The effect of the environment on the mechanical properties of medical grade silicones

Antibacterial Polysiloxane Polymers and Coatings for Cochlear Implants

Within this study, new materials were synthesized and characterized based on polysiloxane modified with different ratios of N-acetyl-l-cysteine (NAC) and crosslinked via UV-assisted thiol-ene addition, in order to obtain efficient membranes able to resist bacterial adherence and biofilm formation. These membranes were subjected to in vitro testing for microbial adherence against S. pneumoniae using standardized tests. WISTAR rats were implanted for 4 weeks with crosslinked siloxane samples without and with NAC. A set of physical characterization methods was employed to assess the chemical structure and morphological aspects of the new synthetized materials before and after contact with the microbiological medium.

Comprehensive characterization of silica-modified silicon rubbers

In this study a commercially liquid silicone rubber was filled with fumed silica particles in different concentrations and evaluated for medical applications. The thermal, morphological and mechanical properties of silicone/silica composite samples were studied before and after aging, flexural tests and immersion in saline environment. Understanding the effect of silica content, aging conditions and thickness (from 0.6 to 2 mm) of the samples on the behavior of these materials in different environments is crucial for applications as implantable devices. Before inducing any mechanical stress, tensile strength was found to increase for samples containing 3 or 5 wt% of fumed silica, depending on the thickness. A similar trend was observed after 10⁶ flexes for tensile strength, storage modulus and hardness at room temperature, which increased with the concentration of fumed silica. Moreover, tensile strength decreased with increasing the thickness of the samples from 0.6 to 2 mm. The thermal degradation was found to start at higher temperature in the case of the composites as compared with neat silicone, however, the glass transition and melting temperatures were only slightly modified by the presence of the silica particles, regardless the mechanical aging. The MTT assay using L929 fibroblasts mouse cells showed a good short-time cytocompatibility for both silicone elastomer and the composite with 3 wt% fumed silica. Similarly, the measurement of the cytokine secretion revealed no inflammatory response.

Color and structural changes of a maxillofacial elastomer: the effects of accelerated photoaging, disinfection and type of pigments

PURPOSE

The aim of this study was to investigate the color and structural changes of a maxillofacial silicone colored with 2 different pigments, after photoaging and immersion in disinfectants.

METHODS

Ninety-six cylindrical specimens were fabricated and divided into 3 equal groups. The specimens of the first group consisted of unpigmented silicone (Multisil Epithetik), those of the second group consisted of unpigmented silicone, colored with red functional liquid pigment (Cosmesil Reactive 0.2% wt). The specimens of the third group were fabricated using unpigmented silicone colored with red powder pigment Cosmesil Dry at 0.2% wt. Specimens of each group were divided into 4 equal subgroups (immersed in soap solution, ethanol 95° or distilled water or placed in a photoaging apparatus for 174 hours). Structural changes were examined by infrared spectroscopy (ATR-FTIR) before and after aging. Color changes (ΔΕ*) were measured using the CIE L*a*b* system. Two-way ANOVA and Tukey's test for post hoc comparison were used at a = 0.05.

RESULTS

Infrared spectroscopy showed no structural changes after immersion in solutions and photoaging, for all the materials tested. No statistically significant differences for ΔΕ* among the tested groups were found.

CONCLUSIONS

It can be concluded that no structural changes of pigmented and unpigmented silicone elastomers were observed among all aging procedures. Recorded color changes for the materials tested were within the limits of clinical acceptability after all aging procedures. Immersion in distilled water presented the best color stability, whereas photoaging, the poorest, for all materials.

Biomaterials in cochlear implants

The cochlear implant (CI) represents, for almost 25 years now, the gold standard in the treatment of children born deaf and for postlingually deafened adults. These devices thus constitute the greatest success story in the field of ‘neurobionic’ prostheses. Their (now routine) fitting in adults, and especially in young children and even babies, places exacting demands on these implants, particularly with regard to the biocompatibility of a CI’s surface components. Furthermore, certain parts of the implant face considerable mechanical challenges, such as the need for the electrode array to be flexible and resistant to breakage, and for the implant casing to be able to withstand external forces.

