Determination of green-bond strength in tacky poly(vinyl alcohol) hydrogels

Pressure-sensitive tack is the adhesive property related to bond formation. It is a key issue when formulating hydrogel poultices for transdermal delivery, dressings, and bioelectrodes. Quantitative tack gives an indication of the potential ease and success of application when gels are brought into contact with skin. The effects of different dwell times and constant pressures on bond formation between tacky poly(vinyl alcohol) (PVA) hydrogels and a skin model were explored in the current study; these were correlated with viscoelastic properties in order to elucidate structure-function relationships. A rolling tack test was performed using a novel apparatus capable of simultaneously controlling the pressure and dwell time in a hydrogel/skin-model-probe system. PVA gels were formed via the freeze-thaw technique using Ca²⁺ ions. Lower calcium availability in PVA gels resulted in longer dwell times required to complete bond formation, decreased creep compliance (at 0.01 s) and a decreased G′_{(ω = 40)}/G′_{(ω = 0.01)} ratio, all three leading to a loss in tack strength. All tested gels were found to have pressure-sensitive tack. The results of this study support the applicability of a rheological methodology and a novel tack-testing procedure to quantify green-bond formation in pressure-sensitive-adhesive PVA hydrogels.