Mechanochromic Stretchable Electronics - ACS Applied Materials

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Mechanochromic stretchable electronics Meredith Hyatt Barbee, Kunal Mondal, John Z. Deng, Vivek Bharambe, Taylor V. Neumann, Jacob Adams, Nicholas Boechler, Michael D. Dickey, and Stephen L. Craig ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09130 • Publication Date (Web): 09 Aug 2018 Downloaded from http://pubs.acs.org on August 10, 2018

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ACS Applied Materials & Interfaces

Mechanochromic Stretchable Electronics §

Meredith H. Barbee†, Kunal Mondal‡, John Z. Deng†, Vivek Bharambe , Taylor V. Neumann‡, § ∥ Jacob J. Adams , Nicholas Boechler , Michael D. Dickey*‡ Stephen L. Craig*†

† Department of Chemistry, Duke University, Durham, NC 27708, USA ‡ Department of Chemical and Biomolecular Engineering, NC State University, Raleigh, NC 27695, USA § Department of Electrical and Computer Engineering, NC State University, Raleigh, NC 27695, USA

∥ Department of Mechanical and Aerospace Engineering, University of California, San Diego, La Jolla, CA, 92093, USA

KEYWORDS. liquid metal, stretchable electronics, polymer mechanochemistry, mechanochromism, silicone elastomers

ABSTRACT. Soft and stretchable electronics are promising for a variety of applications such as wearable electronics, human-machine interfaces, and soft robotics. These devices, which are often encased in elastomeric materials, maintain or adjust their functionality during deformation, but can fail catastrophically if extended too far. Here, we report new functional composites in which stretchable electronic properties are coupled to molecular mechanochromic function,

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enabling at-a-glace visual cues that inform user control. These properties are realized by covalently incorporating a spiropyran mechanophore within poly(dimethylsiloxane) to indicate with a visible color change that a strain threshold has been reached. The resulting colorimetric elastomers can be molded and patterned so that, for example, the word “stop” appears when a critical strain is reached, indicating to the user that further strain risks device failure. We also show that the strain at color onset can be controlled by layering silicones with different moduli into a composite. As a demonstration, we show how color onset can be tailored to indicate a when a specified frequency of a stretchable liquid metal antenna has been reached. The multiscale combination of mechanochromism and soft electronics offers a new avenue to empower user control of strain dependent properties for future stretchable devices. Introduction Soft, stretchable, and flexible electronics have the potential to impact the development of wearable electronic devices,1 soft robotics,2-4 and biomedical devices.5 In contrast to traditional electronic components, some of these devices can be stretched significantly (up to many hundreds of percent strain).6-7 Examples include stretchable wires,6 antennas,8 electroluminescent devices,9 light emitting diodes,10-11 and flexible solar cells.12 Taking these devices outside the controlled environment of a laboratory presents several challenges, one of which is avoiding mechanical failure by inadvertently over-extending the device. Several strategies have emerged to help prevent the failure of composites comprising metal components patterned within soft materials.13-14 This includes electronic materials with “Jshaped” stress-strain behavior (i.e. elastomers that are soft at low strains but stiff at higher strains),15 layered elastomer composites that adapt to strain by changing their moduli,16 and the incorporation of a gradient in mechanical properties between metal and polymer components that


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can aid in preventing delamination by distributing stress at interfaces.17-18 Regardless of the approach, all elastomers eventually reach a critical strain beyond which failure is inevitable. Thus, we sought a general strategy that could be utilized to provide feedback to users prior to reaching this point. We focus on using a eutectic gallium indium alloy (EGaIn, mp-15.7°C),19 which can be patterned within silicones.20 As the alloy is liquid at room temperature and the quantity used is typically small compared to the poly(dimethylsiloxane) (PDMS), the PDMS-liquid metal devices will largely take on the mechanical properties of the elastomer, while maintaining the functionality of the metal.8 This combination of properties has led to a range of promising applications including microfluidics, stretchable and soft electronics, sensors, reconfigurable devices, and self-healing components.21-22 These liquid metal wires can be easily over-stretched to the point of failure (Figure 1), breaking the electrical connection and possibly allowing the metal to escape. In addition, certain properties of stretchable electronics, such as the resonant frequency of a liquid metal antenna, vary continuously with the extent of deformation of the device, motivating the desire for devices that can be programmed to provide their own independent signal that a desired frequency is reached.8 A relatively sharp onset of color at a given strain threshold, for example, would provide a useful mechanism for at-a-glance diagnosis that failure is about to occur. We reasoned that such a response could be engineered into EGaInPDMS composites through covalent polymer mechanochemistry, providing easy to read colorimetric signals that signify a critical strain has been attained.


