4. ConclusionsThe recombinant M. extorquens ATCC 55366 strain harboring the phaC2 gene from P. fluorescens GK13 (M. ex-phaC2) wascapable of producing blends of short-chain-length (4 ≤ C ≤ 5) andshort-chain-length/medium-chain-length polyhydroxyalkanoates(PHAs) (4 ≤ C ≤ 6) bearing terminal C–C double bonds in theirside chains. The co-substrate 5-hexenoic acid impacted stronglyon microbial growth, PHA production and monomeric composition, resulting in a trade-off between productivity and functionalmonomeric unit abundance in PHA copolymers. Results from bioreactor studies suggested that data for cumulative methanol addition(main substrate) can be used to predict growth of M. extorquens.Linking co-substrate consumption to methanol addition led to anempirical model that allowed for on-line estimation of 5-hexenoicacid concentration. This model could be used to automate cosubstrate feeding to cultures of recombinant M. extorquens in thefuture. Material and structural analyses confirmed that it was possible to introduce functionality to a highly regarded group of PHAswith desirable thermal properties. As these biopolyesters are autoclavable and amenable to (bio)chemical modification, they couldfind use in medical applications as biodegradable composite materials.