A Power Compensation Strategy for Achieving Homogeneous Microstructures for 4D Printing Shape-Adaptive PNIPAM Hydrogels: Out-of-Plane Variations

Abstract

In the last decade, 3D printing has attracted significant attention and has resulted in benefits to many research areas. Advances in 3D printing with smart materials at the microscale, such as hydrogels and liquid crystalline polymers, have enabled 4D printing and various applications in microrobots, micro-actuators, and tissue engineering. However, the material absorption of the laser power and the aberrations of the laser light spot will introduce a decay in the polymerization degree along the height direction, and the solution to this problem has not been reported yet. In this paper, a compensation strategy for the laser power is proposed to achieve homogeneous and high aspect ratio hydrogel structures at the microscale along the out-of-plane direction. Linear approximations for the power decay curve are adopted for height steps, discretizing the final high aspect ratio structures. The strategy is achieved experimentally with hydrogel structures fabricated by two-photon polymerization. Moreover, characterizations have been conducted to verify the homogeneity of the printed microstructures. Finally, the saturation of material property is investigated by an indirect 3D deformation method. The proposed strategy is proved to be effective and can be explored for other hydrogel materials showing significant deformation. Furthermore, the strategy for out-of-plane variations provides a critical technique to achieve 4D-printed homogeneous shape-adaptive hydrogels for further applications.

Four-Dimensional Micro/Nanorobots via Laser Photochemical Synthesis towards the Molecular Scale

“…Many studies have successfully proved several stimuli-responsive smart materials for micro/nano 4D printing [ 25 , 33 , 37 , 38 ] ( Figure 2 b). Hydrogel [ 39 , 40 , 41 , 42 , 43 ] is a typical kind of functional material with a crosslinking three-dimensional network [ 18 ], and it is hydrophilic and insoluble in solvents. Especially, our recent work report an amphiphilic hydrogel containing both hydrophilic and hydrophobic groups [ 12 ], making the micro/nano 4D products respond to various stimuli rather than simple humidity.…”

“…Using these advantages, the cross-linked network can generate responsiveness under small loading of environmental stimuli, such as temperature, magnetic field, electric field, or multiple stimuli. The intelligent hydrogel networks usually present a mesoporous or fibrous network with unfoldable space to absorb or desorb the solvent and produce swelling or shrinkage, the recent work shows the swelling/shrinkage volume change ratio easily exceeds 200% [ 37 , 38 , 39 , 40 , 41 , 42 ]. Due to hydrogel’s desirable biocompatibility [ 47 ] and mechanical properties, micro/nano 4D hydrogel can be further combined with in vitro cell culture, vascular dredging [ 48 ], or be used to emulate the cargo/reorganization of organisms [ 49 ].…”

“…Numerous current works emphasize the programming strategies of 4DM/NR biomimetic function [ 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 ], allowing the artificially made 4DM/NRs to realize high-level motions beyond nature. To spatiotemporally program reverse 4D behaviors, the shape-programming strategy is crucially important and not inferior to structural design or smart materials.…”

“…In dual/multi-layer design [ 79 , 80 ], shape-morphing happens by using different materials, forming a soft–rigid structure, where the rigid layer works as a skeleton, and the soft layer works as a muscle [ 81 ] to generate direction bending. However, according to existing analyses [ 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 79 , 80 , 81 , 82 , 83 , 84 , 85 , 86 , 87 , 88 , 89 , 90 , 91 , 92 , 93 , 94 , 95 , 96 , 97 , 98 , 99 , 100 , 101 , 102 , 103 , 104 , 105 , 106 , 107 , 108 , 109 , 110 ], the mechanical mismatch between multiple layers is a pitfall for repeated shape-morphing in actual usage.…”

“…The increase in environmental temperature or under the irradiation of light causes liquid crystal molecules to change from a liquid crystal state [58] to an isotropic state, resulting in thermally induced defor- Many studies have successfully proved several stimuli-responsive smart materials for micro/nano 4D printing [25,33,37,38] (Figure 2b). Hydrogel [39][40][41][42][43] is a typical kind of functional material with a crosslinking three-dimensional network [18], and it is hydrophilic and insoluble in solvents. Especially, our recent work report an amphiphilic hydrogel containing both hydrophilic and hydrophobic groups [12], making the micro/nano 4D products respond to various stimuli rather than simple humidity.…”