ĭirect laser writing by two photon polymerization (2PP) has been recently explored as a 3D fabrication technology of choice for the generation of soft micro-machines, offering the exciting opportunity of translating the extensive library of macro-scale stimuli-responsive hydrogels to the micron (and even sub-micron) scale. As response rate is generally dependent on the square of the hydrogel's characteristic dimensions, the reduction of hydrogel size to a micron regime can yield a profoundly accelerated response time. To date, PBA-functionalized hydrogels have primarily been explored on the macro-scale, exhibiting response times to reach equilibrium swelling typically in the order of minutes to hours. When immobilized in a hydrogel network, this can be used to generate a physical response, either through repulsion of the negatively charged boronate esters that leads to the expansion of the hydrogel network or a shrinking actuation caused by linkage of two boronic acids with a single sugar molecule that acts as an additional crosslinking point. In the presence of 1,2- and 1,3-diols in aqueous media, PBAs yield a boronate ester. A pertinent example can be seen in the use of phenylboronic acids (PBAs) as promising candidates for the realization of sugar-responsive (D-fructose, D-galactose, D-mannose, and D-glucose) hydrogels. This represents one of the major challenges for stimuli-responsive hydrogels which rely on the diffusion of analytes or bioanalytes. However, the main drawback of osmotically driven hydrogel actuators is their low actuation speed, where actuation force and response time are inextricably linked. To this end, stimuli-responsive networks have provided opportunity to detect and respond to external stimuli such as biomolecules, water, temperature, light, and pH, among others. Of particular interest to this work are stimuli-responsive hydrogels that undergo changes in volume as a result of osmotic pressure, leading to the diffusion of water in and out of the hydrogel network. magnetic or electric fields) b) hydrogel structures with cavities driven by hydraulic or pneumatic pressures and c) stimuli-responsive hydrogels actuated by changes in osmotic pressure. magnetic particles or free ions) which respond to external fields (e.g. categorized stimuli-responsive hydrogel actuators into three types: a) hydrogel matrices embedded with active elements (e.g. Through the integration of such stimuli-response, their application has spanned biofabrication, additive manufacturing, and even soft robotics. Hydrogels have epitomized smart materials due to their ability to respond to external stimuli by converting it into other forms of energy. By combining the flexibility of 2PP with an easily adaptable photoresist, an accessible fabrication method is showcased for sophisticated and chemo-responsive 3D hydrogel actuators. Moreover, microstructures with programmable actuation (i.e., bending and opening) are fabricated using the same photoresist within a one-step fabrication process. A phenylboronic acid-based photoresist compatible with 2PP is presented to fabricate stimuli-responsive microstructures with accelerated response times. This offers a remarkable solution for achieving fast response hydrogel systems that have been often hindered by the volume-dependent diffusion times of analytes to receptor sites. Herein, the flexibility of 2PP is exploited to showcase a novel sugar-responsive, phenylboronic acid-based photoresist. Rapid swelling hydrogel microstructures are advantageous for osmotically driven stimuli-response, where actuation speed, that is reliant on the diffusion of analytes or bioanalytes, can be optimized. To achieve the complexity of biomimetic structures, two photon polymerization (2PP) has provided a means of fabricating intricate 3D structures from stimuli-responsive hydrogels. Stimuli-responsive hydrogels have attracted much attention owing to the versatility of their programmed response in offering intelligent solutions for biomimicry applications, such as soft robotics, tissue engineering, and drug delivery.
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