Corn- and Soy-Derived Materials: Properties and Potential Clinical Applications - Medical Devices and Biomaterials for the Developing World - page 60

Preparation of Selected Biomaterials
Corn-Derived PLLA
Lactic acid is a naturally occurring compound that is produced from sugars such as
glucose via fermentation of pyruvate. One of its optical isomers, L-lactic acid, is
commonly found in the human body and in plants, animals, and microorganisms
(Galactic 2012). One common source of lactic acid, and the one considered in this
study, is corn starch. The most notable component of starch-based materials is amy-
lose, which contributes to both the flexibility and the digestibility of the biomaterial.
The starch/amylose content of a given material also has an effect on the tensile
strength of the finished product (Kim et al. 1998). After lactic acid is isolated in one
form or a mixture of its isomeric forms, it can be polymerized in the laboratory via
a ring-opening mechanism (Fig. 4.2 ). Stannous octoate is used as a catalyst for this
process, and lauryl alcohol is used to determine the length of the polymer. The
molecular weight of the polymer is determined by the relative concentrations of
these two reagents (Ikada and Tsuji 2000).
In its polymerized form, L-lactic acid is referred to as poly(L-lactic acid), or
PLLA. Like other forms of PLA, PLLA is an aliphatic thermoplastic polymer,
which means that it softens reversibly when heated and hardens when cooled
(Balasubramaniam et al. 2007). Polymers such as starch (at a price of $1.50/kg) can
be mixed with synthetic polyesters ($4/kg) to produce a blended alternative at a
much lower cost. In a given unit, almost 50% of the polyester may be replaced by a
polymer without sacrificing quality (Malhotra et al. 2008). PLLA can be processed
into various forms such as fiber and film. Mikos et al. (1994) demonstrated that
foams of varying porosity can be created using PLLA-salt composites of varying
salt concentrations, which are molded and then purified via dissolution of the salt.
The versatility, potential porosity, and biodegradability of PLLA signal its potential
as a material with clinical applications. PLLA is also a renewable biomaterial; the
fiber can be recycled via de-polymerization to L-lactic acid and re-processing (back
to PLLA) after purification.
While PLLA has been identified as a material of interest, its current usage is limited
to cosmetic fillers and surgical elements that either do not have direct contact with the
body or are intended to degrade on a relatively short time scale, such as sutures
(Goldman 2011). A possible constraint in the use of PLLA-based materials is the toxic-
ity of the by-products of degradation, especially when the implant is present in the body
for a long time such as may be the case for an implanted biomaterial. These by-products
include remnants from the polymerization process and low-molecular-weight leachables,
or materials that separate from the surface of the material and/or enter the environment
of the cells (Ikada and Tsuji 2000). Elimination of the potentially dangerous degrada-
tion phase may mitigate the risk of infection and provide another option for implants.
Assuming biocompatibility and given the many benefits of the material, a goal for
future research may well be adaptation of PLLA, or other naturally derived materials,
in one of its many forms for use in long-term grafts or implants.
The PLLA used in this experiment was obtained from Biomer and processed in
the laboratory of Dr. Manjusri Misra and Dr. Amar Mohanty at the University of Guelph.