Nitric oxide-releasing PHEMA/polysilsesquioxane photocrosslinked hybrids

Abstract

Polymeric materials capable of releasing nitric oxide (NO) locally have potential uses in various biomedical applications. One of the main challenges in this field is obtaining materials that allow the modulation of NO release rates through the incorporation of different NO donor molecules. Herein, we describe the synthesis of materials composed of poly(2-hydroxyethyl methacrylate) (PHEMA) crosslinked by a polysilsesquioxane (PSS) network through sol-gel polymerization and photocrosslinking. Increasing the PSS content from 5 to 20 wt% led to an increase in the glass transition temperature from 107 °C to 133 °C. Swelling studies in phosphate buffer saline solution and ethanol revealed that higher siloxane content reduced the solvent uptake of the hybrids, while surface contact angle measurements confirmed that all compositions remained hydrophilic (60-70°). These hybrids enabled, for the first time, the incorporation of two structurally distinct NO donors, hydrophilic S-nitrosoglutathione (GSNO) and hydrophobic S-nitroso-N-acetyl-dl-penicillamine (SNAP), via absorption from aqueous and ethanolic solutions, respectively. Computational modeling showed that GSNO forms multiple hydrogen bonds with PHEMA hydroxyl groups, while SNAP interacts hydrophobically with its methyl groups. Real-time NO measurements showed that SNAP spontaneously releases NO at a flow rate 2 to 10 times higher than that of GSNO in the first 30 min after hydration of the hybrids, likely due to its weaker intermolecular interactions and higher mobility upon hydration. The hydrophilic nature of the hybrids, tunable NO release, and lack of cytotoxicity toward cultured endothelial cells position them as promising candidates for the manufacturing of antithrombotic blood-contacting medical devices.

Fig. 1. Chemical structures of S-nitrosogutathione (GSNO) and S-nitroso-N-acetyl-d,l-penicillamine (SNAP).

Fig. 2. Schematic of HEMA-TES precursor synthesis and hydrolysis–condensation process. (a) Carbamate bond formation between 2 hydroxyethyl methacrylate (HEMA) and 3-isocyanatopropyltriethoxysilane (IPTES), producing the HEMA-TES precursor with both methacrylate and siloxane functional groups. (b) Acid-catalysed hydrolysis of triethoxysilane groups, yielding HEMA silanol and ethanol. (c) Subsequent condensation of silanol moieties leads to polysilsesquioxane (PSS) network formation in the hybrid material.

Fig. 3. Spectroscopic confirmation of HEMA-TES formation. (a) FTIR ATR spectra showing gradual disappearance of the isocyanate (NCO) stretching band at 2265 cm⁻¹, along with the emergence of characteristic carbamate bands at 1526 cm⁻¹ (N–H stretch) and 1719 cm⁻¹ (CO stretch), during the reaction. (b) Kinetic plot (A_t/A₀vs. time) for NCO consumption, demonstrating reaction completion in ∼7 h. (c) ¹H NMR spectra of HEMA-TES (top), IPTES (middle), and HEMA (bottom), highlighting the new carbamate proton peak at 7.3 ppm in HEMA-TES, confirming conjugation.

Fig. 4. Schematic representation of the structure of the photo-crosslinked PHEMA–PSS network.

Fig. 5. Spectroscopic and thermal characterization of PHEMA–PSS hybrids. (a) FTIR-ATR spectra of HEMA (black), HEMA-TES (blue), and HT5 hybrid (red). In the HT5 spectrum, the vinyl CC stretching band at 1636 cm⁻¹ is nearly absent, confirming effective photopolymerization and crosslinking. (b) Differential scanning calorimetry (DSC) thermograms of HT5 (black), HT10 (red), and HT20 (blue) hybrids. Glass transition temperatures (T_g), determined from thermogram inflection points or the minima in the first-derivative curves, systematically increase from 107 °C (HT5) to 119 °C (HT10) and 133 °C (HT20), reflecting enhanced network rigidity due to increasing polysilsesquioxane crosslinker content.

Fig. 6. Swelling and surface wettability of PHEMA–PSS hybrids. (a) Swelling kinetics of HT5, HT10, and HT20 hybrids in PBS at 37 °C, showing equilibrium swelling values reached within 12 h. (b) Swelling kinetics in ethanol at 25 °C, where swelling progressed over the full 24-hours period without reaching equilibrium. (c) Contact angle measurements over time (0–120 s) with ultrapure water droplets on HT5, HT10, and HT20 hybrid surfaces (*p < 0,0).

Fig. 7. (a) Total NO loading, (b) real-time NO release, and (c) cumulative NO release of hybrids HT5 and HT20 loaded with 40 mmol L⁻¹ of GSNO and SNAP solutions.

Fig. 8. Average number of hydrogen bonds between PHEMA chain and (a) neutral GSNO and (b) neutral SNAP during 1 ns molecular dynamics simulation at the GFN-FF theoretical level. (c) Final structures from MDs of neutral GSNO–PHEMA/PSS complex and (d) neutral SNAP—PHEMA/PSS complex. Distances are reported in angstroms.

Fig. 9. (a) Viability of HUVECs under control conditions or after incubation with HT5 discs, with or without SNAP (20 mM or 40 mM), for 6 h or 24 h. (b) Cell viability under the same conditions after 24 h of nutrient deprivation (starvation) as a stressor. Data are expressed as percentage relative to control. N = 3. Values are presented as mean ± SEM. One-way ANOVA: P > 0.05.