Robotics is the science of designing and constructing machines capable of movement, perception, and cognition to assist humans in performing various tasks. Inspired by living organisms, using soft matter in robot design has gained significant attention in recent decades. The inherent compliance of soft bodies allows them to adapt to complex environments, enabling innovative applications in fields such as healthcare, agriculture, and the food industry [1-10]. Given the potential of soft robots, various functional materials, such as liquid crystal elastomers, pneumatic actuators, and light-driven systems, have been explored as actuators due to their ability to deform in response to diverse external stimuli. However, the intrinsic compliance and nonlinearity of soft materials pose significant challenges in achieving precise and effective deformation control, which limits their practical effectiveness in real-world applications. A widely adopted approach to addressing this challenge is using bilayer structures in soft robot design. Inspired by natural phenomena such as the opening of pea pods, a bilayer structure consists of two layers--an top and a bottom layer--adhered at their interface [11], as illustrated in Figure 1A. When one layer undergoes expansion, a mismatch strain arises at the interface.

Microrobots show great potential in biomedical applications such as drug delivery and cell manipulations. However, current microrobots are mostly fabricated as a single entity and type and the tasks they can perform are limited. In this paper, modular microrobots, with an overall size of 120 $\mu$m $\times$ 200 $\mu$m, are proposed with responsive mating components, made from stimuli-responsive hydrogels, and application specific end-effectors for microassembly tasks. The modular microrobots are fabricated based on photolithography and two-photon polymerization together or separately. Two types of modular microrobots are created based on the location of the responsive mating component. The first type of modular microrobot has a mating component that can shrink upon stimulation while the second type has a double bilayer structure that can realize an open and close motion. The exchange of end-effectors with an identical actuation base is demonstrated for both types of microrobots. Finally, different manipulation tasks are performed with different types of end-effectors.