On the foundation of thermal "Switching": The culture substrate governs the phase transition mechanism of thermoresponsive brushes and their performance in cell sheet fabrication

Thermoresponsive Brush Coatings for Cell Sheet Engineering with Low Protein Adsorption above the Polymers' Phase Transition Temperature

Thermoresponsive polymer coatings on cell culture substrates enable noninvasive cell detachment and cell sheet fabrication for biomedical applications. Optimized coatings should support controlled culture and detachment of various cell types and allow chemical modifications, e.g., to introduce specific growth factors for enhanced gene expression. Furthermore, the sterilization and storage stability of the coatings must be assessed for translational attempts. Poly(glycidyl ether) (PGE) brush coatings with short alkoxy side chains provide a versatile platform for cell culture and detachment, but their polyether backbones are susceptible to oxidation and degradation. Thus, we rationally designed potential alternatives with thermoresponsive glycerol-based block copolymers comprising a stable polyacrylate or polymethacrylate backbone and an oligomeric benzophenone (BP)-based anchor. The resulting poly(ethoxy hydroxypropyl acrylate-b-benzophenone acrylate) (pEHPA-b-BP) and poly(ethoxy hydroxypropyl methacrylate-b-benzophenone methacrylate) (pEHPMA-b-BP) block copolymers preserve the short alkoxy-terminated side chains of the PGE derived structure on a stable, but hydrophobic, aliphatic backbone. The amphiphilicity balance is maintained through incorporated hydroxyl groups, which simultaneously can be used for chemical modification. The polymers were tailored into brush coatings on polystyrene surfaces via directed adsorption using the BP oligomer anchor. The resulting coatings with thickness values up to ∼3 nm supported efficient adhesion and proliferation of human fibroblasts despite minimal protein adsorption. The conditions for cell sheet fabrication on pEHPA-b-BP were gentler and more reliable than on pEHPMA-b-BP, which required additional cooling. Hence, the stability of pEHPA-b-BP and PGE coatings was evaluated post gamma and formaldehyde (FO) gas sterilization. Gamma sterilization partially degraded PGE coatings and hindered cell detachment on pEHPA-b-BP. In contrast, FO sterilization only slowed detachment on PGE coatings and had no adverse effects on pEHPA-b-BP, maintaining their efficient performance in cell sheet fabrication.

"Fuzzy hair" promotes cell sheet detachment from thermoresponsive brushes already above their volume phase transition temperature

Thermoresponsive poly(glycidyl ether) (PGE) brushes have shown to be viable substrates for the culture and temperature-triggered detachment of confluent cell sheets. Surface-tethered PGEs with a cloud point temperature (T_CP) around ~30 °C exhibit phase transitions well-centered within the physiological range (20-37 °C), which makes them ideal candidates for cell sheet fabrication. However, PGEs with T_CPs at ~20 °C also afford the detachment of various types of cell sheets, even at room temperature (20-23 °C), i.e., above the polymers' T_CPs. In this study, we investigate the phase transition of PGE brushes tethered to polystyrene (PS) culture substrates with varying grafting density and T_CP to arrive at a mechanistic understanding of their functionality in cell sheet fabrication. Using quartz crystal microbalance with dissipation (QCM-D) monitoring, we demonstrate that brushes fabricated from PGEs with T_CPs at ~20 °C display volume phase transition temperatures (VPTTs) well below room temperature. Although the investigated coatings obviously do not exhibit marked thermal switching in terms of brush hydration and layer thickness, their physical properties at the brush-water interface, as ascertained by QCM-D and AFM measurements, undergo subtle changes upon cooling from 37 °C to room temperature which is sufficient to promote cell sheet detachment. Thus, it appears that discreet rehydration of the outmost brush layer, resembling "fuzzy hair" at the brush-water interface, renders the surfaces less protein- and cell-adhesive at room temperature. This minor structural change of the interface allows for the reliable detachment of human dermal fibroblast sheets already at 20 °C well above the VPTT of the brushes.

Polyethers Based on Short-Chain Alkyl Glycidyl Ethers: Thermoresponsive and Highly Biocompatible Materials

The polymerization of short-chain alkyl glycidyl ethers (SCAGEs) enables the synthesis of biocompatible polyethers with finely tunable hydrophilicity. Aliphatic polyethers, most prominently poly(ethylene glycol) (PEG), are utilized in manifold biomedical applications due to their excellent biocompatibility and aqueous solubility. By incorporation of short hydrophobic side-chains at linear polyglycerol, control of aqueous solubility and the respective lower critical solution temperature (LCST) in aqueous solution is feasible. Concurrently, the chemically inert character in analogy to PEG is maintained, as no further functional groups are introduced at the polyether structure. Adjustment of the hydrophilicity and the thermoresponsive behavior of the resulting poly(glycidyl ether)s in a broad temperature range is achieved either by the combination of the different SCAGEs or with PEG as a hydrophilic block. Homopolymers of methyl and ethyl glycidyl ether (PGME, PEGE) are soluble in aqueous solution at room temperature. In contrast, n-propyl glycidyl ether and iso-propyl glycidyl ether lead to hydrophobic polyethers. The use of a variety of ring-opening polymerization techniques allows for controlled polymerization, while simultaneously determining the resulting microstructures. Atactic as well as isotactic polymers are accessible by utilization of the respective racemic or enantiomerically pure monomers. Polymer architectures varying from statistical copolymers, di- and triblock structures to star-shaped architectures, in combination with PEG, have been applied in various thermoresponsive hydrogel formulations or polymeric surface coatings for cell sheet engineering. Materials responding to stimuli are of increasing importance for "smart" biomedical systems, making thermoresponsive polyethers with short-alkyl ether side chains promising candidates for future biomaterials.