Intelligent micromachines, which autonomously change their morphological conformations in response to various external stimuli, such as pH, temperature, and magnetism, have long been considered to have a substantial impact on emerging fields, such as bionics, drug delivery, tissue engineering, artificial muscle, nanomotors, and micro/nano sensing and actuation. Among various types of external stimuli-responsiveness, the light-responsiveness has many unique advantages due to its excellent intrinsic properties, such as remotely and precisely driven capability and multidimensional light field modulation (such as wavelength, frequency, intensity, polarization state, and spatiotemporal distribution of energy), which is regarded as one of the most promising and indispensable measures for the implementation of intelligent micromachines for various applications.However, artificial light-driven intelligent micromachines with a low actuation threshold, rapid responsiveness, and designable and precise 3D transformation capability remain unachievable to date.
Recently, the research team led by Prof. Wei Xiong from Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology,proposes a single-material and one-step 4D printing strategy for the additive nanomanufacturing of agile and programmable light-responsive intelligent micromachines with a precise 3D shape-morphing capability.The success of the strategy relies on two advances, including the development of carbon nanotube-doped N-isopropylacrylamide (NIPAM) composite (CNNC) smart hydrogels and the structural design and assembly of asymmetric mechanical metamaterial unit cells for large and precise 3D shape-morphing. The homogeneous doping of single-walled carbon nanotubes (SWNTs) inside NIPAM simultaneously increases the light absorption coefficient, thermal conductivity, and mechanical modulus of the crosslinked network of the composite hydrogel, hence resolving the dilemma of the well-known material trade-off between the mechanical modulus and the response sensitivity of stimuli-responsive hydrogels. Moreover, they design and assemble microscale mechanical metamaterial unit cells to achieve the one-step-one-parameter femtosecond laser direct writing (FsLDW) construction of a heterogeneous smart metamaterial with a rigid outer frame and a weakly cross-linked soft inner gel. The one-step-one-parameter FsLDW printing method not only possesses a nearly arbitrary complex 3D structuring capability and excellent shape fidelity but also enables programmable and precise 3D shape-morphing functionality with a large dynamic range.The proposed 4D printing strategy open an avenue for the development of advanced intelligent micromachines which have prosperous applications in biomedicine, bionic structures, and soft robotics.
Fig. 1 Schematic of the single-material and single-step 4D printing method.
Prof. Xiong’s group developed and verified that micromachines assembled by different unit cell structures can fully exploit the mechanical effects and properties of voxels at the micro- and nano-scale to achieve anisotropic actuation.As compared with the results of the cube composed of an array of buckyballs, and the other is a solid cube with the same dimensions. Both structures have the same processing laser fluence and slicing parameters during FsLDW. Notably, the buckyball with a certain spatial structure produced a variable layer spacing of 400 nm to 2.4 µm due to the 3D hollow microstructure, providing attachment points and spaces for the weakly cross-linked flexible CNNC hydrogel enveloped by a highly cross-linked external frame. Consequently, the as-written composite structure with a rigid frame of buckyball mechanical metamaterial and a flexible filling core of weakly cross-linked CNNC hydrogel exhibited an excellent shape-fidelity in aqueous environment. While, in comparison, the same cubic structure made by densely packed voxels just exhibited a small degree of structural deformation even after swelling in water. Moreover, the two typical cubic structures exhibited clearly different shrinkage behaviors upon the light stimulation.
Fig 2 The shrinkage performance of different unit cell structures with the sameFsLDWparameters
Based on this 4D printing strategy,researchershave printed 3D microclamps, unidirectional microheart valves, and a beating micro-heart, and revealed their rapid and reliable light-stimuli responsiveness and a spatiotemporal controllability in shape transformation upon light stimulation. This laser-based 4D printing approach can be also applied to fabricate various intelligent hydrogel-based micromachines that are responsive to many stimuli other than light, providing a large design freedom and manufacturing convenience for the development of advanced intelligent micromachines in various fields, such as biomedicine, tissue engineering, and micro/nano integrated systems.
Fig. 3 Functional demonstration of intelligent micromachines upon light stimulation.
This research work has been recently published on journalAdvanced Functional Materials. Ph.D. students Chunsan Deng and Yuncheng Liu of Wuhan National Laboratory for Optoelectronics are the co-first authors of the paper, and Prof. Wei Xiong is the corresponding author. The research affiliation are Huazhong University of Science and Technology and Hubei Optics Valley Laboratory. The work was supported by theNational Key R&D Program of China, National Natural Science Foundation of China, Innovation research project of Optics Valley Laboratory and Knowledge Innovation Program of Wuhan-Shuguang.
Article link: https://doi.org/10.1002/adfm.202211473