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Figure 1. A. Liquid metal wires composed of EGaIn in a PDMS microchannel (left) and one that has failed (right). B. Spiropyran is ring-opened to MC under applied mechanical force. Polymer attachments are shown with blue circles. C. Spiropyran with alkene functionality is covalently incorporated into Sylgard 184, taking advantage of the platinum catalyzed hydrosilylation that forms the polymer network. Upon deformation, mechanical force is transduced to the SP and the material turns purple.

In recent years, covalent polymer mechanochemistry has been demonstrated to provide a range of constructive responses to mechanical forces in polymers. These include: catalysis,23-25 luminescence,26 small molecule release,27 stress-induced strengthening,28 and mechanochromism.29-30 In particular, spiropyran (SP) (Figure 1b) can function as a colorless mechanophore that undergoes a force-coupled ring-opening reaction to its longer, conjugated, purple merocyanine (MC) isomer, and has been widely used as a visual colorimetric indicator of strain.31 Spiropyran can be added as a cross-linker to commercially available silicone elastomer kits (e.g. Sylgard 184 ®, Dow Corning, and DragonSkin ® and EcoFlex ®, Smooth-on) without substantially altering their mechanical properties.32 For instance, SP, with terminal alkenes connected via esterification shown in figure 1C, can be covalently connected into the PDMS network using the platinum-catalyzed hydrosilylation reaction used to cure the elastomer.32-33


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After the material recovers its initial shape, MC reverts quickly back to SP, which is favored in the absence of force.32, 34 The ability to incorporate SP into soft materials has provided access to mechanochemically active soft devices such as writing tablets,33 soft robots,35 and electroactive displays.36 Here, we show that a multi-scale combination of molecular and device engineering provides composite materials that are suitable for mechanochemically active stretchable electronics, in particular PDMS-encased liquid metal antennas. The desired response is incorporated directly into the silicone component of the device through material synthesis. As a result, such devices require no additional energy input for signal generation or any sensor beyond the naked eye, and their function and properties should in general be otherwise indistinguishable from their conventional analogs. Visible color changes are obtained at device strains that are relevant for applications of flexible, stretchable electronics and are shown to be tunable through simple composite design. We expect such mechanochemical approaches to complement other strategies to couple optical and mechanical properties in soft devices.37-38 Results and Discussion We begin with a liquid metal wire encased in an SP-PDMS elastomer, as shown in Figure 1. Silicones are common substrates and encasing materials for these applications because they are commercially available, easy to process, and have excellent mechanical properties. There are a variety of strategies for incorporating stretchable electronics into elastomers,39 and here we utilize 3D printing and vacuum filling.20, 40 When stretched, the elastomeric device exhibited mechanochromism (Figure 2). The color persisted for as long as the device was stretched above a critical value, but faded quickly (90% strain, it is useful as an indicator that failure may occur with greater strain. The reported elongation at break for Sylgard 184 is 140% strain,41 and we observe that failure of the devices usually occurs between 125-160% strain. Some devices break at lower strains, which we attribute to defects in fabrication. The observed color change occurs reversibly and repeatedly (see Figure S17) as long as the device does not fail, maintaining the stress warning capability potentially throughout the lifetime of the elastomer (comparable SP-PDMS samples in our lab that are as much as 6 years old still exhibit mechanochromism).32 To make this technology more “user-friendly,” we sought a more instructive colorimetric response. Sylgard 184 was cast around the partially cured SP-PDMS, in a manner similar to that previously published for EcoFlex42 and patterned here in the shape of the word “STOP” (Figure 3), then the entire device is fully cured. Devices cast around partially cured STOP patterns survived to significantly higher strains than those cast around fully cured patterns. We attribute the improvement in extensibility to increased chain entanglement and potential covalent crosslinking at the interfaces between the partially cured SP-PDMS and Sylgard 184. The word cannot be seen prior to deformation, and color change occurs at a similar strain (90%) to that observed in SP-PDMS dogbone samples. As in the dogbones, the appearance of the word STOP also occurs reversibly and repeatedly (see Supporting Information).


