Professor McGill University Montreal, Quebec, Canada
Introduction: Granular hydrogels have recently garnered significant attention in tissue engineering and bioprinting for their shear-thinning and self-healing properties, as well as their tunable porosity. These hydrogels, comprising microgel particles larger than 10 microns, primarily depend on frictional interactions between the particles for structural integrity. Traditional approaches to enhance these interactions have involved complex methods like interparticle cross-linking, which adds complexity to the fabrication process, or embedding the microgels in a hydrogel matrix, which can compromise their intrinsic porosity. In this study, we enhanced the mechanical stability and integrity of these hydrogels using a leaching technique that increased the surface roughness and porosity of PEGDA/PEG microgels, thereby improving their frictional interactions. This technique involved fabricating microgels with a mean diameter of 80 microns using a water-in-oil batch emulsion method, followed by a leaching process to remove PEG from the microgel structure.
Materials and
Methods: To fabricate the microgels with increased surface roughness (Figure 1), we synthesized a solution with the following composition: 10% w/v poly(ethylene glycol) diacrylate (PEGDA), 5% w/v poly(ethylene glycol) (PEG), and 0.4% w/v lithium phenyl-2,4,6-trimethylbenzoylphosphinate (LAP). Microgels with a mean diameter of 80 microns were then fabricated using a water-in-oil (W/O) batch emulsion technique, with the oil phase composed of light mineral oil and 2% v/v Span80. Photopolymerization was conducted at a wavelength of 405 nm for 5 minutes. The microgels, suspended in the oil phase, were separated by centrifugation at 4400 RPM for 10 minutes. To remove any residual oil droplets, the microgels were washed with a PBS solution containing 0.1% v/v Tween20. This washing process was repeated with pure PBS, followed by centrifugation at 4400 RPM for 10 minutes, and repeated three times. The microgels were then immersed in PBS, with the solution being replaced once a day for two days. This process facilitated the leaching of PEG from the microgel structure, introducing microporosity and increasing surface roughness. To achieve jamming of the microgels, the microgel hydrogels underwent centrifugation at 14100 RCF for 10 minutes, and the supernatant was carefully aspirated.
Results, Conclusions, and Discussions: This increased the microgels' surface roughness and introduced microporosity, essential for enhancing frictional interactions. To compare the effects of leached PEG, the control group of microgels lacking PEG was fabricated under similar conditions. To evaluate the impact of enhanced surface roughness on hydrogel properties, comprehensive rheological studies were conducted. These studies highlighted a 200% increase in initial viscosity at low shear rates, from 235 to 705 Pa·s, while preserving their shear-thinning behavior (Figure 2. A). Additionally, the hydrogels demonstrated self-healing capabilities, while showing 100% increase in storage modulus during low cyclic strain cycles (Figure 2. B). The yield strain remained stable at approximately 20%, with only a slight increase in the PEG-leached samples (Figure 2. C). This data suggests that in static conditions, the augmented porosity and roughness facilitate interlocking of the microgels, thereby boosting friction and viscosity. Conversely, under dynamic conditions with increased strain, these interlocked states dissipate, allowing the hydrogel to revert to its shear-thinning properties. Injectability and printability tests were performed using a 27G needle, focusing on the hydrogel's ability to form fibers rather than droplets upon injection. The tests also assessed the hydrogel’s capability to stack layers without merging, by measuring the contact angle between layers. Results showed that both sample groups formed fibers. However, the hydrogel with leached PEG produced thinner fibers and demonstrated a lower contact angle between layers when injected perpendicularly (Figure 3), suggesting improved mechanical stability and enhanced frictional interactions. This enhancement likely contributes to better shape stability and prevents layer merging under gravitational forces. Overall, the leaching technique effectively increased the frictional interactions between microgels by enhancing their surface roughness, leading to improved mechanical stability and integrity without compromising the inherent properties of granular hydrogels.
