Author(s): Pereira, João Ricardo Antunes dos Santos Soares
Date: 2016
Persistent ID: http://hdl.handle.net/10362/19513
Origin: Repositório Institucional da UNL
Subject(s): Domínio/Área Científica::Engenharia e Tecnologia::Outras Engenharias e Tecnologias
Poly(hydroxyalkanoates) (PHAs) are a class of biodegradable and biocompatible polyesters with potential applications in biomedicine (Sodian et al., 2000). PHA are produced inside bacterial cells in the form of granules as carbon and energy storage compounds. There are three different PHA polymers, namely, short-chain-length PHAs (scl-PHA), medium-chain-length PHAs (mcl-PHA) and long-chain-length PHAs (lcl-PHA), depending on the size of their monomers. These polymers are formed during fermentation processes where bacteria grow with some defiant conditions (i.e. limitation in phosphorus, nitrogen, trace elements and/or oxygen), but with an excess supply of a suitable carbon source (Lee, 1996; Zinn and Hany, 2005). However, the use of effective carbon sources for production of PHA represent high costs to the fermentation process (Cruz et al., 2016). Therefore, the use of wastes or by-products as carbon sources, such as waste glycerol generated by the biodiesel industry, is important to reduce substantially the production costs of mcl-PHA (Muhr et al., 2013). In this work, a group of known mcl-PHA-accumulating bacteria, was studied for the accumulation of mcl-PHA using waste glycerol, from the biodiesel industry as the sole carbon source. Pseudomonas chlororaphis was found to be the most suitable candidate to produce mcl-PHA from glycerol, with a final CDW concentration of 3.28 g/L, among the tested strains, and with a polymer composed by hydroxyhexanoate (HHx), hydroxyoctanoate (HO), hydroxydecanoate (HD) and hydroxydodecanoate (HDd) monomers. Pseudomonas chlororaphis was studied in bioreactor cultivation experiments, in three different cultivation modes, namely, batch, pulse feeding and repeated batch and productivities of 0.026, 0.023 and 0.025 g/L h, respectively, were achieved. The polymer was recovered from the bacterial cells by Soxhlet extraction with chloroform. For the purification process, a standard technique with the re-dissolution of the mcl-PHA in chloroform followed by precipitation in ice cold ethanol was used. However, chloroform is a hazard solvent and cannot be used for industrial production. Hence, alternative recovery methods were also studied, using less hazardous solvents and the highest purity values were achieved by acetone, ethyl acetate and hexane solvents. Mcl-PHAs have unique mechanical properties that were studied by preforming an axial tensile test in a mcl-PHA film. In this assay, a tension at break values between 2.91 to 4.89 MPa were achieved in four replicates, together with an elongation at break of 212.39 to 296.87% and a Young modulus of 0.60 to 0.90 MPa. Since the main objective in this work was to develop a new fully natural bio-based adhesive to be used in medical applications, mcl-PHA, due to his tacky behaviour, presents its self as perfect candidate for this issue. So for that purpose some adhesion tests, using mcl-PHA and porcine skin as a model substrate, were made. It was performed a tension test, in three replicates, where it was attained values of tension at separation between 45.02 and 90.13 kPa. It was also made some shear tests, in three replicates and a tension at separation values of 10.02 to 12.87 kPa were achieved.
