Merging of Thin- and Thick-Film Fabrication Technologies: Toward Soft Stretchable “Island–Bridge” Devices

DOI: 10.1002/admt.201600284 fabrication of such materials onto deterministically stretchable designs thus requires reliance on other fabrication techniques. The key focus of the present work was to develop a strategy for combining lithography (thin-film) and screen-printing (thick-film) techniques to realize deterministic, high-performance stretchable devices. Screen printing has been widely used toward large-scale, cost-effective incorporation of a myriad of materials onto numerous substrates for various applications.[19,20] However, most of the printable inks form either rigid or flexible films. Developing stretch-enduring inks is challenging as only a handful of elastomeric binders and functional materials can be homogeneously dispersed to achieve highperformance stretchable inks. While lithographic and printing techniques have been the primary methods used for fabricating wearable devices, their distinct and complementary advantages and characteristics have not been combined. The described new hybrid fabrication process, combining the printing of functional ink materials onto lithographically stretchable deterministic patterns, represents an attractive route that can address the challenges of each individual technique. For example, while lithography has been exploited for realizing complex stretchable systems, they are limited to thin films (<10 μm), which are not suitable for devices requiring high loading of active materials (e.g., energy harvesting and storage devices). In addition, there are limited choices of materials that can be vacuum deposited and solution etched. On the other hand, screen printing enables sufficient loading of a variety of active materials into thick (20–50 μm) films, but it suffers from meeting required layout resolutions and performance. In the hybrid system, the thick film resides over rigid isolated islands interconnected with free-standing stretchable serpentine bridges. The device can thus afford to include a wide range of rigid and lithographically incompatible materials without concern of device failure, since most of the strain is accommodated by the serpentine structures while leaving the thick-film islands unharmed. The hybrid system thus combines the best of two worlds. The hybrid pattern has been realized by first lithographically fabricating the entire “island–bridge” layout of the device in gold onto an elastomeric substrate, followed by printing the ink layer on the islands (Figure 1A). Briefly, a polyimide-coated copper film was bonded to a polydimethylsiloxane-covered glass slide. Thereafter, copper electrodes were patterned via standard lithographic technique. Subsequently, gold electrodes were patterned onto the copper design. Later, a top layer of polyimide was patterned to define the active electrode area and finally transferred to an EcoFlex layer. Subsequently, the device was transferred to the printer where electrodes were screen printed with inks. The detailed fabrication process is described in the Biointegrated soft electronic devices are expected to play crucial roles in consumer electronics,[1] healthcare,[2] and energy[3] domains to significantly transform our lifestyle. However, mating of conventional rigid electronic devices with soft biological tissues leads to significant compromise in performance.[4] The rapidly emerging field of soft, stretchable electronics has the potential to address this issue by ensuring conformal contact between wearable devices and the human body.[5] Researchers have mainly focused on using two approaches for realizing stretchable devices: deterministic[6] and random[7] composites. The deterministic composite route, also known as the “island–bridge” approach, involves lithographic fabrication of the device components onto rigid islands connected by serpentine bridges and ultimately bonding the device to a soft, stretchable elastomeric substrate.[8] When subjected to external strain, the underlying elastomeric substrate and the serpentine structures accommodate most of the stress, thus leaving the crucial device components unharmed.[5,9] On the other hand, random composite-based stretchability relies on the random incorporation of functional material within or on the elastomeric matrix to develop stretchable systems.[10] Deterministically stretchable devices have an edge over their random composite counterparts since the performance of random composite devices diminishes by incorporating the functional components within/on elastomeric substrates.[11] In contrast, deterministic systems allow fabrication of complex, stretchable devices with performance similar to conventional rigid devices.[12] Additionally, lithographically fabricated deterministic stretchable systems can possess features with a few micrometer and even sub-micrometer dimensions, thus leading to compact, multifunctional devices. However, widespread applications of such devices are hindered since there are several materials that are incompatible with the lithographic fabrication route. For example, many devices rely on nanomaterials,[13] polymer composites,[14] carbonaceous,[15] biological,[16] lowtemperature,[17] and solvent-sensitive[18] materials. Integrating these materials in microstructured forms on elastomeric substrates, for intimate contact with biological tissues, will open up new paradigms for wearable devices. High-precision scalable www.advmattechnol.de