(German title: Verbundvorhaben (FSP-Klebstoffe) Biomimetischer Klebstoff aus ligninhaltigen Pflanzenresten (BioBond))

The aim of the research project "Biomimetic adhesive from lignin-containing plant residues" is to imitate the adhesive force of mussels on smooth surfaces in order to develop an adhesive that is free of toxic substances (e.g. formaldehyde) and is based on renewable raw materials. By combining lignin with the adhesive properties of mussels, a cost-effective and strong adhesive is to be developed that is biodegradable. 

Mussels have the ability of reversible adhesion to smooth and moist surfaces. The primary component of mussel glue, L-DOPA, has also been found to have good adhesive properties on metal oxide compounds. L-DOPA is an amino acid with a catechol group, which is formed by hydroxylation of the hydroxyphenyl ring of the canonical amino acid tyrosine. The literature describes that adhesives based on the polymerisation of catechol groups (e.g. tannins or demethylated lignin) with polyimines perform similarly to phenol-formaldehyde resins. 

L-DOPA is expensive and therefore not a suitable basic building block for a cost-effective adhesive. Lignin, on the other hand, forms the structural basis of green plants and trees and is therefore inexpensive and readily available. Lignin is a phenol-containing polymer that is difficult to access enzymatically and is therefore currently mainly utilised thermally. Commercially available kraft lignin is a by-product of pulp production and is therefore chemically sulphated, which can impair its microbiological degradability. A sulphate-free lignin can be obtained from the plant waste using an organosolv process that has already been established at the Institute of Bioprocess Engineering. Optimisation of the Organosolv process with other solvents and mediators can lead to better degradability of lignin.

The formation of the adhesive forces of an adhesive, known as curing, often occurs through the radical polymerisation of monomers. Laccases and peroxidases are known to catalyse the formation or degradation of polymers (e.g. lignin) in nature through the formation of radicals. Laccases are oxidoreductases that catalyse the oxidation of phenolic substances by reducing oxygen. The phenol group is oxidised to a radical, while oxygen is reduced to water. An innovative adhesive is to be developed by enzymatic polymerisation of promising mussel components (including L-DOPA) with lignin.