Mechanics of pH-Responsive Hydrogel Capsules.

While soft hydrogel nano- and microstructureshold great potentialfor therapeutic treatments and in vivo applications, their nanomechanicalcharacterization remains a challenge. In this paper, soft, single-component,supported hydrogel films were fabricated using pendant-thiol-modifiedpoly(methacrylic acid) (PMASH). The influence of hydrogelarchitecture on deformation properties was studied by fabricatingfilms on particle supports and producing free-standing capsules. Theinfluence of the degree of thiol-based cross-linking on the mechanicalproperties of the soft hydrogel systems (core–shell and capsules)was studied using a colloidal-probe (CP) AFM technique. It was foundthat film mechanical properties, stability, and capsule swelling couldbe finely tuned by controlling the extent of poly(methacrylic acid)thiol modification. Furthermore, switching the pH from 7.4 to 4.0led to film densification due to increased hydrogen bonding. Hydrogelcapsule systems were found to have stiffness values ranging from 0.9to 16.9 mN m-1over a thiol modification range of5 to 20 mol %. These values are significantly greater than those forpreviously reported PMASHplanar films of 0.7–5.7mN m–1over the same thiol modification range (Best et al., Soft Matter2013, 9, 4580–4584). Films on particlesubstrates had comparable mechanical properties to planar films, demonstratingthat while substrate geometry has a negligible effect, membrane andtension effects may play an important role in capsule force resistance.Further, when transitioning from solid-supported films to free-standingcapsules, simple predictions of shell stiffness based on modulus changesfound for supported films are not valid. Rather, additional effectslike diameter increases (geometrical changes) as well as tension buildupneed to be taken into account. These results are important for researchrelated to the characterization of soft hydrogel materials and controlover their mechanical properties. [ABSTRACT FROM AUTHOR]