Femtosecond laser direct writing is a microfabrication technology that focuses a pulsed laser beam on the surface or interior of a material, causing changes in material propeties. The unique processing of femtosecond-laser direct-writing technology enables the processing and preparation of arbitrarily shaped three-dimensional structures. Hydrogels can be regarded as soft materials in liquid environments because of their special molecular morphology and conformational changes. In response to external environmental stimuli, they have the ability to deform and are widely used in the preparation of micro-actuators. Hydrogels can respond to two common stimuli: pH and temperature. However, at the micro and nano scales, the actuators prepared by these two hydrogel materials have common problems, such as poor mechanical strength and simple morphological structure. Therefore, researchers have begun to consider combining the advantages of hard and soft materials to develop a micro-actuator. The pH hydrogels require changes in the pH value of the liquid environment, thereby complicating their use in composite micro-actuators in fields such as cell harvesting and biomedicine. Therefore, to cover more scenarios, hard materials should be effectively combined with thermosensitive hydrogels.

In this study, a strategy for the fabrication of thermosensitive micro-actuators is proposed based on femtosecond laser printing, using hard photoresist and soft thermosensitive hydrogel materials, thereby combining the advantages of high mechanical strength of hard materials and stimulated deformation of soft materials. The photoresist used in the experiment is a hard material, almost unaffected by the ambient temperature after processing and molding. The other material, thermosensitive hydrogel, is a soft material that is affected by ambient temperature, shrinking in volume when the critical temperature is exceeded and returning to its original state below the critical temperature. Using the programmability and ultra-high precision of femtosecond laser printing, the photoresist and hydrogel structures are processed step-by-step, whereby the deformation force generated by the hydrogel shrinkage causes the photoresist to deform synchronously. Therefore, the temperature can be reversed to drive the micro-actuators. The hydrogel required additional formulation according to materials and dosage, as follows: 400 mg N-isopropylacrylamide (NIPAM) as monomer, 30 mg diphenyl (2, 4, 6-trimethylbenzoyl) phosphine oxide (TPO) as photoinitiator, 30 mg N, N′-methylenebisacrylamide (MBA) as crosslinking agent, and 450 μL ethylene glycol as solvent. During preparation, several materials are mixed, heated in a water bath, and sonicated until completely dissolved, finally adding 50 mg polyvinylpyrrolidone (PVP) as a thickener.

We prepare a bimaterial microvalve consisting of four identical thermosensitive hydrogels and photoresist sheets, as shown in Fig 6. For ambient temperatures lower than the critical deformation temperature of the thermosensitive hydrogel, the gap in the middle of the valve is the smallest, an approximate square with a side length of 11.4 μm, such that the valve is considered to be closed. By adding hot deionized water, the temperature of the surrounding solution is instantly raised, whereby the hydrogel shrinks and the photoresist sheet is bent simultaneously, and the square side length in the middle of the valve changes from 11.4 μm to 21.4 μm, which is regarded as the valve being open; the valve area is about 3.5 times that of the valve in the closed state. In addition, inspired by the defense and movement of shells in nature, we prepare a bionic shell actuator at micro and nano scales with a hard photoresist as the shell and a soft hydrogel as the muscle, using it to complete the capture of particles, as shown in Fig 7. In the experiment, a silica microsphere with a diameter of 20 μm is successfully captured using the reproducible open and closed characteristics of the bionic shell. Both micro-actuators are characterized by fast response, excellent mechanical properties, and reusability. The strategy of composite processing of thermosensitive hydrogels and photoresists provides a new idea for the preparation of actuators for microfluidics and particle capture.

In this study, based on a femtosecond laser high-precision three-dimensional processing system, a strategy is proposed for the development of micro-actuators combining photoresist and thermosensitive hydrogels with two different properties. The system is successfully applied to microvalves and particulate capture devices. The prepared microstructured devices are in the micron range and display good mechanical properties, fast response speed, and high programmability. The opening and closing of temperature-controlled microvalves can increase the channel area by up to 3.5 times, which can be used for fluid flow control or particle screening. In the experiment, the bionic shell device uses hard photoresist as the shell and soft hydrogel as the muscle, to imitate the opening and closing of natural shells, thereby successfully capturing silica microspheres with a diameter of 20 μm. The bimaterial micro-actuator fabrication method proposed in this study can be widely used in microfluidic systems, particle capture, and other fields, thereby providing new ideas for future applications. However, owing to the influence of the deformation mechanism of hydrogels, the micro-actuators proposed in this paper can only work in a liquid environment. Therefore, in the future, we can continue to explore alternative materials for thermosensitive hydrogels, such as liquid crystal elastomers and air hydrogels, to expand the application of bimaterial micro-actuators.