Aishling Dunne

Thiol-Ene Photo-Click Collagen-PEG Hydrogels: Impact of Water-Soluble Photoinitiators on Cell Viability, Gelation Kinetics and Rheological Properties

Thiol-ene photo-click hydrogels were prepared via step-growth polymerisation using thiol-functionalised type-I collagen and 8-arm poly(ethylene glycol) norbornene-terminated (PEG-NB), as a potential injectable regenerative device. Type-I collagen was thiol-functionalised by a ring opening reaction with 2-iminothiolane (2IT), whereby up to 80 Abs. % functionalisation and 90 RPN% triple helical preservation were recorded via 2,4,6-Trinitrobenzenesulfonic acid (TNBS) colorimetric assay and circular dichroism (CD). Type, i.e., either 2-Hydroxy-1-[4-(2-hydroxyethoxy) phenyl]-2-methyl-1-propanone (I2959) or lithium phenyl-2,4,6-trimethylbenzoylphosphinate (LAP), and concentration of photoinitiator were varied to ensure minimal photoinitiator-induced cytotoxicity and to enable thiol-ene network formation of collagen-PEG mixtures. The viability of G292 cells following 24 h culture in photoinitiator-supplemented media was largely affected by the photoinitiator concentration, with I2959-supplemented media observed to induce higher toxic response (0.1 → 0.5% (w/v) I2959, cell survival: 62 → 2 Abs. %) compared to LAP-supplemented media (cell survival: 86 → 8 Abs. %). In line with the in vitro study, selected photoinitiator concentrations were used to prepare thiol-ene photo-click hydrogels. Gelation kinetics proved to be largely affected by the specific photoinitiator, with LAP-containing thiol-ene mixtures leading to significantly reduced complete gelation time (τ: 187 s) with respect to I2959-containing mixtures (τ: 1683 s). Other than the specific photoinitiator, the photoinitiator concentration was key to adjusting the hydrogel storage modulus (G’), whereby 15-fold G’ increase (232 → 3360 Pa) was observed in samples prepared with 0.5% (w/v) compared to 0.1% (w/v) LAP. Further thiol-ene formulations with 0.5% (w/v) LAP and varied content of PEG-NB were tested to prepare photo-click hydrogels with porous architecture, as well as tunable storage modulus (G’: 540–4810 Pa), gelation time (τ: 73–300 s) and swelling ratio (SR: 1530–2840 wt %). The photoinitiator-gelation-cytotoxicity relationships established in this study will be instrumental to the design of orthogonal collagen-based niches for regenerative medicine.

Solvato-morphologically controlled, reversible NIPAAm hydrogel photoactuators

Photo-actuator hydrogels were generated using a N-isopropylacrylamide-co-acrylated spiropyran-co-acrylic acid (p(NIPAAm-co-SP-co-AA)) copolymer, in 100–1–5 mole ratio. Different ratios of deionised water : organic solvent (tetrahydrofuran, dioxane and acetone) were used as the polymerisation solvent. By changing the polymerisation solvent, the pore size and density of the hydrogels were altered, which in turn had an impact on the diffusion path-length of water molecules, thus influencing the swelling and photo-induced shrinking kinetics of the hydrogel. We successfully demonstrated that the polymerisation solvent has a significant effect on the curing time, the elasticity and morphology of the resulting hydrogel. The highest shrinking ratio was obtained for hydrogels produced using 4 : 1 acetone : deionised water (CI) as the polymerisation solvent, with the hydrogel reaching 39.56% (±2.37% (n = 3)) of its hydrated area after 4 min of white light irradiation followed by reswelling in the dark to 61.95% (±5.76% (n = 3)) after 11 min. Conversely, the best reswelling capabilities were obtained for the hydrogels produced using 1 : 1 tetrahydrofuran : deionised water (AIII), when the shrunk hydrogel (61.78 ± 0.26% (n = 3)) regained 91.31% (±0.22% (n = 3)) of its original size after 11 min in the dark. To our knowledge, this is the largest reported photo-induced area change for self-protonated spiropyran containing hydrogels. The shrinking/reswelling process was completely reversible in DI water with no detectable hysteresis over three repeat irradiation cycles.

Micro-Capillary Coatings Based on Spiropyran Polymeric Brushes for Metal Ion Binding, Detection, and Release in Continuous Flow

Micro-capillaries, capable of light-regulated binding and qualitative detection of divalent metal ions in continuous flow, have been realised through functionalisation with spiropyran photochromic brush-type coatings. Upon irradiation with UV light, the coating switches from the passive non-binding spiropyran form to the active merocyanine form, which binds different divalent metal ions (Zn2+, Co2+, Cu2+, Ni2+, Cd2+), as they pass through the micro-capillary. Furthermore, the merocyanine visible absorbance spectrum changes upon metal ion binding, enabling the ion uptake to be detected optically. Irradiation with white light causes reversion of the merocyanine to the passive spiropyran form, with simultaneous release of the bound metal ion from the micro-capillary coating.

Photoswitchable layer-by-layer coatings based on photochromic polynorbornenes bearing spiropyran side groups

Herein, we present the synthesis of linear photochromic norbornene polymers bearing spiropyran side groups (poly(SP-R)) and their assembly into layer-by-layer (LbL) films on glass substrates when converted to poly(MC-R) under UV irradiation. The LbL films were composed of bilayers of poly(allylamine hydrochloride) (PAH) and poly(MC-R), forming (PAH/poly(MC-R)) n coatings. The merocyanine (MC) form presents a significant absorption band in the visible spectral region, which allowed tracking of the LbL deposition process by UV-vis spectroscopy, which showed a linear increase of the characteristic MC absorbance band with increasing number of bilayers. The thickness and morphology of the (PAH/poly(MC-R)) n films were characterized by ellipsometry and scanning electron microscopy, respectively, with a height of ∼27.5 nm for the first bilayer and an overall height of ∼165 nm for the (PAH/poly(MC-R))5 multilayer film. Prolonged white light irradiation (22 h) resulted in a gradual decrease of the MC band by 90.4 ± 2.9% relative to the baseline, indicating the potential application of these films as coatings for photocontrolled delivery systems.

Solvato-morphologically controlled, reversible photo-actuated hydrogels, operative in neutral environments

In this study photo-actuator hydrogels operating in neutral environments were generated using N-isopropylacrylamide-co-acrylated-spiropyran-co-acrylic acid, p(NIPAAM-co-SPA-co-AA) copolymer, in a 100-1-5 mole ratio. Acrylic acid dissociates in neutral environments, triggering spiropyran (SP) moiety conversion to the more hydrophilic protonated merocyanine form (MC-H+). MC-H+ allows water uptake, causing hydrogel expansion. Exposure to white light causes the expanded hydrogel to contract due to the photonic-conversion of MC-H+ to the more hydrophobic SP form. Different ratios of deionised water: organic solvent (tetrahydrofuran (THF), dioxane and acetone) were used as the polymerisation solvent. Varying the polymerisation solvent mixtures resulted in hydrogels with different pore sizes and therefore different swelling/shrinking properties and photo-actuation kinetics.

Stimuli-controlled fluid control and microvehicle movement in microfluidic channels

Integration of stimuli-responsive materials into microfluidic systems provides a means to locally manipulate flow at the microscale, in a noninvasive manner, while also reducing system complexity. In recent years, several modes of stimulation have been applied, including electrical, magnetic, light and temperature, among others. To achieve remote control of flow in microfluidics using external stimulation, two main approaches have emerged in the recent years: