Page 85 - Biodegradable Polyesters

3.8 Application Opportunities of Bacterial Polyhydroxyalkanoates 63

PHA production strains that are able to grow to high cell density within a short
period of time on lower cost substrates under less demanding fermentation con-
ditions, such as microaerobic conditions. A synthetic strain containing the mini-
mum genome could help increase the substrate to PHA yield. Simple purification
and extraction technology employing controllable lysis of high PHA content con-
taining bacteria accumulated large PHA granules should be developed. Such a
process could radically decrease the cost of centrifugation, filtration, and extrac-
tion as it could be coupled with inorganic aqueous treatment. However, the use
of simple substrates such as glucose only for production of copolymers including
PHBV, PHBHHx, copolymers of scl- and mcl 3-hydroxyalkanoates, and P3HB4HB
will reduce the PHA cost attributed to co-substrates including propionate, fatty
acids, or 1,4-butanediol. The PHAs produced should have a controllable molecu-
lar weight to meet various applications. In particular, the use of continuous, mixed
culture fermentation without sterilization is a newly developed technology to sig-
nificantly lower PHA production cost.
Low-cost PHAs would not only benefit the PHA material application as bio-
plastics, but promote the application of PHAs as biofuels as well. There is a large
potential for compromise in this area, as low-cost PHAs could also be obtained
from activated sludge and wastewater fermentation, so it will not run into the con-
troversy of food versus fuel or fuel versus arable land. Plant production of PHAs
could become a reality, as indicated by some promising results, in the foreseeable
future.

3.8.6.2 Unusual PHAs with Special Properties
Unusual PHAs containing various functional side groups such as double bonds,
hydroxyl- and/or carboxyl-groups should be produced as these PHAs have not
only intrinsic novel properties, but easily modified side groups as well, which
greatly expand their applications. Now, as it is possible to use low-specificity PHA
synthases for the production of scl- and mcl PHA copolymers through screening
or molecular evolution, it has become a reality to design and produce PHAs with
the expected structures. PHAs containing designed functional side groups allow
chemical modification to change the polymer properties, which allows for the
expansion of PHA applications.
We now have the capacity to produce PHAs containing various blocks such
as PHB-b-PHV or PHB-b-PHBV, and PHB-b-PHA. These block PHAs have been
found to show new properties. More PHB block copolymers are under develop-
ment and they have the potential to generate more unique applications.
Thanks to cost-competitive PHAs being developed, low-cost applications
should also be developed. These include the new processing technology that can
exploit the existing extruders and other molding machines used to make products
from common plastics such as polyethylene and polypropylene. In addition,
the blending of PHAs with cheaper materials such as starch and cellulose will
reduce the cost even further without the loss of degradability and sustain-
ability.