A second way to categorize hydrogels is by the starting point for synthesis (production). First, a polymer network may be prepared from monomers (small molecules that may be linked in a repeating fashion to form high-molecular weight polymers). A network is made by copolymerizing hydrophilic monomers with crosslinkers (polyfunctonal monomers). Networks made with PEG and PPO monomers are good carriers for drug delivery. Second, a polymer network may be prepared starting with prepolymers (oligomers [low-molecular weight polymers] that are capable of further polymerization). Polyurethane networks may be prepared using prepolymers. A non-medical application of a polyurethane hydrogel is entrapment (immobilization) of microbial cells for wastewater treatment. Third, polymer networks may be prepared starting with polymers; a network structure is made by crosslinking hydrophilic polymer chains. An example is chitosan (a linear polysaccharide polymer) crosslinked with glutaraldehyde. Two applications are scaffolding for soft tissue engineering and adhesive for peripheral nerve repair.