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A new modified silicone was obtained by physical entrapment of a hydrophobic lipid, isopropyl myristate (IPM), to improve encapsulation properties and corrosion resistance for medical electronic implants. It was demonstrated that IPM loaded silicone films were stable in an aqueous environment with no leaching from the silicone matrix, though slight initial leaching was observed in buffered BSA. An unexpected difference between water transport for films in contact with water vapour vs those in contact with liquid water was identified, showing increased permeability to water vapour, possibly the result of differences in water organisation at the hydrophobic film interface. An improvement in the mechanical properties of the modified material was also achieved including enhanced scratch resistance and adhesion.
Incorporation of IPM further resulted in significant improvement of cell biocompatibility compared with the unmodified polymer suggesting that the IPM combination could be a viable basis for implant device packaging.
Keywords: silicone coating, isopropyl myristate, water vapour transport, liquid water transport, biocompatibility
Silicone elastomers are widely used as biomaterials for coating of medical devices and implants due to their well known, and commonly accepted, biocompatibility and bioinertness. Nevertheless, basic materials properties could still be further optimised  . The generic advantage of these elastomers, however, is that they can respectively function as durable dielectric insulators, a barrier against matrix induced contamination and as stress relieving shock/vibration adsorbers over a wide range of humidity and temperature  . The production of catheters, stents, cardiac leads, tracheal intubators  soft contact lenses  and plastic surgery implants  is commonly silicone based, and now silicones have become the industry standard for encapsulating implantable electronic devices  . However, the major concern in these applications is resistance to water transport, particularly through relatively thin barrier layers, leading to device reliability issues through hydration, corrosion and degradation processes compounded by possible delamination  . Thus, it was reported that the main failure of a visual prosthesis was attributable to the ingress of moisture; such moisture could originate as water vapour as well as condensed water  caused by the failure of adhesion between encapsulant and microelectronic device, so creating voids within which condensation can occur  . Eventually, when enough water diffuses through an encapsulant to create a continuous water path at the device interface, the presence of entrained ions and any electrical bias promotes electrocorrosion  . This accelerates the surface degradation that puts at risk device functionality over the longer term.
Several solutions at molecular level exist to reduce water permeability through polymeric membranes. First and the most important is control through modification of chemical structure of polymer influencing for example chain polarity and hydrophobicity  ,  ,  . The permeability of polymers towards gases decreases with increasing polymer polarity, largely due to the higher activation energy for diffusion in a polar polymer  . Conversely, polar polymers are poor water barriers because water is soluble in such polymer phases  .
Polymer symmetry promotes closer chain packing and crystallinity, both of which also decrease permeability  ,  . The orientation of polymer molecules has a further effect on permeability17, the extent depending on the type of polymer and the degree of orientation; this can also lead to anisotropic permeability behaviour. Inter-chain crosslinking can also reduce permeability  , especially where the original non-crosslinked phase can be swollen by the penetrant. Finally, the addition of plasticiser to a polymer causes increased segmental mobility, effectively lowering the TÂg value and so causing a reduction in barrier properties  .
Entrapment of an additional hydrophobic agent, a synthetic lipid as part of a silicone membrane to mimic human skin  has proved a useful means of testing drug transport. In particular it appears that for skin it is intercellular lipid matrix that is essential to the barrier properties of skin, and drug permeation is radically altered after extraction of skin lipid  .
It was considered, however, that incorporation of the highly hydrophobic isopropyl myristate into a silicone matrix would not only reduce surface water repellency, but through reduced water solubility in the bulk phase, reduce water transport.
A further possible benefit of lipid incorporation into silicone was a slower ageing of the material in vivo. One cause of ageing of silicone breast implants, for example, is identified to be lipid infiltration from tissue resulting in loss of mechanical properties and failure of the elastomer  . Pre-entrapment of synthetic lipid should saturate the matrix with a defined, controlled agent and help reduce the further impact of body lipid ingress, so slowing down any effect on ageing.