As these implants are in the immediate vicinity of the middle-ear mucosa and of the junction to the perilymph of the cochlea, the risk exists – at least in principle – that bacteria may spread along the electrode array into the cochlea. The wide-ranging requirements made of the CI in terms of biocompatibility and the electrode mechanism mean that there is still further scope – despite the fact that CIs are already technically highly sophisticated – for ongoing improvements to the properties of these implants and their constituent materials, thus enhancing the effectiveness of these devices.

This paper will therefore discuss fundamental material aspects of CIs as well as the potential for their future development.

The effect of accelerated ageing on performance properties of addition type silicone biomaterials

The UV-protection provided to addition type silicone elastomers by various colorants, such as conventional dry earth pigments, as well as the so called "functional or reactive" pigments, was investigated. Moreover, the effect of a UV light absorber and a silica filler was also explored. Under the experimental parameters of this work, the exposure of silicone to UV radiation resulted in some changes of the IR absorbance, thermal decomposition after 400 degrees C, T(g) and tensile properties, whereas the storage modulus of samples was not affected. The obtained spectroscopic data, as well as the results of TGA and storage modulus, were interpreted by assuming that chain scission takes place during aging, whereas the improvement of tensile strength allows the hypothesis of a post-curing process, initiated by UV radiation. Therefore, the increase of T(g) could partly be due to the above reason and, furthermore, to the contribution of a rearrangement of chain fragments within the free volume of the elastomeric material. Regarding the evaluation of various coloring agents used in this work, the obtained results show that dry pigments are more sensitive to accelerated ageing conditions in comparison with functional liquid pigments. Moreover, the hydrophobic character of silicone matrix is enhanced, with the addition of this type pigments because of the vinyl functional silanes groups present in their chemical structure. Finally, it should be noted that the incorporation of silica nanofiller did not seem to prevent the silicone elastomer from degradation upon UV irradiation, but showed a significant reinforcing effect.

Frequency dependence of viscoelastic properties of medical grade silicones

Cylinders of medical grade silicone elastomers, (29 mm in diameter and 13 mm thick), immersed in physiological saline solution at 37 degrees C, were investigated by dynamic mechanical analysis (DMA). A sinusoidal cyclic compression of 40 +/- 5 N was applied over a frequency range, f, of 0.02-100 Hz. Values of the storage, E', and loss, E'', moduli for the cylinders were found to depend on f; the dependence of E' or E'' on the logarithm (base 10) of f was represented by a third-order polynomial. Above about 0.3 Hz, the cylindrical specimens appeared to be undergoing the onset of a transition from the rubbery to the glassy state. There was no significant difference between results obtained at 37 and 23 degrees C; pretreatment of specimens in physiological saline at 37 degrees C for 24 h and 29 days had no appreciable effect on the results.

Accelerated aging for testing polymeric biomaterials and medical devices

Elevated temperature is frequently used to accelerate the aging process in polymers that are associated with medical devices and other applications. A common approach is to assume that the rate of aging is increased by a factor of 2(DeltaT/10), where DeltaT is the temperature increase. This result is a mathematical expression of the empirical observation that increasing the temperature by about 10 degrees C roughly doubles the rate of many polymer reactions. It is equivalent to assuming that the aging process is a first order chemical reaction with an activation energy of 10R/log(e)2, where R is the universal gas constant. A better approach would be to determine the activation energy for the process being considered but this is not always practicable. The simple approach does not depend on the temperature increase, provided that it is not so great that it initiates any physical or chemical process that is unlikely to be involved in normal aging. If a temperature increment theta were to increase a given polymer reaction rate n times, then an elevated temperature would increase the rate of aging by a factor of n(DeltaT/theta).