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International Journal of Bioprinting 4D heterojunction shape reconfiguration by two-photon polymerization

curvature by releasing turgor across the leaf . Antheraea and as obtained, became enabled to trigger a series
[3]
pernyi silk fiber contracts and combines amorphous of versatile high-freedom 4D behaviors at nanometer
silk fibers, which spin into crystallization for tough silk accuracy.
fibers. Shape-reconfigurable materials have inspired Herein, triallyl isocyanurate (TAIC), a biocompatible
intensive effort in bioscience , automatic robots/organ crosslinking agent, strengthens the polymeric
[4]
regeneration , self-deployed components/actuators [6,7] , bonding between extender N-isopropylacrylamide
[5]
and even self-adaptive optics/photonics without joints or (NIPAM) [34,35] and polyethylene glycol-diacrylate
[8]
hinges. The self-driven structural reorganization becomes a (PEG-DA) [24,33] (Figure S1) due to their desirable
new frontier of current four-dimensional (4D) prototyping miscibility. The monolayer nanostructured interactive
and manufacturing. hydrogels (MNIHs) were a composite network of NIPAM/
Soft-interactive hydrogels have been regarded as PEG showing amphiphilicity, enhanced responsiveness,
[9]
the most promising self-driven robotics , which render and desirable structural integrity. A cationic photoinitiator
[10]
stationary three-dimensional (3D) structures with (methylene blue , MB, a derivative of phenothiazine with
[36]
[37]
morphological mutation by small loads of stimuli, a high photon absorption ratio ) triggers the confined
[11]
possessing desirable ductility, biocompatibility, optical two-photon polymerization (TPP) in a sub-micrometer
[27]
clarity, lightweight, and easy synthesis (Table S1) [12-16] . laser-focus voxel (Videoclip S1) at tunable optical
To date, our literature review confirms several pioneered parameters (Table S2). MNIHs featured hierarchical
hydrogel artificial grippers , self-perspiring actuators [13,14] , polymeric degrees, succeeding in harnessing tunable
[12]
deployable cages or scaffolds [15,16] , drug delivery , tissue mechanics [38,39] at high-order directions much better than
[17]
regeneration , adaptive optics/photonics , simple the reported isotropic PNIPAM hydrogels. Our home-built
[8]
[5]
microrobot collector , and soft machines [20,21] . However, mask-free transfer-free fsLDW system [40,41] fully utilized
[19]
there are some intrinsic bottlenecks of massively applying the hydrogel’s properties [42-45] owing to higher-complexity
hydrogels originating from their sluggish expansion shape morphing function at ultrafine resolution. fsLDW-
or poor controllability in direction/stiffness [22,23] due to fabricated MNIHs exhibited self-driven high freedom
the isotropic shape reconfiguration. Macroscopic 4D contraction, torsional twisting, reverse distortion, large-
biomimetic hydrogels using inks are still at an early stage angle rotations, directional gripping, and structural
and are physically inaccessible to microenvironments of coloration, and allowed researchers to select stimuli.
lab-on-chip devices , microbiological tissues (blood Some physically-damaged MNIHs recovered themselves
[24]
vessel, capillary, or artery) , or micro/nano (μ/n) like having self-consciousness, ensuring long-term use
[25]
electromechanical systems because of bulky volume and for extreme conditions. To simplify the discussion, all
poor volumetric programming resolution. Therefore, it is hydrogels at μ/n footprint were rapidly prepared using
strategically important for miniaturizing the self-driven 4D fsLDW, and referred to as MNIH.
hydrogels at high resolution and high shape controllability
to develop embedded bio-applications. 2. Materials and methods
The last decade witnessed significant strides in two- 2.1. Precursor preparation
photon polymerization (TPP) [26,27] toward on-demand As illustrated in Figure S1, NIPAM (C H NO, 0.6696 mg,
11
6
4D μ/n applications. Despite the progress, high-freedom 99.99+% purity, Mw = 113.16, extender), PEG-DA
shape reconfigurations remain a formidable prerequisite ((C H O).(C H O)n.(C H O), 1.8 mL, 99.999+% purity,
3
3
4
3
3
2
for practical robotics. The intensively-used stress-releasing Mw = 700, crosslinker) were added to potassium persulfate
method generates only unidirectional deformations (K S O , 0.001 mg, Mw = 270.32) for a room-condition
[28]
2 2
8
without resilience. TPP fabrication of compound pre-polymerization. Subsequently, TAIC (C H N O ,
3
3
15
12
machines [15,18] necessitates tedious preparation/scanning Mw = 249.27, a crosslinking agent, 0.04 mg) was ground and
steps, increasing time/material cost to hinder producibility. added into the mixture with MC (C H ClN S), and then
18
16
3
Unfortunately, weak interfacial coupling between multi- vortexed and sonicated to homogenous precursor; sediment
layers leads to mechanical mismatch and low strengths. was removed before fsLDW (Figure S1). The precursor-
To substantially overcome these limitations, we upgrade containing materials were obtained commercially and
the shape controllability (Figure 1) by monolayer used without further purification.
heterojunction scanning for biomimetic reversible multi-
dimensional shape reconfiguration. Here, the conventional 2.2. Morphological characterization
femtosecond laser direct writing (fsLDW) procedure was The indium tin oxide (ITO)-coated glasses operated as the
facilitated by incorporating hydrogel-nature materials [29-33] , substrates for the as-fabricated MNIHs, which provided a
Volume 9 Issue 3 (2023) 15 https://doi.org/10.18063/ijb.678