Biomaterials for MEMS

Biodegradable Elastomeric Polymers and MEMS in Tissue Engineering 39

(T

g

) will also affect the degradation rate due to the increased water diffusion rate

into the material. In order to prevent any changes in the elasticity of a material,

it is important to maintain the T

g

below the normal body temperature. In the

case of an elastomer, both the T

g

and mechanical properties are affected by the

degree of crosslinking. A higher crosslinked elastomer will normally have a slower

degradation rate, stronger mechanical strength and smaller elongation rate.

3.3 BIODEGRADABLE ELASTOMERIC POLYMERS

3.3.1 Polyesters

The use of elastomers in medical applications originates back to the beginning of

the rubber industry. Since then, numerous materials have played a major role in

medical technology.

37

Polyesters are the most widespread category of polymers

used in biomedical applications. The ester bond is important because it allows

for degradation through hydrolytic cleavage in the presence of water. Unlike

enzymatic degradation, this form of degradation is advantageous because of the

minimal site–to–site and patient–to–patient variations.

A polymer used in tissue engineering applications should show good degrad-

ability and biocompatibility when presented in vivo. Due to these requirements,

glycolic and lactic acid based poly(α–hydroxy acids) such as poly-L-lactide acid

(PLLA) and poly(lactic-co-glycolic acid) (PLGA) have gained attention in the past

few decades as suitable polyesters for various medical applications. Their use

can be seen in drug delivery systems, scaffolds for tissue regeneration, resorbable

sutures, staples, and orthopedic fixation devices.

38

However, these α–hydroxyl acid polymers are inappropriate for soft tissue ap-

plications because of their stiff nature. Due to this major drawback, researchers are

advancing towards a new category of polyesters whose mechanical properties can

be tuned for particular soft tissue engineering applications such as blood vessels,

heart valves, ligaments, and tendons. Polyesters that possess elastic properties

to meet the requirements for soft tissue engineering are shown in Table 3.2. The

following section will focus on the polyester elastomers that have been used in the

field of soft tissue engineering.

3.3.1.1 Polyhydroxyalkanoates (PHAs)

In the early 1920’s, the bacteria bacillus megaterium was recognized for producing

poly(3–hydroxybutyrate) (PHB), which is the most common polymer among the

polyester class. Since then, more than 150 different monomer combinations have

been used in the formation of different polymers within the PHA family.

39

Four

different pathways have been revealed for the synthesis of PHA through the

process of biosynthesis, which has been mentioned in detail elsewhere.

40

Due to

advancements in the field of genetic engineering, researchers have also used plants

as the production house for PHB-related polymers.

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40 R. Tran et al.

Table 3. 2 Mechanical properties of polyester elastomers in recent research.

Mechanical Properties

Polymer Name Youngs Modulus(MPa) Elongation at Break(%) Tensile Strength (MPa)

PHB 2500 3 36

P4HB 70 1000 50

PGS 0.056–1.2 40–448 0.2–0.5

PGSA 0.048–1.37 47–170 0.54–0.5

POC 1.85–13.98 117–502 2.93–11.15

PEC 0.25–1.91 140–1505 0.51–1.51

PPSC 0.6–1.23 226–432 0.87–2.12

POM Not reported 3.86–14.34 7.32–25.6

PAMC 0.05–1.8 55–450 0.29–0.88

CUPE 4.14–38.35 222.66–337.558 14.6–41.07

Several groups have also reported the chemical synthesis of poly(3–

hydroxyalkanoates) (P(3HB)) through the process of a ring opening of β–

butyrolactone (BL) in the presence of aluminum, zinc, and tin based catalysts.

42−44

However, these reactions did not yield high molecular weight polymers. To

overcome this limitation, Hori et al. utilized the distannoxane complexes as

an excellent catalyst for the ring-opening polymerization of (R)–b–butyrolactone

((R)–BL) and BL to produce P[(R)–3HB] and P(3HB) of high molecular weights

and in high yields.

45

By using different combinations of various monomers, researchers have

successfully produced PHAs with a wide range of mechanical properties and

degradation profiles. For example, poly(3–hydroxybutyrate) is a stiff polymer

with a Young’s Modulus of 2500 MPa and 3% elongation where as poly(4–

hydroxybutyrate) is an elastic polymer with a Young’s Modulus of 70 MPa and

1000% elongation. In terms of their biocompatibility, PHA elastomers are biosyn-

thetic polymers and require serious consideration on their purity.

46

In the early 1990’s, Akhtar et al. reported a prolonged acute inflamma-

tory response and severe chronic inflammatory response from PHA films im-

planted in vivo.

47

William et al. proposed the idea of using a depyrogenation

technique through the use of an oxidizing agent that resulted in the reduction in

the amount of endotoxins. In addition, William and co-workers also understood

the problems associated with the use of solvents while extracting the polymer.

The group found that a higher purity could be obtained if the polymer was

extracted with hexane or acetone instead of the traditional chlorinated solvents.

In order to support their study, the research group performed in vivo tests by

placing several different types of implants such as microspheres, tubes, and pellets

subcutaneously in mice. The histological results revealed the formation of a thin

fibroblast capsule (four to six cell layers), and the absence of macrophages at the

implant sites.

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