Microhardness of starch based biomaterials in simulated physiological conditions

Abstract

In this work the variation of the surface mechanical properties of starch-based biomaterials with immersion time was followed using microhardness measurements. Two blends with very distinct water uptake capabilities, starch/cellulose acetate (SCA) and starch/poly(ε-caprolactone) (SPCL), were immersed in a phosphate buffer solution (PBS) at 37.5 °C for various times. The microhardness of the blends decreased significantly (∼50% for SPCL and ∼94% for SCA), within a time period of 30 days of immersion, reflecting the different hydrophilic character of the synthetic components of the blends. The dependence of microhardness on the applied loading time and load was also analysed and showed a power law dependency for SCA. Water uptake and weight loss measurements were performed for the same immersion times used in the microhardness experiments. The different swelling/degradation behaviour presented by the blends was related to the respective variation in microhardness. Moreover, complementary characterization of the mechanical properties of SCA and SPCL was accomplished by dynamic mechanical analysis (DMA) and creep measurements. Microhardness measurements proved to be a useful technique for characterizing the mechanical behaviour near the surface of polymeric biomaterials, including in simulated physiological conditions.

Introduction

The idea of substituting synthetic polymers with natural polymers in any application and, in this way, using a renewable source, is extremely appealing. Starch is the major polysaccharide constituent of photosynthetic tissues and of many storage organs in plants [1]. Starch-based biomaterials are totally biodegradable, with an associated low cost when compared with other biodegradable polymers, are available in large quantities and, therefore have an enormous potential for environmental and clinical applications. In the last few years these kinds of systems have been proposed in our research group for different biomedical applications [2], [3], [4], [5], [6], [7], [8] including for replacement materials, controlled delivery systems, hydrophilic cements and, more recently, for scaffolds in tissue engineering applications. The proposed systems are blends of starch with ethylene–vinyl alcohol copolymer (SEVA-C), cellulose acetate (SCA), poly(ε-caprolactone) (SPCL) and poly(lactic acid) (SPLA) [2], [3], [4], [5], [6], [7], [8]. The combination of biocompatibility, suitable mechanical and degradation properties constitutes one of the main advantages of the starch-based blends that have been developed, showing that they have potential to be used as scaffolds in tissue engineering [9], [10].

Moreover, biomaterials interact with their environment at the cellular level and it is the surface of the material that directly interacts with proteins and cells [11], [12], [13]. Consequently, the surface mechanical properties of an implant are very important in determining cell responses [11], [12] and the implant behaviour will strongly depend on these properties [13]. For instance, the surface topography and surface mechanical strength of an implant can be critical to its success, because it has been shown that cell adhesion and spreading depend on these factors [12]. Being a hydrophilic polysaccharide, starch and its blends may present considerable water uptake and it is expected that the mechanical features will be different in the dry state when compared with the hydrated state, the latter being obviously more relevant in clinical applications. Some works have reported the influence of water on the mechanical properties of biomaterials (see e.g. Refs. [14], [15]). However, in this case the mechanical behaviour of the surface of the material with which the cells and tissues will interact may be different from the bulk properties, especially when the hydration equilibrium is not achieved. A simple technique such as microhardness can provide a possible way to measure the actual hardness of the surface layer, which is difficult to measure by traditional techniques, such as tensile or flexural tests.

So, the aim of the present work was to use microhardness to evaluate the changes in the surface mechanical properties of starch-based blends with different swelling capabilities (SCA and SPCL), when they are immersed in a solution for different periods of time. Although some microhardness studies of starch can be found in the literature [16], [17], [18], as far as we know, this is the first time that this technique is used to characterise starch-based biomaterials after being immersed in simulated physiological fluids at body temperature.

In order to complement the characterization of the mechanical properties of the starch-based blends, their creep and dynamic mechanical properties were also investigated. Such kinds of tests allow the intrinsic viscoelastic behaviour of these polymeric systems to be analysed and could also provide information on whether any correlation exists between the bulk viscoelastic properties of the materials and their microhardness.

Parallel swelling and weight loss measurements were performed in order to study how the degradation of these starch-based biomaterials is affected by the hydrophobic or hydrophilic character of the other component and how this influences the respective mechanical properties. As reported in the literature [19], [20], hydrophilicity is a determining parameter in the degradation behaviour of starch-based materials.