Poster M16 - 4D-Printable Photocrosslinkable Polyurethane-Based Inks for Biomanufacturing

Introduction: 4D printing has transformed conventional 3D printing, where a printed construct can reconfigure in response to a specific stimulus. We introduce a digital light processing (DLP) ink based on polyethylene–glycol polyurethane for light-assisted 4D printing of tissue scaffolds and smart actuators. We synthesize a hydrophilic copolymer with tunable molecular weight. A practical strategy is mixing different monomers. We fine-tune the molecular weight without compromising water solubility, printability, and printing resolution. The ink demonstrates exceptional biocompatibility, making it ideal for biomanufacturing. We embed bioactive glass particles into the ink to enhance the biocompatibility of the printed smart structures. The ink can be used in tissue engineering and actuator applications, making the ink a promising avenue for future research.

Materials and

Methods: Isopropanol (IPA) was used as a washing reagent. 1,6-hexamethylene diisocyanate (HDI) was used as a crosslinker, hydroxyethyl acrylate (HEA) was used as a photocrosslinking agent, polyethylene-glycol (PEG) was used as a monomer, dibutyltin dilaurate (DBTDL) was used as a catalyst, Comarin 6 was used as a florescent dye, Ponceau 4R food dye was used as a photo-absorber, lithium phenyl-2,4,6-trimethyl-benzoyl-phosphinate (LAP) was used as photoinitiator and dimethylformamide (DMF) was used during synthesis. PEG-polyurethane (PU) copolymers were synthesized through a two-step polymerization technique. The ink, photo-initiator, and additives such as photo absorber, microparticles, and fluorescent additives were dissolved in a selective buffer for printing. The ink was prepared by mixing synthesized PEG-PU-DA (having 15 wt% copolymer), with water, stirring for 5 min till reaching a transparent solution of low viscosity ( < 5 Pa.s). The ink was mixed with LAP for a custom-built DLP printer that allowed printing at a very high-intensity light (700 mW/cm2 at 380 nm, Figure 1).

Rectangular samples (1.5 × 1 mm) of thickness 100 µm were fabricated at different light exposure times upon a single-step crosslinking (Figure 2). The thickness of the samples was controlled by limiting the cure depth by keeping the substrate and the build platform at a gap of around 100 µm. The samples were dried in a vacuum oven and immersed in water. Finally, for the actuator demonstration, a simple rectangular structure was produced by printing two layers, each 100 µm thick, using low-intensity light (2 mW cm−2, Figure 3).

Results, Conclusions, and Discussions: The concept of 4D shape transformation of a DLP printed film is illustrated in Figure 1. Post-printing process, the crosslinked gel is dried (Figure 1a-ii) and then immersed in water, resulting in inhomogeneous swelling (Figure 1a-iii). Upon drying, the gel reverts to its initial 2D structure. The idea of our approach is inspired by shape transformation observed in plants. Figure 1b shows an analogy to a similar behavior in the leaves of the Mimosa–pudica plant. Water is removed from the leaf base upon exposure to an external stimulus such as finger proximity. The mechanism forms a gradient water distribution, leading to leaf folding in response, making them flaccid (Figure 1b-ii). Once the stimulus is removed, the leaf gradually restores its initial water distribution, eliminating the gradient and returning the fresh leaves to their original shape (Figure 1b-iii).

We demonstrated the fabrication of solid and porous tubular scaffolds with pH-responsive properties in Figure 2. After washing and drying the resulting flat rectangular hydrogels, they were immersed in water, leading to a remarkable self-folding response (Figure 2a). Using our approach, we created high-resolution and robust tubular scaffolds with diameters as low as 190 µm diameter. The films were printed, washed, dried, and immersed in PBS to induce self-folding and form porous tubular scaffolds (Figure 2b). In the DLP printed patterns in Figure 3, we observed a transient shape transformation started with folding (toward the 2nd) and subsequent structure unfolding (Figure 3a-b). This sequential folding–unfolding results from different swelling kinetics between the 1st (less crosslinked) and 2nd (more crosslinked) layers, caused by variations in their different crosslinking densities. The different crosslinking times applied during printing lead to distinct crosslink densities, diffusion rates, and relaxation times between the 1st and 2nd layers. The time-lapse images in Figure 3a-b illustrate the folding and unfolding processes over time. The closure and opening of these structures occurred on different timescales. We also investigated the sensitivity of our printed films to moisture (Figure 3c).

In conclusion, we synthesized hydrophilic PEG-based polyurethane copolymers for light-assisted printing, where we tuned molecular weight distribution by adjusting the copolymerization reaction time.

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