Photo-reversible adhesive polyacrylic acid/Fe³⁺ hydrogels for the efficient recovery of starch grains from archaeological lithic artifacts

Starch rains entrapped in valleys, fractures, and pores of stone tools are underestimated bio-archives that can provide valuable information about the environment, diet, and technological advancements of ancient populations. This study aimed to develop innovative smart photo-reversible adhesive materials to recover starch grains from ground stone tools, particularly those that are part of museum collections, and to subsequently release them for archaeologically relevant analyses. A polyacrylic acid (PAA) hydrogel crosslinked with Fe³⁺ cations was selected for its adaptability to stone surfaces, strong affinity for starch grains, and photo-reversible adhesive behavior. Nine PAA*Fe stitching gels were synthesized and tested. The sampling approach included two steps: (i) extraction of starch grains from stone tool proxies through adhesion to the gels, and (ii) recovery of the entrapped starch by UV-induced fluidification of the gel network at 16 °C. By lowering the elastic and adhesive properties of the gel and the temperature during UV irradiation, starch grains could be recovered without altering their morphology or chemical properties, as investigated by Optical Microscopy (OM) analysis, Scanning Electron Microscopy (SEM), Fourier Transform Infrared (FTIR) spectroscopy, X-Ray Diffraction (XRD) and Thermogravimetric Analysis (TGA). The most suitable PAA*Fe gel formulations were selected by correlating their rheological properties to their extraction ability (i.e., starch yields). The results demonstrated efficient starch recovery with minimal starch grain alteration, offering a promising method for future archaeological applications.