Theoretical considerations on mechanisms of harvesting cells cultured on thermoresponsive polymer brushes

Poly (N-isopropylacrylamide) (PNIPAM) brushes and hydrogels serve as temperature-responsive cell culture substrates. The cells adhere at 37 °C and are detached by cooling to below the lower critical solution temperature T(LCST) ≈ 32 °C, an effect hitherto attributed to change in PNIPAM hydration. The article proposes a mechanism coupling the change of hydration to integrin mediated environmental sensing for cell culture on brushes and hydrogels in serum containing medium. Hydration is associated with swelling and higher osmotic pressure leading to two effects: (i) The lower osmotic pressure in the collapsed brush/hydrogel favors the adsorption of serum borne extracellular matrix (ECM) proteins enabling cell adhesion; (ii) Brush/hydrogel swelling at T < T(LCST) gives rise to a disjoining force f(cell) due to confinement by the ventral membrane of a cell adhering via integrin-ECM bonds. f(cell) places the integrin-ECM bonds under tension thus accelerating their dissociation and promoting desorption of ECM proteins. Self consistent field theory of PNIPAM brushes quantifies the effect of the polymerization degree N, the area per chain Σ, and the temperature, T on ECM adsorption, f(cell) and the dissociation rate of integrin-ECM bonds. It suggests guidelines for tuning Σ and N to optimize adhesion at 37 °C and detachment at T < T(LCST). The mechanism rationalizes existing experimental results on the influence of the dry thickness and the RGD fraction on adhesion and detachment.