covalent bonds is typically hard to remove. Adhesion through physical interactions may be detachable, but usually requires solvents to act at the bonding front [25,30] ; the operation can be time-consuming and environmentally harmful. Some traditional adhesives are chemically modified to be detachable upon a change in temperature (e.g., epoxy) [31,32] or an exposure of light (e.g., pressure-sensitive adhesive), [23,33] but they are usually cytotoxic and ineffective for wet materials like hydrogels and living tissues. Some bioinspired adhesion systems also use noncontact stimuli like temperature or magnetic field to trigger detachment. [34][35][36] Nevertheless, their adhesions rely on specific materials with special surface geometry, or generate low adhesion energy (1-10 J m −2 ). Achieving both strong adhesion and easy detachment has been a challenge.Here we describe an approach to achieve both strong adhesion and lighttriggered easy detachment. We first describe the principle of strong and photodetachable adhesion using two hydrogels as adherends (Figure 1). Each hydrogel aggregates water molecules and a covalent polymer network. The polymer networks in the two hydrogels have no matching functional groups for bonding, so that the two hydrogels by themselves adhere poorly. We achieve strong adhesion by spreading an aqueous solution of polymer chains on the surfaces of the two hydrogels, and triggering the polymer chains to cross-link into a third polymer network in situ, in topological entanglement with the preexisting polymer networks of the two hydrogels. The third polymer network acts as a molecular suture that stitches the two preexisting polymer networks of the hydrogels together. This process is called topological adhesion, or topohesion for short. [25] We achieve photodetach by functionalizing the stitching polymer network for photodetach, and triggering the network to dissociate upon an exposure to light of a certain frequency range.The principle described above requires two triggers. The first trigger, which we call the topohesion-trigger, causes the stitching polymer chains to cross-link into a new polymer network in topological entanglement with the preexisting polymer networks of the two hydrogels. The second trigger, which we call the photodetach-trigger, causes the stitching network to dissociate in response to light of certain frequency range. Conceivably the two triggers can be realized with various chemistries. Here we demonstrate topohesion and photodetach using two facts of chemistry: 1) Fe 3+ ions and carboxyl groups form coordination complexes, [37,38] and 2) the coordination complexes

Exploring the abundant resources in the ocean requires underwater acoustic detectors with a high-sensitivity reception of low-frequency sound from greater distances and zero reflections. Here we address both challenges by integrating an easily deformable network of metal nanoparticles in a hydrogel matrix for use as a cavity-free microphone. Since metal nanoparticles can be densely implanted as inclusions, and can even be arranged in coherent arrays, this microphone can detect static loads and air breezes from different angles, as well as underwater acoustic signals from 20 Hz to 3 kHz at amplitudes as low as 4 Pa. Unlike dielectric capacitors or cavity-based microphones that respond to stimuli by deforming the device in thickness directions, this hydrogel device responds with a transient modulation of electric double layers, resulting in an extraordinary sensitivity (217 nF kPa−1 or 24 μC N−1 at a bias of 1.0 V) without using any signal amplification tools.

Achieving adhesion between hydrogels and diverse materials in a facile and universal way is challenging. Existing methods rely on special chemical or physical properties of the hydrogel and adherends, which lead to limited applicability and complicated pretreatments. A stitch‐bonding strategy is proposed here by introducing a polymer chain with versatile functional group and triggerable crosslinking property inspired by catechol chemistry. The polymer chain can stitch the hydrogel by forming a network in topological entanglement with the preexisting hydrogel network, and directly bond to the adherend surface by versatile chemical interactions. Through this, the polymer chain solution works as a universal glue for facile adhesion of hydrogels to diverse substrates like metals, glasses, elastomers, plastics, and living tissues, without requiring any chemical design or pretreatment for the hydrogel and adherends. The adhesion energy between polyacrylamide hydrogel and diverse substrates can reach 200–400 J m−2, and it can reach ≈900 J m−2 with a toughened polyacrylic acid polyacrylamide hydrogel. The mechanism of stitch‐bonding strategy is illustrated by studying various influence factors.

During operations, surgical mesh is commonly fixed on tissues through fasteners such as sutures and staples. Attributes of surgical mesh include biocompatibility, flexibility, strength, and permeability, but sutures and staples may cause stress concentration and tissue damage. Here, we show that the functions of surgical mesh can be significantly broadened by developing a family of materials called hydrogel–mesh composites (HMCs). The HMCs retain all the attributes of surgical mesh and add one more: adhesion to tissues. We fabricate an HMC by soaking a surgical mesh with a precursor, and upon cure, the precursor forms a polymer network of a hydrogel, in macrotopological entanglement with the fibers of the surgical mesh. In a surgery, the HMC is pressed onto a tissue, and the polymers in the hydrogel form covalent bonds with the tissue. To demonstrate the concept, we use a poly(N-isopropylacrylamide) (PNIPAAm)/chitosan hydrogel and a polyethylene terephthalate (PET) surgical mesh. In the presence a bioconjugation agent, the chitosan and the tissue form covalent bonds, and the adhesion energy reaches above 100 J⋅m−2. At body temperature, PNIPAAm becomes hydrophobic, so that the hydrogel does not swell and the adhesion is stable. Compared with sutured surgical mesh, the HMC distributes force over a large area. In vitro experiments are conducted to study the application of HMCs to wound closure, especially on tissues under high mechanical stress. The performance of HMCs on dynamic living tissues is further investigated in the surgery of a sheep.