Resorbable polymers are becoming more and more popular in medicine as they are prone to entire degradation into products easily disposable from the body. Among others, polyesters of lactide acid, e.g. polylactide belong to this group (1), (2), (3), (4), (5). The process of degradation and resorption depends on numerous factors – the most important ones are: properties of the polymer as such (its molecular structure, polydispersity index, hydrophilic or hydrophobic character, crystallinity, chemical stability of polymer chains, presence of catalysts, additives, pollutants and softening agents), geometry of the implant and the conditions of implantation, e.g. temperature, pH (6), (7), (8), (9). Moreover, nowadays implants are expected to be multifunctional – they are to play a certain mechanical role (e.g. plates or screws) but also grant assumed biological features (10). Adding modifiers might ensure these functions. Bioactive ceramic additives are beginning to play a role in modifying biological properties of resorbable polymers used in bone surgery (including polylactide) (6), (11), (12), (13), (14). They facilitate the growth of bone tissue that is supposed to gradually replace the resorbable implant (15). Yet the additives may affect the process of polymer degradation. This influence depends not only on the kind of additives but also on their form, size and volume fraction in the composite (16), (17). These relationships are not fully explained, especially in multiphase systems. Moreover, despite many years of investigation of composites based on resorbable matrix, especially PLA modified by calcium phosphate ceramic, the problem of obtaining the composites with satisfying properties (mechanical, degradation rate) is still current, which is confirmed by the recent publication in this area (18), (19), (20), (21). Due to the long duration of polylactide degradation, it is difficult to observe the whole process in laboratory conditions. That is why, the accelerated degradation (taking place in harsh environmental conditions) is studied, especially for the sake of preliminary assessment of clinical utility. In vitro testing is the first stage of analysis which assesses the influence of different factors (e.g. modifiers) on the material’s properties (22).
The aim of the research was to describe the mechanisms of the accelerated degradation of resorbable implants designed as mini-plates for maxillofacial surgery. Those PLA matrix composites were modified with bioactive additives: micrometric tricalcium phosphate (TCP) and nanometric hydroxyapatite (HAP). The influence of the applied modifiers on the degradation rate was also established. Procedures of accelerated degradation test were performed on the basis of standard indications.
The following materials were tested: PLA – poly(L-lactide), PLA/TCP – a composite based on PLA matrix with the addition of 7 wt.% TCP and PLA/TCP/HAP – a composite based on PLA matrix with the addition of 7 wt.% TCP and 0.3 wt.% HAP. PLA was produced by PURASORB® Purac company (Netherlands), β-TCP produced by CHEMA-ELEKTROMET (Poland, Rzeszów) was used in micrometric size and HAP produced by CHEMA-ELEKTROMET (Poland, Rzeszów) was used in nanometric size. Volume fractions of additives were selected experimentally, taking into account samples bioactivity, mechanical properties and technological aspects. The composition of the materials was optimized in the preceding research. From the literature it was known that introduction of TCP into the polymer matrix gives such a composite bioactivity and the more TCP that is added the better bioactivity. From our research, on the other hand, it was revealed that when the mechanical properties of such a composite are taken under consideration, the maximum amount of TCP is limited. When the addition of TCP is higher than 7 wt.%, mechanical properties of the composite significantly decrease. Thus, to better modify and change bioactive and mechanical properties, nanohydroxyapatite was used as a second modifier. Similar tests were conducted to experimentally verify the optimal amount of HAP. It was discovered that even such small amounts as 0.5–1 wt.% of nano-HAP changed the mechanical properties of the material. Composite PLA/7 wt.%TCP/0.3 wt.%HAP became more ductile compared to PLA/7 wt.%TCP which was rather brittle. Also, it was revealed that the addition of more than 1 wt.% of nano-HAP makes the homogenization in the injection molding machine more difficult. Based on these results three materials compositions were selected for further research.
For sample preparation the vertical injection molding machine Multiplas V4-S-15N (Taiwan) was used. The materials – polymer granules and bioceramic additives – were firstly mechanically mixed in plastic containers and such a feedstock was applied to the hopper of the injection molding machine. Afterwards it was homogenized in the heated barrel with a reciprocating screw in the temperature range 165–175°C. Then the material was injected into the mold and was shaped as “paddles”.
