Preparation and characterization of high-performance poly (lactic acid) nanocomposites for biomedical applications
Pinto, Artur Daniel Moreira
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Poly(lactic acid) (PLA) has several applications in biomedicine (sutures, scaffolds, implants, drug micro/nanoencapsulation). This aliphatic polyester is prepared from lactic acid (therefore derived from 100 % renewable sources, e.g. corn or sugarcane), biodegradable, biocompatible and has a low cost.  In order to make this material more attractive to the mentioned applications, as a valid alternative to petrochemical plastics, some properties should be improved, namely mechanical resistance and gas permeability.  Graphene, the elementary structure of graphite, is an atomically thick sheet composed of sp2 carbon atoms arranged in a flat honeycomb structure, possesses remarkable mechanical strength (Young’s modulus = 1 TPa, tensile strength = 130 GPa), an extremely high surface area (theoretical limit: 2630 m2·g-1) and is impermeable to gases. Graphene oxide (GO) is similar to graphene, but presents several oxygen-containing functional groups (e.g. hydroxyls, epoxides, and carbonyls). The presence of these polar groups reduces the thermal stability of the nanomaterial, but may be important to promote interaction and compatibility with a particular polymer matrix.   It has been shown in studies with mice that GO is biocompatible  up to blood concentrations of 10 mg·kg-1 ; since only small amounts of graphene oxide are needed to reinforce poly(lactic acid), these nanocomposites could be used in food/medicines protection materials  and biomedical technology.  Effective mechanical reinforcement of polymeric materials using very small wt. % of GO has been reported by several authors. Wang and co-workers, improved the Young´s modulus of chitosan by 51% and the tensile strength by 91% incorporating 1 wt. % GO . An increase of about 75% in polypropylene’s Young´s modulus and yield strength was achieved at 0.42 wt. % GO loading by Song and co-workers.  Cao and co-workers increased Young´s modulus of PLA by 18 % with only 0.2 wt. % of reduced GO.  Graphene oxide and graphene had been reported as efficient drug carriers  . PLA is also used for this purpose ; development of hybrid vehicles for drug targeting can take advantage of both materials properties and originate synergistic effects.  Also several graphene based biosensors are being developed , these sensors can be used, for example, to detect drug concentrations on target places. Recent studies show that graphene substractes promote adherence of human mesenchymal stromal cells and osteoblasts , which can lead to better performance on tissues recovery using scaffolds containing graphene and graphene oxide. Due to their great potential several approaches are under study for future applications of these nanomaterials in biomedical engineering and biotechnology.  In this work, nanocomposite poly(lactic acid) (PLA) thin films were produced incorporating small amounts (0.2 to 1 wt. %) of graphene oxide (GO) and graphene nanoplatelets (GNP). Films were prepared by solvent-casting. Mechanical properties were evaluated for plasticized (by residual solvent) and unplasticized films. Plasticized nanocomposite films presented yield strength and Young´s modulus about 100 % higher than pristine PLA. For unplasticized films improvements in tensile strength and Young´s modulus were about 15 % and 85 %, respectively. For both film conditions, a maximum in mechanical performance was identified for about 0.4 wt. % loadings of the two filler materials tested. Permeabilities towards O2 and N2 decreased respectively three and fourfold in films loaded with both GO or GNP. The glass transition temperature showed maximum increases, in relation to unloaded PLA films, of 5 ºC for 0.4 % GO, and 7 ºC for 0.4 % GNP, coinciding with the observed maximums in mechanical properties. The incorporation of GO and GNP on PLA at low loadings (0.4 wt. %), don´t affect cellular proliferation at the surface of the resultant nanocomposites. This allows GNP and GO dispersion on polymers in order to obtain high performance materials for biomedical applications. Future work will be focused on improving the synthesized materials mechanical and gas barrier performance by optimizing the surface oxidation level and using different manufacture processes. Other biological assays will be performed to assure composite biocompatibility and to characterize it´s “in vitro” and “in vivo” behavior.