This study is about the synthesis of modified glycol-based hydrogel adhesives, which could be used by human skin-attached devices such as smart sensors and wearable devices. With normal adhesives, it is difficult for them to adhere to substrates with low surface energy, specially to human skin (22~30 dyne/cm). Hydrogel adhesives were prepared by modifying glycol oligomers in various methods. The hydrogel adhesives made from modified glycol were examined by adhesive strength on substrates having various surface energies included the human skin.
Glycols are a type of diol containing two hydroxyl groups at both ends of its chemical structure which have been used widely in various industries. Poly(ethylene glycol diacrylate) (PEGDA) is one of the polymers made from glycol and is widely being used in industries.
PEGDA has an ester group in its chemical structure, which came from condensation reaction of reactants. The ester group usually can be decomposed easily by water and/or acids. This decomposition property of PEGDA has been used as an advantage in medical treatments extensively.
However, in the industry such, as transdermal (skin) attachable biosensors or wearable devices, decomposing reaction was considered a disadvantage since they are required to be attached long time or repeatedly several times on the human skin.
In this research, poly(ethylene-ran-propylene glycol) (PEP) random copolymer having molecular weight around 12,000 g/mol was used in the polymerization. PEP was condensed with acrylic anhydride to produce poly(ethylene-ran-propylene di-methacrylate) (PEPDMAESTER) containing ester groups. The final hydrogel adhesives were produced through UV curing the (PEPDMAESTER). The prepared adhesive which contains byproduct acid was exposed to acid decomposition and was monitored. After about 30 days, it was found that a part of the adhesive showed a liquid form as like before cross-linking.
PEPDMAURETHANE and PEPDMAETHER were prepared by reacting the hydroxyl groups with block-isocyanate or glycidyl methacrylate (GMA) to produce a glycol-based hydrogel resin. It contained acrylic groups for UV curing and showed a low degree of decomposition. While the prepared urethane or β-hydroxy-modified glycol hydrogel was exposed to the same condition and left for about 90 days, and showed no decomposition at all.
The contact angle of modified glycol-based hydrogel resins was measured on an silicon-coated release film with a surface energy of 30 dyne/cm to confirm the wettability of the materials with low surface energy, such as human skin. The deionized water dropped on the film and showed a contact angle of about 114˚, which showed that the film exhibited hydrophobicity. PEPDMAESTER, PEPDMAURETHANE, and PEPDMAETHER showed contact angles of 82.7˚, 78.7˚, and 71.9˚, respectively. It confirmed a good wettability of modified glycol-based hydrogel resins for LSE materials.
The adhesive properties of hydrogel resins were measured on the various substrates which have different surface energy. PEPDMAESTER adhesive showed peel strength in the range of 0.3 ~ 0.7 kgf/25mm on the. The hydrogel adhesives containing the β-hydroxy group exhibited peel strength in the range of 0.5 ~ 0.6 kgf/25mm, and urethane modified hydrogel showed peel strength in the range of 0.7 ~ 1.3 kgf/25mm.
It was confirmed that the modified hydrogel adhesive prepared in this study was significantly retarded in the decomposition reaction and exhibited good adhesive strength to LSE materials without using extra layer such as acrylics and silicons.
In addition, the hydrogel adhesives developed in this research are expected to be used in various industry area that require adhesion to LSE surfaces. In the applications on elastic and flexible substrates, such as human skin, the hydrogel adhesives developed in this research are expected to be served as fundamental and important components.