**4. Modification of PLA**

254 Biomedical Science, Engineering and Technology

**Propionyl-CoA transferase**

**LA-polymerization enzyme**

Fig. 2. Mechanism for the bio-synthesis of LA polyester. In the bio-process, the LA monomer is converted into LA-CoA ,which is recognized by the LA-polymerizing enzyme recruited

**Lactate (LA)**

**LA-CoA**

**LA polyester**

**Microbe**

Synthesis methods Advantages Disadvantages

easy to control

PLA

Biosynthesis One-step, efficient, non-

Table 2. Comparison of PLA synthesis methods.

One-step, economical and

High molecular weight

Efficient, non-toxic, no pollution , high molecular

toxic, no pollution, low

weight PLA, etc.

cost, etc.

Melt polycondensation High reaction temperature,

**Bio-process Chemo-process**

**Substitution**

**CO2**

Solution

polycondensation

Ring-opening polymerization

New solutions (new catalysts, polymerization conditions, etc)

**Degradation**

from microbial PHA synthase (Tajima et al., 2009).

**Fermentation**

**Sugar**

**P**

**h**

**otosy**

**nthesis**

**O**

**PLA**

**LA**

**H3C**

Impurities, side reactions, pollution, low molecular weight

sensitivity to reaction conditions, low molecular

Under development

Under development

Requires strict purity of the lactide monomer, related high

weight PLA

PLA

cost

**O CH3**

**Metal catalysts**

**O O**

The major drawbacks of PLA limiting its applications are its poor chemical modifiability and mechanical ductility, slow degradation profile, and poor hydrophilicity. In order to be suitable for specific biomedical applications, the PLA has been modified mainly concerning two aspects: Bulk properties and surface chemistry. To achieve this, both chemical modification and physical modification have been tried, involving the incorporation of functional monomers with different molecular architectures and compositions, the tuning of crystallinity and processibility via blending and plasticization, etc., which are described in the following sections.
