Printable Metal-Polymer Conductors for Highly Stretchable Bio-Devices

摘要

class="head no_bottom_margin" id="sec1title">IntroductionFusion of electronics with biology and medicine demands electronic devices that are supple, stretchable, and compatible with human tissues, such as the skin (, ) and the brain (, ), especially in the fields of health monitoring and disease treatment (, , ). Flexible circuits are currently realized by depositing a thin layer of metal like copper and gold on flexible substrates such as polyimide. To further endow such flexible circuits with stretchability, wavy or serpentine structures are designed to counteract deformations (, ). The stretchability of these strategies is limited (about 150%), and the fabrication processes of such structures are often complicated. Another approach is developing conductive materials that could bear large deformations such as carbon nanomaterials, silver inks, and liquid metals (LMs) (, , , , ). Most of these materials are hard to be patterned with microstructures in large scale. Among these materials, LMs, especially eutectic gallium indium alloy (EGaIn, melting point 15°C), stand out for their excellent performances of high conductivity and unequaled stretchability (). Besides, EGaIn is much less toxic (href="#bib18" rid="bib18" class=" bibr popnode">Lu et al., 2015) than other metals that may potentially be useful for these applications, such as mercury (also liquid at room temperature) or silver (its nanowires can withstand stretch). However, the huge surface tension (>400 mN/m) (href="#bib3" rid="bib3" class=" bibr popnode">Dickey, 2014) of LM impedes its direct patterning by using straightforward technologies such as stencil and ink-jet printing (href="#bib28" rid="bib28" class=" bibr popnode">Zheng et al., 2014, href="#bib27" rid="bib27" class=" bibr popnode">Wang et al., 2015), making its patterning limited to very few substrates that can be wetted by LMs. In addition, such strategies have high LM consumption (the thickness of the LM pattern is usually larger than 100 μm), which further limits their wider applications. Another common method, injecting the LM into microfluidic channels or hollow wires, can yield conductors with high stretchability (>1,000%). However, these methods cannot fabricate complex conductive patterns that require the microfluidic channels or hollow wires to be continuous from the beginning to end (href="#bib13" rid="bib13" class=" bibr popnode">Kubo et al., 2010, href="#bib29" rid="bib29" class=" bibr popnode">Zhu et al., 2013). By contrast, bottom-up approaches to fabricate LM into particles is a good way to minimize the surface tension. Some reported strategies usually deposit a layer of liquid metal particles (LMPs) (not conductive due to the oxide layer on the particles) on the surface of elastomers and use a marker to mechanically sinter LMPs (break the oxide layer to release the conductive LM) to obtain a desirable conductive pattern (href="#bib14" rid="bib14" class=" bibr popnode">Lin et al., 2015, href="#bib1" rid="bib1" class=" bibr popnode">Boley et al., 2015; href="#bib23" rid="bib23" class=" bibr popnode">Ren et al., 2016, href="#bib20" rid="bib20" class=" bibr popnode">Mohammed and Kramer, 2017). However, conductive patterns fabricated by these strategies cannot bear large deformations like stretching in practical applications, because stress in deformations will also sinter the LMPs in regions where conductive patterns are not desirable, causing short circuit in the electronics. Besides, using a marker to sinter the LMP to obtain electronics has low efficiency and low utilization of particles. These strategies using LMPs are not applicable to mass-manufacturing.Here, we report printable and mass-manufacturable metal-polymer conductors (MPCs) by fabricating LM into LMPs (liquid core-oxide shell structure), and embedding the patterned LMPs on the surface of polymers by casting and peeling off steps, instead of using a marker or a nozzle, to result in microstructured, conductive path. The theoretical calculation indicates that the stress on the particles during stripping is much larger than the yield stress of gallium oxide, causing the release of the LM to form conductive paths. We used screen printing or microfluidic patterning strategy to pattern any 2D MPC pattern on various substrates in different thicknesses with high resolution (15 μm), high efficiency, and low cost. The printable MPC is highly conductive (8 × 10³ S/cm), robust, and stretchable, which can keep conductivity as high as 2,316 S/cm at a strain of 500%. Because MPC patterns allow exposed LMs on the surface of polymer substrates (instead of completely buried within polymers), electronic components could be easily mounted. We used the printable MPC for highly stretchable circuits, motion sensors, wearable glove keyboards, and electroporation of live cells because of its biocompatibility. This potentially widely applicable approach will greatly increase the stretchability of electronic devices and sharply decrease the production cost of printed electronics, which will significantly promote the development of wearable or implantable electronic devices.