The testing was performed in accordance with EN ISO 10993-13:2010 and ISO 13781 norms. The accelerated degradation was conducted in distilled water at 70°C. The observation periods lasted, respectively: 1 day and 1, 2, 4 weeks. All tests were done in quadruplicate – four samples of each kind were used for every period of time. Data are presented as mean value±standard deviation. Statistical analysis was performed by one-way ANOVA and the Tukey test. The results with P-values <0.05 were considered as statistically significant. Microstructural observation was done with a scanning electron microscopy (SEM) Nova 200 NanoSEM (FEI Europe Company). The microstructure observation was conducted on breaches obtained by breaking the paddles. Thermal testing was performed by means of differential scanning calorimetry (DSC) with a DSC1 manufactured by METTLER TOLEDO (Switzerland). The tests took place at temperatures of 0°–220°C, at the speed of 10°C/min, in aluminum pots, in a nitrogen atmosphere. To calculate the crystallinity index (χ [%]=ΔHm/ΔHm0), the thermal effect of melting the tested sample (ΔHm) and pure crystalline sample of PLA=93 J/g (ΔHm0) was assumed. Viscosity was measured with the capillary viscometer Ubbelohde. The polymer solutions were prepared by dissolving the samples in chloroform, where the initial concentration was 0.8 g/100 ml. The measurements were taken at 25°C for six consecutive concentrations: 0.8, 0.6, 0.4, 0.3, 0.2, and 0.1 g/100 ml (quintuplicate for every concentration). The Mark-Houwink equation (a=0.73; K=0.545*10−3) was applied to determine the viscosity average molecular weight. Mechanical parameters were established in a tensile test performed by means of a universal testing machine Zwick 7000 type 1435 (Germany). In accordance with the norm, the wet paddles were tested. As scheduled, the initial samples (non-incubated) were immersed in distilled water for 45 min at 37°C before the tests.
Mechanical testing proved that adding TCP significantly reduced plasticity of the material (Figures 1 and 2). Initial samples of the two-component composite were more fragile; in fact, no plastic area was observed. It happened because TCP increased the crystallinity index of the polymer, which equaled 50% for PLA and PLA/TCP/HAP and 60% for PLA/TCP (the values calculated from DSC). Importantly, the presence of TCP/HAP did not make initial materials more fragile and it did not increase their crystallinity index. The materials became more fragile in the degradation process, which led to the increase in polymer crystallinity. It was very evident for the PLA/TCP composite and less visible for the PLA/TCP/HAP one. For pure polymer the plasticity first increased probably due to the water absorption which acts as plasticizer for the polymer matrix. The tensile strength of all the materials significantly decreased after the incubation (Figure 2). A particularly sharp decrease was observed for the two-component composite (PLA/TCP). Adding calcium phosphates slightly increased the Young’s modulus, yet the degradation process eliminated these differences. HAP increased the tensile strength and the structural stability of the composite material as compared to PLA/TCP, which implies that HAP may improve homogeneousness of the composite with TCP. The selected results of SEM testing confirmed the alterations in the fractures’ character during the degradation. The fracture became more fragile as the degradation advanced and the process was especially prominent for the composite samples (Figure 3). The images of the fractures in initial samples showed the microstructural differences resulting from the presence of phosphate additives.
As the degradation proceeded, the disintegration of polymer chains and decrease in molecular weight of the material occurred (Figure 4). The results of the testing also indicated that the presence of TCP alone significantly accelerated the polymer degradation, while the degradation of the PLA/TCP/HAP composite was similar to pure PLA. The molecular weight changes are relevant to the changes in polymer crystallinity. The DSC curves for pure PLA and the PLA/TCP/HAP composite are presented in Figures 5 and 6. For both materials the melting peak sharpened as the incubation was prolonged, which was connected with gradual restructuring and the increase in crystallinity. Only for the initial samples was the peak coming from cold crystallization (at 160°C) was present in the curves. It implies that for the incubated samples all processes of organizing the structure happened during the incubation at 70°C. Additionally, along with the polymer degradation its melting temperature decreased (the peaks shifting to the left). The presence of TCP/HAP influenced the thermal effect connected with cold crystallization (the peak marked with an arrow in Figure 6). The “splitting” peak was observed for the composite, which proves that the additives may facilitate organizing of the polymer structure, and – acting as crystallization seeds – leads to the two-phase crystallization.
The mass changes of samples are presented in Figure 7. After initial water absorption, mass decrease is visible. There are no visible differences in the rate of mass decrease, as it was observed in the case of molecular weight, however, in most cases they are statistically significant. The influence of additives is also different. After 14 days the mass decrease is slightly bigger for pure PLA than for composites, however, after 28 days the biggest decrease is visible for PLA/TCP/HAP.
