Design of New Biomaterials from lactic acid

Polymeric biomaterials derived from lactic acid have extensive uses in medical applications, especially in the context of biodegradable sutures. They are widely used in the architecture of cardiac tissue, as support for drugs, and biodegradable fixation devices for the repair of small broken bones such as the ones in the hands, joints and feet.

These orthopaedic implants are gradually metabolised and naturally assimilated by the body. Their mass is progressively transferred into the broken bone, helping the healing process and thereby eliminating the need for a second intervention.

These new materials are obtained through molecular catalysis, and require breaking the cyclic dimer of lactic acid “lactide” to obtain polylactic acid (PLA). The lactide is a renewable natural resource that occurs as a by-product of the fermentation of biomass with high starch content, such as maize, wheat, or sugar beet. As in every polymerisation process, a catalyst is required and in this case the active compound must be a metal.

Consequently, this catalytic process has been studied with different metals such as tin, yttrium, titanium, aluminium and other lanthanides. However, since on some occasions residues of the catalyst can be incorporated into the polymer, it is important to preserve the biocompatibility and zero toxicity of the PLA by insuring that the metallic catalyst used is biologically benign and does not have a negative impact on tissue. These medical uses have favoured the use of metals like magnesium, calcium or zinc, all of them common inside the human body.

On a different front, PLAs are being investigated as a possible raw material of many manufactured products, since they present similar and in some cases better properties than traditional polymers that are derived from the bioresistant poly (a-olefin), with the significant added benefit of biodegradation.

While their production costs were considered too high in the past, recent developments in the treatment and production combined with the contrasting ecological hazard represented by petroleum derived polymers have brought these types of biodegradable polymers to very competitive positions.

One of the most recent and relevant examples that confirm this growing expansion, is the joint endeavour by Cargill. Inc., and The Dow Chemical Co., who have recently announced the mass production of many tons of PLAs.

The scientific community shows a growing interest to find catalysts that are capable of producing such biomaterials with well defined microstructures, since this defines the mechanical properties, the biodegradability, and the overall usability of the material.

With this in mind, the research group from the URJC, formed by Dr Andrés Garcés and Carlos Alonso and coordinated by Dr Luis Fernando Sánchez-Barba, is working in collaboration with the UCLM to develop different families of catalysts based on magnesium and zinc and stabilised by ligands like heteroscorpionate of they type “NNN”, capable of polymerising the ε-caprolactone and the lactide in a controlled manner. These are extremely active initiators with a chemical formula of [M(R)(NNN)] (M = Mg, Zn) that achieve a productivity of 21.000 Kg of poly-ε- caprolactone (PLC) produced per mol of Mg each hour at room temperature.

Moreover, some of these initiators allow for a controlled growth of the PLA’s microstructure. This is linked to the influence that the heteroscorpionate exerts during the process of opening the cyclic dimmer, which in turn grants control over the future specifications and applications of the produced material such as a high molecular mass, crystallinity as well as high fusion temperature (165ºC), all of it generating a great interest from industry.

This study has been published in the latest editions of the Inorganic Chemistry & Organometallics magazine.