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Figure 3: The STOP elastomers have Sylgard 184 cast around SP-PDMS so that the word stop is visible after deformation, but prior to failure. A dashed line is added at the edge of the elastomer for improved contrast.

While we chose the word “STOP” for this application, different architectures are possible. Additive manufacturing has been used previously to make plastic materials that have mechanochromic regions.43-45 Recently, the ability to 3D print silicone elastomers based on Sylgard 184 has also been demonstrated,46-47 and the methodology should be compatible with the partial curing strategy, opening many new possibilities for functional geometries. We noted batch-to-batch variation in the mechanochromic color distribution of the STOP devices. Whereas the color in the word STOP shown in Figure 3 is concentrated at the interface between the components (even though the spiropyran is distributed throughout the SP-PDMS around the word stop), other batches of STOP elastomers resulted in much more even color distribution (see Supporting Information for additional photos). We imagine that this is due to effects such as subtle differences in the extent of partial curing of the SP-PDMS, relative thickness of the two layers, and temperature and curing time during fabrication. An investigation of these factors is beyond the scope of this work, but the variation hints at the possibility of


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controlled color patterns that are tailored to specific applications, for example by concentrating strains at the interface between components. Having demonstrated that mechanochromism could be used to warn of looming failure, we next set out to demonstrate that the device strain at color onset could be tuned through the composite architecture. For applications with liquid metal antennas, which have a resonant frequency that varies continuously with elongation, it would be useful to have a colorimetric indicator that indicate a desired, functional resonant frequency has been reached. Likewise, other strategies for making stretchable conductors, such as serpentine solid metals, often reach a point of failure before the silicone. Thus, strategies to tune the strain at which color appears are desirable. Our strategy is to use a softer elastomer (DragonSkin 30, modulus 538 kPa)48 in a layered composite with stiffer SP-PDMS (Sylgard 184, modulus 3.9 MPa)49 that is mechanochromic, as shown in Figure 4A and B. Devices were made using the same sequential, partial curing strategy used in the STOP elastomers, and the length of the SP-PDMS was varied within otherwise identical devices (Figures 4a and 4b). Using stress-strain data and images collected during tensile testing, we measured the strain within the SP-PDMS region of the device as a function of macroscopic device strain (Figure 4c). For a fixed total section length, as the length of SP-PDMS region Ls decreases relative to the softer DragonSkin region, the displacement required to reach the threshold strain for coloration increases because the device becomes effectively softer with the increased fraction of soft material, resulting in less stress being experienced in the SP-PDMS region for a given applied total device strain. This dependence can be estimated in advance by treating the composites as springs in series (see Supporting Information). The experimentally observed total device strain at the onset of


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∗ ∗ agrees well with the predicted values ߝ௖௔௟௖ , as shown in Figure 4d. For the coloration ߝ௘௫௣ ∗ particular geometry and elastomers chosen here, strain at color onset ߝ௘௫௣ is observed to range

from ~120-160% strain. This method could be extended to an even wider range of strains depending on the elastomers and composite geometries chosen.


