Hydration Forces in Ultrathin Films of Cellulose

Ultrathin ( d < 20 nm) films of cellulose were fabricated by regeneration of trimethylsilylcellulose films, which were deposited on silicon wafers by the Langmuir−Blodgett (LB) or the spin-coating method. Equilibrium film thickness with respect to relative atmospheric humidity was quantitatively measured by ellipsometry to obtain relationships between thickness and osmotic pressures within the hydrated films (force−distance curves). The hydration of cellulose LB films and that of spin-coated films under equilibrium were found to be independent from initial dry film thickness and surprisingly similar in terms of (a) maximum swelling ratio (ρ max ∼ 1.6), (b) power law exponent under high-pressure conditions (>5 × 10 7 Pa), and (c) exponential decay length (λ 0 = 0.32 Å) under low-pressure conditions (<5 × 10 7 Pa). Fast water uptake kinetics across an abrupt change of the relative humidity (osmotic shock) was monitored as changes in film thickness with time, yielding a characteristic time constant of τ = 14 s. Furthermore, microstructures of cellulose films were processed through deep UV photolithography, and their local three-dimensional profiles could be obtained by imaging ellipsometry. Selective immobilization of native cell membranes onto such patterns will enable us to design biocompatible microtemplates to accumulate native cell membranes on solid surfaces.