A further concern regarding development of new materials for biomedical applications is the regulatory approval process.  Although synthetic polymer chemistry is a powerful and convenient materials modification tool, this leads to major regulatory challenges when any new chemistry is applied. Concerns would exist, for example, regarding residual synthetic side products, their bioeffects and the longevity of any facile surface chemical modification in the face of a chronic tissue degradative response at the implant locus. Both medical grade silicones and IPM have the advantage that they are already approved for clinical use, and use of physical entrapment eliminates the problems of covalent binding, with the uncertainty of unknown reaction side products. Such a modified material should thus experience a shorter approval pathway.
II. Materials and methods
Medical grade unrestricted use (for >30 days in vivo) silicone rubber (MED-4211 or MED6215). Nusil Technology Ltd, USA) part A was dissolved in heptane (VWR Int., UK) at a concentration of 20% w/v and stirred until solution homogeneity was obtained (2-3h). Next, isopropyl myristate (Fluka, UK) was added (with stirring) in varying amounts as required (specified under results). For preliminary studies, the following lipids were added: cholesterol, behenyl oleate, 4-methylumbelliferyl stearate, all purchased from SigmaAldrich and used without further purification. Finally, silicone part B of the was added in the ratio 1:10 (A:B) to part A was added and stirred. After 1h the material was ready for use for sample preparation. 4ml was cast on to a polystyrene Petri dish followed by 65Â°C incubation for 4h. After that, silicone films were gently removed from the Perti dish and placed on a glass plate for further heat treatment at 100Â°C for 2h and then at 150Â°C for 45min.
Contact angle measurements were performed by a contact angle rig (CAM 200, KSV Instruments Ltd., Finland) set up with at least five readings for each sample, using water as a wetting agent by the sessile drop method.
Visualisation of IPM
Lipid soluble dye Sudan Red (SR), purchased from Fluka, UK, was used to track IPM in silicone and to visualise any leaching. SR was dissolved in IPM (50mg/ml) which was then mixed with the silicone rubber as a visual marker and the combination taken through the standard procedure for silicone membrane production. After immersing the membranes in water for 14 days they were analysed by optical microscopy.
Water vapour transport
Water vapour transmission was determined by fixing a silicone membrane over the open mouth of a 50ml glass conical flask containing a known amount of water (~10ml). Membranes were additionally sealed at the sides with Parafilmïƒ’. Measurements of water vapour transport, based on gravimetric determination - vaporised water  , were taken after one week exposure to ambient air.
Liquid water transport measurement
A specially designed device was used to determine water transmission from liquid water through the silicone membranes. It consisted of two parts: a screwed, open circular (r=22.5 mm) upper clamp and a lower component of a known amount of hygroscopic agent (anhydrous CuSO4, ~2g, Fluka, UK) in a weighting boat. A silicone membrane was mounted between the two parts with paste sealant to ensure a leak-proof seal. After one week of immersion of the construct in distilled water at temperature 37ï‚°C, the weight of CuSO4 was remeasured to determine hydrate formation and therefore water transport through the membrane.
Membranes both for water vapour and liquid water transport measurements were vacuum dried at 60ï‚°C for 4h to remove any residual water prior to studies.
A silicon wafer coated with a single layer of Nusil MED 6215 silicone (used as a control sample) was coated with additional layers of either unmodified or 1% IPM modified silicones (MED4211 or alternatively MED 6215). The scratch resistance of such laminates was then determined by the following experimental setup: 1) a wafer was attached to a steel plate by double side adhesive tape 2) a specially designed device with a bevelled stainless steel needle was mounted on a tensile tester such that the needle pushed into the wafer with a constant force 3) the device was moved with a constant vertical velocity of 25mm/min to make a scratch. Scratch resistance was determined by measurements of the width of the created fissure using an optical microscope.
The same samples as described in the scratch test were cut into 50x10 mm squares and then a silicone layer strip was removed up to ~20mm and clamped on the upper part of a tensile tester, while the remaining silicon adherent sample was clamped to the lower surface. Once the sample was mounted, increasing force was applied and measured at the moment when the additional silicone flap started to delaminate. The value of this force was a direct indicator of the strength of adhesion. Each sample was measured in triplicate.
The test chosen was the direct cell contact assay, carried out in conformity with ISO10993-5:1999 using 3T3 mouse fibroblasts. Since the end-point of the standard test is a relatively subjective observation of cell morphology, it has become common practice to supplement this with a quantitative evaluation of cell proliferation. Here we used the rezazurin-based AlamarBlueÂ® dye reduction assay.
