Microactuators, which can transform external stimuli into mechanical motion at microscale, have attracted extensive attention because they can be used to construct microelectromechanical systems (MEMS) and/or microrobots, resulting in extensive applications in a large number of fields such as noninvasive surgery, targeted delivery, and biomedical machines. In contrast to classical 2D MEMS devices, 3D microactuators provide a new platform for the research of stimuli-responsive functional devices. However, traditional planar processing techniques based on photolithography are inadequate in the construction of 3D microstructures. To solve this issue, researchers have proposed many strategies, among which 3D laser printing is becoming a prospective technique to create smart devices at the microscale because of its versatility, adjustability, and flexibility. Here, we review the recent progress in stimulus-responsive 3D microactuators fabricated with 3D laser printing depending on different stimuli. Then, an outlook of the design, fabrication, control, and applications of 3D laser-printed microactuators is propounded with the goal of providing a reference for related research.

Directed droplet manipulation is paramount in various applications, including chemical micro-reaction and biomedical analysis. The existing strategies include some kinds of gradients (structure, inherent wettability, and charge density), whereas they suffer from several limitations, such as low velocity, limited volume range, poor durability, and inefficient environmental suitability. Moreover, active bi-directional reversal of omni-droplets remains challenging because one kind of microstructure at a single scale cannot acquire two kinds of net results of mechanical interaction. Herein, we report an active and directional steering of omni-droplets utilizing bi-directional (vertical and horizontal) vibration on slippery cross-scale structures consisting of macro millimeter-scale circular arc arrays and micro/nanometer-scale slant ratchet arrays, which are fabricated by femtosecond laser patterned oblique etching and lubricant infusion. The physical mechanism of active droplet steering lies in the relative competition between the forces under vertical and horizontal vibration, which mainly arise from the circular arc arrays and slant ratchet arrays, respectively. Various steering modes, including climbing and programmable manipulation, can be realized. Our work is applicable to a wide range of potential applications, including circuit on/off and droplet-based chemical micro-reaction, particularly in the field of high-throughput omni-droplets operation.

Microcrystals with defined sizes, morphologies, and locations provide a core opportunity for applications in microelectronic devices and microoptical components. Herein, a simple method was developed to obtain microcrystals with controlled sizes and locations by capillary-assisted localized crystallization, which utilizes “discrete micropillar rings” (DMRs) to mediate the solution evaporation process. Being different from conventional hydrophilic/hydrophobic modified surfaces, DMRs can guide the movement of three-phase contact lines, confine the solution residing inside the rings with a high concentration ratio, and help to crystallize microparticles (NaCl or CaCO₃) without any further surface modification. Microcrystal sizes can be tuned in a wide range (from the order of ∼μm² to that of ∼100 μm²) by changing the geometric parameters (height, pillar interval, and diameter) of pillar arrays. This facile, scalable, and low-cost technique for generating microcrystals with controlled size and location inside the DMRs holds great promise for micro-electronic and micro-optoelectronic applications.