The research on accelerated degradation (in harsh conditions of the experiment) ensured observation of the subsequent stages of hydrolytic polymer degradation and the assessment of relative time correlations between those stages. The tests made it possible to know a mechanism of accelerated hydrolytic degradation of poly-l-lactide modified with bioactive particles. At the first stage, water penetrates the implant, attacking the amorphic phase bonds and reducing the long polymer chains to shorter fragments. The result of this phenomenon is regrouping of polymer chains and the crystallinity increases. As the matrix of the material is stable (due to the stability of crystalline phase), only a decrease in its molecular weight occurs, while the mechanical parameters and mass remain unchanged. The reduction of physical properties takes place later, when the implant undergoes further observable fragmentation caused by water. The mechanism of PLA degradation obtained in accelerated degradation is similar to the one reported for the real time degradation (23), (24). It confirms the usefulness of the short time degradation in an accelerated environment to analyze the influence of the modifying phase.
The applied bioactive additives do not change the mechanism of polylactide degradation, yet they clearly alter the rate of this process. The influence of modifiers on implant degradation is especially important in their application as mini-plates in maxillofacial surgery, primarily due to their mechanical function. Decrease of implants mechanical properties and the rate of bone healing should be adjusted. The changes in the degradation rate of polymer modified with bioactive particles are connected with the presence of interphase boundaries in the composites, which eases diffusion of liquids inwards in the material and promotes degradation (25). It is particularly visible in the case of PLA/TCP. At the initial stage of incubation, before the accelerated degradation of polymer chains and macroscopic disintegration of the material, ceramic particles are nucleation agents, accelerating and stimulating the matrix organization and appearance of crystalline areas. This is clearly confirmed by structural analysis of the tested samples. The influence of the micrometric TCP addition on polymer crystallinity was also observed by Siqueira et al. (18). The strongest effect of TCP on the increase of PLA crystallinity degree was observed for 1 wt.% of modifier. However, 5 wt.% was less effective and 8 wt.% caused the decrease of crystallinity. It can be explained that the presence of a larger amount of ceramic particles hinders the reorganization of polymer chains. The properties of the composite with TCP and HAP are very different. A small number of HAP nanoparticles may hinder the process of polymer chain organization and in consequence, the crystallinity of tri-phases composite is lower than PLA/TCP. The amount of nanomaterials for which their influence is most often intensified depends on their type and many authors observed different results (26), (27). Pielichowska et al. observed the most effective influence of nano-HAP on the composite properties with the amount of 0.5 wt.%, as compared to the 1, 2.5 and 10 wt.%. The effect of nano-additive is similar to the one observed for micro-additive, however, their volume fractions differ. The studies obtained in this paper, prove that simultaneous addition of TCP and HAP changes their influence and the crystallinity of PLA/TCP/HAP is comparable to pure PLA. The differences in PLA crystallinity caused by TCP and TCP/HAP influence the mechanical parameters of the analyzed composites. The PLA/TCP composite is more fragile, which is related to the crystallinity increase. Moreover, its strength is the lowest because of TCP agglomeration. Besides, nano-HAP can improve the adhesion at the composite interphases. As a result of these changes, the TCP addition facilitates the polymer degradation more than the presence of TCP/HAP. In the three-component composite, TCP as a resorbable material makes it possible to create a fully-degradable composite. Its role is to deliver ions of calcium and phosphorus, while a slight addition of nanometric HAP improves the mechanical properties, besides granting the bioactive features and accelerating mineralization.
The influence of TCP and HAP addition on polymer degradation related to its crystallinity changes and the molecular weight decrease is slightly different from the sample mass changes. The mass decrease is the effect of diffusion and removal of degradation products. However, due to bulk degradation of PLA and autocatalytic process (23), the changes in polymer molecular weight are visible earlier than the mass decrease. The rate of these processes is related to the presence of polymer-ceramic particles interphases (25), therefore the water absorption in the first stage of incubation was more intensive for composites. The mass decrease after 28 days is more visible for PLA/TCP/HAP but the difference is about 1% comparing with PLA and PLA/TCP. This result suggests that removing of degradation products is similar in all tested materials, despite differences in polymer molecular mass changes.
The applied modifiers TCP and HAP did not affect the mechanism of polylactide degradation.
Adding TCP alone influenced the structural changes in the polymer more significantly than the addition of TCP/HAP, thus affecting the faster degradation. It is to be expected that the degradation mechanism and the influence of TCP and HAP modifiers on this process will be sustained in real-life conditions, although its kinetics will be different.
This research was financially supported by the research project: PARP No 126.96.36.199 in cooperation with “MEDGAL”, Białystok.
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About the article
Published Online: 2016-10-08
Published in Print: 2016-11-01