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International Journal of Bioprinting 3D-printed thermosensitive hydrogel based microrobots

In recent decades, scientists have devoted themselves with zeolitic imidazolate framework-8 (Fe@ZIF-8), the
to the development of thermosensitive hydrogels that thermosensitive PNAGA-based microstructures can
exhibit swelling properties at temperatures close to the achieve locomotion with the control of a magnetic field,
human body. Due to the lack of preparation technology acting as microrobots in water, which can be potentially
and theoretical support [20-22] , so far, only a few hydrogels applied in drug delivery and object transportation.
with rapid swelling performance near body temperature
have been successfully developed. Among them, Sun 2. Materials and methods
et al. reported poly-N-acryloyl glycinamide (PNAGA)-
[16]
type thermosensitive hydrogels, which produced obvious 2.1. Materials
swelling effect at 45°C, and PNAGA composites have been N-acryloyl glycinamide (NAGA; purity: 98%) was
widely used in tissue engineering [23,24] , gripper [16,25] , cancer purchased from Alfa (Zhengzhou, China), and water-
therapy , etc. soluble two-photon photo-initiator P2CK and Fe@ZIF-8
[26]
were prepared based on the protocols described in a research
However, the fabrication process of PNAGA is paper . Crosslinker poly (ethylene glycol) diacrylate
[38]
mainly based on the ultraviolet (UV) polymerization (PEGDA-575; 99.5%) was purchased from Sigma-Aldrich
method [16,26-29] ; therefore, it is not easy to obtain PNAGA- (Shanghai, China). N,N′-methylene bisacrylamide
based 3D structures. This issue could be solved by (MBAA, purity: 99%), ethanol and isopropanol (purity:
introducing an advanced 3D printing technology called 98%) were purchased from Energy Chemical (Anhui,
two-photon polymerization (2PP) as the fabrication method China). All reagents were used directly without further
of PNAGA. 2PP, one of the most versatile and precise 3D purification. Deionized (DI) water from ultra-pure water
printing techniques, uses models generated by computer- machine was utilized throughout the experiments.
aided design (CAD) tools to fabricate 3D materials with
sub-100 nm resolution , manifesting great advantages 2.2. Instruments and measurements
[30]
for fabricating PNAGA based hydrogels. First, 2PP utilizes The 2PP 3D printing of PNAGA was conducted by
femtosecond laser pulses of a near-infrared (NIR) laser Nanoscribe GmbH. The micro-level Fourier-transform
beam to provide a nonlinear energy distribution at the infrared spectroscopy (FTIR) spectrum of PNAGA-100
center of the laser’s focal point, resulting in the formation of was obtained using spotlight 200i (PerkinElmer FTIR
aggregated 3D micro/nanostructures . Second, 2PP does microscopy). The surface morphologies of these
[31]
not require harsh processing environments (such as deep thermosensitive hydrogels were examined using Apreo2
UV exposure or reactive ion etching) and harsh synthesis S Lovac. Elastic modulus and energy storage modulus
conditions (strong oxidants, toxic metal catalysts) . of PNAGA hydrogels were recorded by Discovery Series
[32]
Third, 2PP 3D technology can fabricate parts with complex Hybrid Rheometer (DHR; TA Instruments, New Castle,
geometries in a relatively short period of time, exhibiting USA). The swimming experiments of PNAGA-based
the advantages of unprecedented precision, flexibility, high magnetic microrobots were conducted using Magnebotix
processing quality, and easy functionalization, and thus, (MFG-100) and a self-built Helmholtz coil magnetic
this technology has been widely used in manufacturing control system. The 3D-printed microstructures were
research on the regeneration of biological microdevices examined by inverted fluorescence microscopy IX73P2F.
and tissues such as bone, muscle, skin, and neurons, as well The doxorubicin (DOX) release was determined by ELISA.
as organs such as trachea, liver, kidney, and heart [33-37] .
2.3. Thermosensitive measurement of 3D-printed
Given the various advantages of 2PP 3D printing PNAGA microstructures
technology and the urgent need for enriching the synthetic PNAGA hydrogel-based microstructures were fabricated
approaches of PNAGA-based hydrogels to endow them according to different recipes (Table 1) using 3D Direct
with wider applications, we first report micro-sized Laser lithography (Nanoscribe GmbH). In detail, NAGA
PNAGA thermosensitive hydrogels prepared by 2PP 3D (50 mg; 100 mg; 200 mg; 300 mg), PEGDA-575 (200 μL)
printing technology in this work. Our PNAGA hydrogels and P2CK (1 mg) were dissolved in 0.8 mL DI water,
exhibit better swelling performance at 45°C than that at respectively. The mixture was stirred at room temperature
room temperature, at which PNAGA-100 microstructure for 10 min. Then, 15 μL mixture buffer was spin-coated on
manifests the best and fast swelling property with a the glass substrate. The precursor solution was dipped in
growth rate of 22.5%. Particularly, PNAGA-100-based a round gasket on the glass and printed via 2PP (780 nm).
thermosensitive microstructures show higher drug release Purified 3D structures were observed after developing in
efficiency at 45°C (close to body temperature) than room DI water three times for 5 min each time. The sizes of these
temperature, indicating that they are promising candidates PNAGA microstructures for thermosensitive measurement
for drug release in the human body. After being decorated are 3D cubic structures with a side length of 100 μm and a

Volume 9 Issue 3 (2023) 273 https://doi.org/10.18063/ijb.709