Samples of 10 x 10 mm were cut for testing. Both positive and negative toxicity polymer samples were included in the test. The negative toxicity controls were 13 mm Thermanox coverslips (Nalgene Nunc, USA, type 174950, lot 551810). ISO 10993-5 standard organo-tin plasticised PVC (Portex UK, type 499/400/000 lot 30375) was used for positive toxicity controls. Cell growth in the presence of test and control materials was compared to growth in standard multi-well tissue culture plates.
The Swiss albino murine fibroblast cell line 3T3 (ECACC Ref No: 93061524) was obtained from mycoplasma-free stocks held within the Institute of Cell and Molecular Biology. Cells were cultured in Eagle minimal essential medium (EMEM, D5546, Sigma, UK) supplemented with 1% penicillin/sterptomycin solution (10,000 U/ml of each antibiotic, Gibco Invitrogen, type 15140-122, lot 1268409) and 10% heat-inactivated calf serum (Biosera, South America origin, cat. No. S 1810/500).
The test materials were immersed overnight in 100% ethanol (Analytical reagent grade, Fisher Scientific, code: E/0605 DF/17, Batch 0613664) in a petri dish and kept overnight within a class II laminar flow cabinet. The materials were then washed three times in phosphate buffered saline and subsequently washed with EMEM containing 3% (w/v) penicillin/streptomycin.
The test and control materials were placed into 24 well plates; each containing three replicates of three of the test materials, and negative and positive controls. Each test material was used in two replicate cell culture plates, so that a total of 6 replicate determinations were carried out for each material. Once the test and control materials had been positioned in the plate, 1 ml of cell culture medium containing 3T3 cells at a density of 1 x 104 cells ml-1 was added to the wells. The tests for this group of materials were carried out in two parts, with duplicate cell culture plates used in each part.
1ml of a 10% solution of AlamarBlue (Biosource, type DAL1100, lot 146581SA) in cell culture medium was added to each well. The plates were placed back into the incubator for 4 hours at 37Â°C to allow reduction of the dye. A sample of the medium was then removed from each well and its optical density measured using a fluorescence plate reader (Bio-TEK Synergy HT) with excitation filters at 530-560 nm and emission at 590 nm. The resulting fluorescence signal is proportional to the amount the AlamarBlue dye that had been converted to the reduced form by the metabolic action of the cells and so is a measure of the number of metabolically active cells.
At the end of each dye incubation period, the dye containing medium was removed, the cells washed once, and then 1 ml of fresh cell culture medium was added. The cells were then returned to the incubator until the next measurement point.
III. Results and discussion
Preliminary lipid screening
The strategy for creating a new material with potential usage as a coating for implantable microdevices was centred here on a modified commercially available silicone. Lipids were selected as modifiers with a possibility for creating a water impermeable barrier through a shift to hydrophobic properties, with the aim of extending coated implant life time.
Four lipids: isopropyl myristate (IPM), cholesterol (CH), behenyl oleate (BO) and 4-methylumbelliferyl stearate (4MB) were investigated.
Contact angle measurement was used for a preliminary assessment of modified material water repellency  .
Contact angle measurements for Nusil MED-4211 silicone modified with various w/v lipids at 1 % w/v are shown in Figure 1. None of the compounds gave a significant increase in contact angle and cholesterol actually reduced contact angle, presumably because of the surface exposure of its hydrophilic hydroxyl group.
Among investigated lipids, IPM was attractive because of its prior use in biomedicine. Thus, it was reported not to cause significant inflammatory change to outer human stratum corneum in a comparative study with rat skin  . Also, it is also widely used in deeper dermis layers to enhance drug flux through the skin, eg of estradiol  . The hydrophobicity of the material gives a good basis for reducing device surface wetting through its inner surface contact with the device; liquid water tracking along the silicone/substrate interface would be less likely to occur.
Figure 1. Water / polymer surface contact angle measurements of lipid (1% w/v) modified MED 4211 silicone membranes with water. Abbreviations: IPM- isopropyl myristate, 4MB- 4-methylumbelliferyl stearate BO- behenyl oleate, CH- cholesterol.
