Ultrathin surface-attached hydrogel coating for potential reversible nanogluing of implants

A multifunctional copolymer of N-(2-hydroxypropyl)methacrylamide (HPMA) and diethyl-2-(methacrylamido)diethylphosphonate as a phosphonate-containing monomer was developed with the aim of applying a self-organizing hydrogel coating to titanium surfaces with the potential to be used for the functionalization of implants. For this purpose, a copolymer was prepared that can bind to the titanium via a phosphonate group. A photocrosslinkable group (phenyl azide) and hydrophilic groups were also incorporated to form a hydrogel. Coatings of these polymers produced by spin coating and subsequent photocrosslinking were characterised by laser ellipsometry, which yielded controlled dry layer thicknesses in the range 40 to 140 nm. Furthermore, the swelling capacity was investigated by in-situ ellipsometry in different aqueous media, resulting in a swelling up to three times the original dry film thickness, depending on their crosslinker content. We also validated long-term stability under physiological conditions and the biocompatibility. Although the hydrogel layers and the silica nanoparticles have negative surface charges, different types of silica nanoparticles are strongly adsorbed on the surfaces. This shows the potential of the hydrogels for bonding with other hydrogels or soft tissue (nanogluing). Preliminary tests have shown that a PDMA hydrogel could in fact adhere to the surface-bound hydrogels by way of nanoparticles.