Viscosity of liquid, intermediate, and viscous inks as a function of shear


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rate. (c) Storage moduli (G’) and loss moduli (G”) as a function of shear stress for three inks. (d) Optical microscope images and height profiles (corresponding to the dashed line-cuts) of interconnected line segments printed by using liquid, intermediate and viscous inks, respectively.

Figure 2. Omnidirectional printing of self-supporting structures. (a) Schematic diagram of direct ink writing along a 3D path. (b) Images of a printed non-coplanar feature involving an array of out-of-plane segments with a steep angle of 60°. (c) Schematic illustration of printing across a large unsupported region. (d) Images of a suspended film that span across gaps of varying lengths.


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Figure 3. Complicated 3D architectures generated by printing in a layerwise sequence. (a) Images of the printing process to create a “T” shaped structure with overhanging parts. (b) Images showing the printing process of a miniaturized bridge. (c) Images of the process to generate an embedded microchannel structure. The microchannel is filled with blue aqueous solution for visualization. (d) Images of an as printed miniaturized vase (left) and after storing under ambient conditions for three weeks.


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Figure 4. Mechanical properties of printed features after curing. (a) Representative tensile stress versus strain curves for transverse specimen, longitudinal specimen and base elastomer (Ecoflex 0030) (b) Images of a miniaturized cup bent under the diameter of 10 mm stainless steel ball (top) and completely recovered after release (bottom). (c) Images of an as printed balloon (left) that is pneumatically inflated to 3.8 (middle) and 5.9 (right) times of its original volume, respectively.


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Figure 5. Liquid-state stretchable conductor filling microchannels with liquid metal. (a) Images of a soft microfluidic chip with embedded hollow (top) and liquid metal filled microchannels (bottom). (b) Optical microscope images of the cross sections of as printed microchannels (top) and liquid metal filled microchannels (bottom). (c) Experimental and theoretical resistance of stretchable conductor as a function of the channel length. (d) Relative resistance as a function of tensile strain.


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Figure 6. Characteristics of liquid-state strain sensors. (a) Images of a printed strain sensor with liquid metal filled microchannel. (b) Relative resistance as a function of tensile strain under stretching and relaxing conditions. Inset: Representative images of device stretched to different strains.


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Figure 7. Characteristics of liquid-state pressure sensors. (a) Schematic of the pressure sensor. (b) Schematic of its cross-sectional images of the pressure sensor with and without cavity. (c) Image of the experimental setup for calibrating the pressure sensor. (d) Schematic to illustrate the mechanical deformation of the pressure sensor under pressure. (e) Output voltage versus pressure under loading and unloading conditions. (f) Real-time output voltage change of the pressure sensor by placing and removing a dummy frog.


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Figure 8. Smart glove with integrated strain sensor system. (a) Schematic diagram to illustrate the overall design of the smart glove. (b) Image of a smart glove consisting of liquid-state strain sensors, liquid-state interconnects, and a flexible connector. (c) Realtime response of the strain sensors at individual fingers to represent different hand gestures.

ASSOCIATED CONTENT

Supporting Information. The following files are available free of charge. 3D models and corresponding printed structures; influence of printing path design on


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the quality of elastomeric structure; Characterizations on printing resolution; response of strain sensor to temperature changes, step deformations, varying strain rates, and large tensile strains; operation mechanism of the pressure sensor; response of pressure sensor to temperature changes and varying weights; microchannel design for the integrated strain sensing glove; printing process for hand-shaped elastomeric structure; mechanical properties of printed elastomers. (PDF) A video of the dynamic process to create suspended filaments. (AVI)

AUTHOR INFORMATION

Corresponding Author

*To whom correspondence should be addressed: [email protected] (X.W.), [email protected] (D.L.), and [email protected] (D.K.).

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

We acknowledge the support from National Key Research and Development Program of China (Grant No. 2018YFB1105400), National Natural Science Foundation of China (Grant No. 61704074, 21790345 and 21406048), Natural Science Foundation of Jiangsu Province (Grant No. BK20160629), and Fundamental Research Funds for the Central Universities.

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Omnidirectional Printing of Soft Elastomer for Liquid-State Stretchable

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