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Figure 4: A. Composite dogbones are made by casting Dragonskin 30 around partially cured SPPDMS. Upon deformation, the SP-PDMS exhibits mechanochromism at a strain determined by Ls. This composite structure can tune the macroscopic strain at which color change occurs. B. Photo of a composite before deformation and under tension. C. Experimentally measured strain


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in the SP-PDMS region ߝௌ௉ି௉஽ெௌ of the composite as a function of total strain on the device ߝ௧௢௧௔௟ determined from image analysis. This data is shown for composites with different SPPDMS lengths, indicated by the legend. D. Experimentally determined total device strain at color ∗ ∗ onset ߝ௘௫௣ and predicted strain at color onset ߝ௖௔௟௖ for the same composites shown in Figure C as a function of Ls. Since the resonant frequency of liquid metal antennas is mechanically tunable as a function of strain, mechanochromism can be used to provide an at-a-glance indication that a desired resonant frequency has been reached. For example, the resonant frequency ݂௥ of a dipole antenna depends upon its physical length L, the speed of light in vacuum, c , and the effective permittivity ߝ௘௙௙. for the DragonSkin 30-air interface, as per the following expression: ݂௥ = ଶ௅

௖

(1)

ඥఌ೐೑೑.

As ߝ௘௙௙. and c do not vary upon elongation, Equation (1) suggests an inverse relation between the resonant frequency and the applied strain (via change in length). The measured and predicted resonant frequency for a DragonSkin 30 liquid metal antenna for varying strain levels is shown in Figure 5. Each composite architecture changes color at a strain that corresponds to a different resonant frequency (Figure 5B), and so one can, in advance, design a composite with a color onset that corresponds to a desired frequency fr.


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Figure 5: A. The experimental resonant frequency fr,exp compared to the simulated resonant frequency fr,calc for a Dragonskin liquid metal antenna of the same initial length as the composites ∗ in Figure C. B. The strain at color onset ߝ௘௫௣ and the resonant frequency at that strain for each of the three composites.

Tuning the color onset with composite architecture has several advantages over other possible strategies. Strain at color onset could possibly be tuned by changing the nature of the material, however, filled silicone elastomers are the only materials to date that exhibit repeatable and reversible mechanochromism without substantial plastic deformation, making our choice of materials limited. In these elastomers, color change occurs at the strain-hardening region. New mechanophores, or different derivatives of the SP mechanophore, could have different reactivity under mechanical deformation, which could influence strain at color onset. Tuning color onset with composite architecture is (in our experience) more accessible and easily implemented, as it allows us to use a single, established material and spiropyran, while also allowing for continuous


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variation in strain at color onset. Although the tests employed here all involve uniaxial tension, prior work suggests that the response should be general to other deformation modes, including compression and bending.38 Conclusions The ability to engineer strain-responsive behavior across molecular, material, and composite length scales offers considerable promise for soft devices. Here, molecular mechanochromism provides an indication that helps prevent damage and aid function. The approach can easily be extended such that any property that depends on strain could have a colorimetric indicator for a specific, functional value. While the silicone elastomers employed here are ubiquitous in a wide range of soft devices, mechanochromism can also be embedded in other elastomers (e.g. polyurethanes).50 As such, these results motivate the development of a broader range of functional mechanochromic elastomers. Finally, we note that while the composites described herein were fabricated through standard molding methods, we expect that the available design space will grow substantially with the development of additive manufacturing methods.43, 45-46, 51 The combination of molecular, material, and composite engineering should therefore be extendable to the predictive design of a myriad of stretchable electronics, with properties tuned to specific functional outputs.

