DOI: 10.1002/adma. 201500954 integration of electronic devices and bridging the abiotic–biotic interface.[32–36] Multielectrode arrays fabricated on ultrathin poly (ethylene terephthalate) substrates use polyrotoxane hydrogel films to improve tissue-device integration while monitoring cardiac function in vivo.[29] Hydrogels can serve as templates for in situ assembly of metallic nanoparticles through metal ion reduction [37] or conducting polymers via oxidative polymerization.[32–34] However, the direct integration of microelectronics with swollen hydrogel substrates is challenging with commonly available microfabrication techniques such as photolithography and transfer printing.[38–41] Hydrated networks prohibit vacuum-based thin film deposition techniques directly on hydrogel substrates. High swelling ratios and hydrated surface environments of the hydrogel substrates also attenuate van der Waals interactions, which are essential for transfer printing of prefabricated microelectronics.[42] Here, we describe an application-specific target hydrogel substrate for transfer printing of electronic microstructures. This approach utilizes hydrogels with adhesion-promoting moieties that permit direct assemble of functional microstructures on swollen target hydrogel substrates via transfer printing. This technique melds thin film patterning and deposition techniques with adhesive highly compliant swollen hydrogel substrates. Adhesion in hydrated environments is a challenging problem that has been solved in part by recent discoveries of adhesion-promoting catechol-bearing materials.[43–46] Catechols bond to inorganic/organic materials in hydrated environments through polarizable aromatic groups, hydrogen bonds, and coordination bonds.[47–49] Hydrogels synthesized from nontoxic poly (2-hydroxyethyl methacrylate)(P (HEMA)) and polyethyleneglycol precursors [50] are materials that comprise biomedical devices used in human trials for many applications including controlled release matrices,[51, 52] soft contact lenses,[53, 54] and artificial corneas.[55] Catechol-bearing HEMA-based hydrogels are suitable target substrates for transfer printing of electronic structures. Dopamine methacrylate (DMA) monomers are copolymerized with HEMA hydrogels and poly (ethyl glycol) dimethacrylate crosslinker to form P (HEMA-co-DMA) hydrogels (Figure 1a). DMA incorporation was characterized using Fourier transform infrared (FT-IR) spectroscopy (Figure 1 b). DMA monomers exhibit strong peaks at 1523 and 1653 cm− 1, which are assigned to NH bending in amides [56] and CC bonds in pendant methacrylates, respectively. The latter peak is abolished after P (HEMA-co-DMA) hydrogel formation through crosslinking via photopolymerization. Peak deconvolution of features from 1580 to 1670 cm− 1 of P (HEMA-co-DMA) indicates that CC stretches at 1602 cm− 1 from aromatic rings in DMA are preserved in P (HEMA-co-DMA) hydrogels. The new peak at 1633 cm− 1 in P (HEMA-co-DMA)

Polymeric substrates are an important component in flexible electronics because they can overcome many limitations associated with inorganic substrates that may be rigid, brittle, and planar.[1–4] Devices fabricated on polymeric substrates can also be light weight,[5] stretchable,[6–8] or biodegradable.[9, 10] These systems are suitable for applications including environmentally friendly sensors,[11, 12] wearable medical devices,[13, 14] and temporary biomedical implants.[15, 16] For example, contact lenses can be impregnated with electronics to improve visual acuity [17, 18] or measure glucose levels in real time.[19] Dissolvable and elastomeric substrates allow conformal coating of …