The recent developments of bioabsorbable polymers have the potential to significantly change medical devices. The polymers, an emerging area, could someday soon perform vital functions in the body and them simply be absorbed after serving their purpose. These implantable polymers degrade with by-products excreted through one of the organ systems. Several families of bioabsorbable polymers include polyesters, poly (amino acids), polyanhydrides, polyortho-esters, polyurethanes, and polycarbonates.

An alternative to traditional materials

Bioabsorbable polymers can safely remain in the body for controlled periods, thereby presenting an alternative to traditional polymers and metal components. Further, bioabsorbable polymers may be extruded into forms that include tubes, rods, monofilament and braided sutures, screws, staples, pins, and plates. Stents and grafts are also possible along with 3D-porous scaffolds, films and coatings, injectable gels, microspheres, and shapes for drug delivery.

Bioabsorbable polymers generate no harmful debris from wear during use and are MRI compatible.

And because the chemical and physical properties are more than slight differences to existing materials, bioabsorbable polymers can significantly change the next generation of medical devices. The high tensile strength and high modulus of some of these polymers make them well suited for load-bearing applications, such as orthopedic fixation and sutures, while the flexibility of others will make them useful in tissue engineering.

Bioabsorbable polymers are prepared by copolymerization of various monomers to modify and improve their properties as applications demand. Some of the most common are the poly(lactide-co-glycolide) copolymers. Blending (mechanically mixing as opposed to copolymerization) the polymers can further expand their properties. For example, blending poly(L-lactide) and poly(D-lactide) at a 1:1 ratio provides more strength and greater thermal and chemical stability than the individual polylactides.

Special provisions should be made for handling these materials on a regular basis. Packages must be back-flushed with an inert gas, such as nitrogen or argon, and the packaging material must have low oxygen and water-vapor-transmission rates. Storage specifications depend on material and a device's final form. Stability studies should also be conducted as part of the design process. Shelf-life can be increased for most systems by storing them at reduced temperatures in their original packaging. Once opened, the ideal storage is under vacuum with an inert purge gas used when opening the storage chambers.

Expectations and opportunities

Bioabsorbable materials will greatly influence the future of the medical-device industry especially for a wide range of medical products in preventive care and treatments. Current applications include wound management, orthopedics, dental, surgical, tissue engineering, and controlled drug delivery.

A few properties for selected absorbable polymers

Material	Tg (C)	Tm (C)	E (GPa)	τ (MPa)	Absorption time (months)
PGA	40	215 to 225	6.0 to 7.0	95	4 to 6
PLLA	60	180 to 190	2.7 to 4.1	65	18 to 36
PDLLA	55	Amorphous	1.0 to 3.0	40	12 to 16
PDO	-10	110 to 120	1.5	Na	6 to 12
PGA-PLLA (90%, 10%)	42	202 to 210	3.0 to 6.0	45	3 to 4
PLLA-PGA (85%, 15%)	55	140 to 150	3.3 to 3.5	65	12 to 18
PGA-PTMC	Na	Na	2.4	Na	6 to 12

A few key properties of bioabsorbable polymers include adjustable absorption rates, varying strengths and stiffness, and multiple extrusion forms. The materials are FDA-approved for medical devices and can be tailored to customer specifications. (In the table, T_g is the glass-transition temperature and T_m is the melting temperature.)