Experimental Section Preparation of pre-polymers: Synthesis of spiropyran and its incorporation into SP-PDMS followed a previously reported procedure and is detailed in the supporting information.32, 52 SPPDMS prepolymer was made by mixing 10 parts Sylgard ® 184 Base with 1 part curing agent. Spiropyran was added at 0.5 wt % in a solution of xylenes. After degassing, the mixture can be


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cast into a mold or onto a PTFE or polyester surface. Sylgard 184 prepolymer and Dragon Skin ® 30 prepolymer were both prepared and degassed according to the manufacturer’s instructions. Preparation of color changing liquid metal wires: To make color changing liquid metal wires, the SP-PDMS prepolymer was cast onto a SU8 coated silicon wafer mold patterned with 4 microchannels (200 µm width and 50 µm tall, Figure 2). After curing overnight at 65°C, this piece was plasma bonded to another piece of cured SP-PDMS. A small hole was made on either end of the microchannel and EGaIn was injected with a syringe to fill the channel.40 The holes can then be sealed with optical adhesive (Norland Optical Adhesive, Norland Products Inc. NJ, USA) to prevent the liquid metal from coming out. The stretchable wires were cut to size with a razor blade for mechanical testing and conductivity measurements. Preparation of “STOP” sign elastomers: To make the “STOP” signs, SP-PDMS prepolymer was cast onto a polyester surface with a rectangular 3D printed mold. The 3D printed mold of the word “STOP” was pressed into the prepolymer. The prepolymer cured for approximately 2 hours at 65°C, until the prepolymer was tacky, yet capable of maintaining its shape. Both molds were removed, then an additional rectangular mold was placed around the partially cured SPPDMS, and Sylgard 184 prepolymer was cast around it. The entire device cured overnight at 65°C, then the stretchable “STOP” signs were cut to size with a razor blade. Preparation of Sylgard 184/DragonSkin 30 composites: Composite elastomers of Sylgard 184 and Dragon Skin 30 were made in a similar manner as previous composites in the literature.42 SP-PDMS prepolymer was cast onto a polyester surface within a 3/64” thick mold cut from polycarbonate. To ensure an even surface of uniform height, a polyester film was placed on top of the sample, followed by an aluminum block. This was partially cured for approximately 1 hour at 80°C, then the polyester film and polycarbonate mold were removed. A second 1/16”


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thick mold cut from polycarbonate was placed around the partially cured sample. Dragon Skin 30 prepolymer was cast into this mold. Again, a polyester film and aluminum block were placed on top before curing overnight at 80°C. Characterization of Sylgard 184/DragonSkin 30 Composites: For tensile testing, samples were cut with either a dogbone shaped stamp cutter or cut using a laser cutter. Photos were taken during tensile testing and image analysis to determine mean pixel intensity for red, green, and blue channels was completed using Fiji ImageJ. For more detailed information about synthesis, fabrication of devices, molds, tensile testing, photography, and image analysis see the Supporting Information. Preparation of liquid metal antennas: The DragonSkin 30 prepolymer was cast onto a mold fitted with a SMA (SubMiniature Version A) connector and cured at 65°C. Then the liquid metal conducting lines were patterned on the surface via direct writing. This process used a custommade 3D printer consisting of a numerically controlled X-Y-Z stage, a pressure actuator (Nordson EFD Ultimus V), and a syringe barrel with an 22-gauge conical polypropylene needle loaded with liquid metal that is attached to the pressure actuator.20, 53 After patterning the liquid metal, Sylgard 184 diluted at 50% volume with toluene was sprayed onto the surface. Multiple coatings resulted in a final layer thickness of ~ 100µm. Characterization of the materials and liquid metal dipole antenna measurements: The permittivity of DragonSkin 30 prepolymer ( ε r ' = 2.75, tan δ e = 0.014 at 1.84 GHz) was characterized using a resonant cavity method.54 The conductivity of EGaIn was considered to be 3.4x106 S/m and used for electromagnetic simulations using HFSS (Ansys Inc.).55 The resonant frequency of the fabricated stretchable dipole antenna was measured using a Vector Network Analyzer (Agilent Technologies).


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ASSOCIATED CONTENT Supporting Information The supplementary document contains experimental details and additional data.

