To prepare an acceptable polyacrylic acid (PAA) hydrogel as an adhesive for dermatological patch, we applied a photopolymerization technique to gel formation.The purpose of this study is to clarify relationships between formulation factors and the gel characteristics of the photocrosslinked PAA hydrogel, and to optimize the formulation of the hydrogel as an adhesive for dermatological patches.Model formulations were prepared according to L8 orthogonal experimental design and a composite spherical experimental design.The polymer concentration in the aqueous solution, the degree of modification with 2-hydroxyethyl methacrylate (HEMA), and the photoinitiator concentration were selected as the causal formulation factors.The gel fraction, the degree of swelling, the probe tack,and the turbidity were measured as the characteristics of the hydrogel.The observed values were analyzed by analysis of variance (ANOVA).The relationships between the formulation factors and characteristics were clarified.Formulation optimization study was conducted by using a novel response surface method incorporating multivariate spline interpolation (RSM-S).A set of causal factors and characteristics was used as a tutorial data.Response surfaces generated by RSM-S represented the nonlinear relationship between the factors and the characteristics, and the observed values for these characteristics of the optimal formulation agreed with the predicted values.A high functional photocrosslinked PAA hydrogel as an adhesive for a dermatological patch was successfully created.