Contact time- and pH-dependent adhesion and cohesion of low molecular weight chitosan coated surfaces

Low molecular weight chitosan (LMW chitosan, ∼5 kDa) potentially has many desirable biomedical applications such as anti-microbial, anti-tumor, and anti-diabetes. Unlike high molecular weight chitosan, LMW chitosan is easily dissolvable in aqueous solutions even at neutral and basic pH, but its dissolution mechanism is not well understood. Here, we measured adhesion and cohesion of molecularly thin LMW chitosan films in aqueous solutions in different buffer pHs (from 3.0 to 8.5) using a surface forces apparatus (SFA). Interestingly, significantly lower adhesion force was measured for LMW chitosan films compared to the high molecular weight chitosan (∼150 kDa) films. Not only the strength of adhesion is lower, but also the critical contact time where adhesion starts to increase with contact time is longer. The results from both the SFA and atomic force microscopy (AFM) indicate that, in physiological and basic conditions, the low cohesion of LMW chitosan due to the stiffness of the chain which cause strong electrostatic correlation energy penalty when they are aggregated. Here, we propose the reduction in cohesion for shorter chitosan (LMW chitosan) as an explanation of its high solubility of LMW chitosan in physiological pHs.