The

supplemental video has 5 clips: 1.) a simple LED circuit with a mechanochromic liquid metal wire under deformation; 2.) mechanochromism of a liquid metal wire; 3.) mechanochromism of a STOP elastomer; 4.) mechanochromism of elastomers displaying “Duke” and “NCSU”; and 5.) a video of the 3D printing of the liquid metal antenna. AUTHOR INFORMATION Corresponding Author *Stephen L. Craig, [email protected], *Michael D. Dickey, [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This material is based on work supported by the US Army Research Laboratory and the Army Research Office under grant W911NF-17-1-0595 to S.L.C. and N. B. S.L.C. and M.H.B acknowledge funding support from the NSF Research Triangle MRSEC (DMR-1121107) and fellowship support from DoD (Air Force Office of Scientific Research, NDSEG Fellowship, 32 CFR 168A to M.H.B.). J.Z.D. thanks Duke University for fellowship support. M.D.D., K. M.,


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and T. V. M. acknowledge funding from NSF-ASSIST Center for Advanced Self Powered Systems of Integrated Sensors and Technologies Center (EEC-1160483) and NC State University. J. J. A. and V. B. acknowledge funding from U.S. Army Research Office under Grant W911NF-17-1-0216. This work was performed in part at the Duke University Shared Materials Instrumentation Facility (SMIF), a member of the North Carolina Research Triangle Nanotechnology Network (RTNN), which is supported by the National Science Foundation (Grant ECCS-1542015) as part of the National Nanotechnology Coordinated Infrastructure (NNCI). We would like to thank the Duke Innovation Lab for the use of their laser cutter. We are also grateful to Russell W. Mailen and Urs Fabian Fritze for helpful conversations. REFERENCES 1. Hammock, M. L.; Chortos, A.; Tee, B. C.; Tok, J. B.; Bao, Z. 25th Anniversary Article: The Evolution of Electronic Skin (E-Skin): A Brief History, Design Considerations, and Recent Progress. Adv. Mater. 2013, 25, 5997-6038. 2. Kim, S.; Laschi, C.; Trimmer, B. Soft Robotics: A Bioinspired Evolution in Robotics. Trends Biotechnol. 2013, 31, 287-294. 3. Majidi, C. Soft Robotics: A Perspective—Current Trends and Prospects for the Future. Soft Robotics 2014, 1, 5-11. 4. Lu, N.; Kim, D.-H. Flexible and Stretchable Electronics Paving the Way for Soft Robotics. Soft Robotics 2014, 1, 53-62. 5. Minev, I. R.; Musienko, P.; Hirsch, A.; Barraud, Q.; Wenger, N.; Moraud, E. M.; Gandar, J.; Capogrosso, M.; Milekovic, T.; Asboth, L.; Torres, R. F.; Vachicouras, N.; Liu, Q.; Pavlova, N.; Duis, S.; Larmagnac, A.; Voros, J.; Micera, S.; Suo, Z.; Courtine, G.; Lacour, S. P. Electronic Dura Mater for Long-Term Multimodal Neural Interfaces. Science 2015, 347, 159-163. 6. Zhu, S.; So, J.-H.; Mays, R.; Desai, S.; Barnes, W. R.; Pourdeyhimi, B.; Dickey, M. D. Ultrastretchable Fibers with Metallic Conductivity Using a Liquid Metal Alloy Core. Adv. Funct. Mater. 2013, 23, 2308-2314. 7. Matsuhisa, N.; Kaltenbrunner, M.; Yokota, T.; Jinno, H.; Kuribara, K.; Sekitani, T.; Someya, T. Printable Elastic Conductors with a High Conductivity for Electronic Textile Applications. Nat. Commun. 2015, 6, 7461. 8. So, J.-H.; Thelen, J.; Qusba, A.; Hayes, G. J.; Lazzi, G.; Dickey, M. D. Reversibly Deformable and Mechanically Tunable Fluidic Antennas. Adv. Funct. Mater. 2009, 19, 36323637. 9. Larson, C. P., B.; Robinson, S.; Totaro, M.; Beccai, L.; Mazzolai, B.; Shepherd, R. Highly Stretchable Electroluminescent Skin for Optical Signaling and Tactile Sensing. Science 2016, 351, 1071-1074.


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Mechanochromic Stretchable Electronics - ACS Applied Materials

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