IPM as a viable hydrophobic modifier was confirmed by a significant concentration dependent increase in contact angle up to at least 5% w/v IPM (Fig 2). At 10% IPM, likely surface inhomogenity may have led to an anomalous decrease in contact angle, a result of the surface migration of IPM as its dispersion in silicone becomes less stable at high concentration.
Figure 2. Effect of IPM concentration on the water / polymer surface contact angle of modified silicones.
After preparation, IPM modified silicone membranes were rinsed and then immersed in distilled water for 24h. Subsequently, they were dried with tissue and weighed. Results reflecting weight change are summarized in Table 1.
Mean % weight loss [n=5]
Table 1. Percentage weight loss after immersing in distilled water for 1day with subsequent vacuum drying. Silicone deposited on a glass slide
Results show significant experimental variability and no dependence of IPM concentration. However, the confirmed loss in weight for silicone alone suggests that some of the loss in weight with IPM modification is the result of residual material losses in the silicone itself rather than due simply to IPM leaching; possibly IPM loss was from traces on the surface and not from the bulk. The results also indicate that silicone polymerization went to near completion exceeding 99.4% (the maximum mean weight loss seen in Table 1).
After the above preliminary washing stage, in order to investigate the aging effect of silicone films they were immersed in water and reweighed periodically. There was no additional weight loss seen over a 3 week period (data not shown), indicating that IPM did not leach out from the silicone matrix and so was effectively retained even though only physically dispersed in the silicone.
Another test of IPM retention stability was a microscopy comparison of test membranes before and after immersing in a static water bath. Based on images in Figure 3, where sudan red is the IPM marker, it is clear that storing membranes in water did not cause IPM leaching nor to significant intra-membrane dispersive change. The images taken from matched membrane locations show little or no change in distribution or morphological features for the IPM after 14 days. It is also worth noting that at low concentration (0.5% w/v) IPM is dispersed uniformly, whereas at higher concentrations it created large aggregates and clusters.
Dye only 0.5%w/v IPM
2%w/ v IPM
Before immersing in water solution
After 14 days of immersing in water solution
Figure 3. Detection of IPM leaching using lipid soluble dye - Sudan Red, at light microscope, zoom 10X.
Modified silicone membranes were also exposed to phosphate buffer saline (pH 7.4) and bovine serum albumin (BSA) (40mg/ml) solution at 37â-¦C for 30 days to evaluate any protein influence on IPM stability. Figure 4 shows that with increasing of IPM concentration, the weight loss of silicone membranes immersed in BSA increased. It can be concluded that BSA has an additional effect and that as an amphiphile it appears to solvate the IPM assisting its release from the silicone matrix. It is known both that BSA has surfactant properties and that it can penetrate a polymer matrix, particularly a loosely packed polymer layer, regardless of its thickness provided there is polymer sufficient chain flexibility. 
Figure 4. IPM modified silicones membrane immersed in phosphate buffer with BSA solution for 30 days, at pH 7.0.
The most important feature of an encapsulating material, and the primary aim of the silicone IPM modification, is water resistance. One aspect to this is water uptake, ie water residing within the polymer at saturated equilibrium, the other is water transport through the silicone membrane phase; a more direct indicator of membrane barrier property. A further contributor of water resistance is the strength of adhesion to the substrate8; adhesion should be strong enough to avoid condensed water tracking alone between coating and device following delamination.
Water vapour transport
Permeation of a gas or vapour through a polymer film is thought to involve the following stages: adsorption of the permeating species onto the polymer surface; solubilisation in the polymer matrix; diffusion through the film down a concentration gradient; and desorption from the opposite surface10.
Remarkably, an upward trend of increased water vapour transport was seen with increased IPM concentration can be observed with the exception of at IPM 1% w/v concentration (Fig 5). A statistical analysis was performed using one-way ANNOVA coupled with Dunnett's Multiple Comparison Test to evaluate differences between water vapour permeability of materials (GraphPad Prism Version 5.01). Confidence taken at levels 0.05, 0.1, 0.01 showed that the increased water vapour transport through IPM modified membranes was statistically significant and correlated with an increase in IPM concentration.
As far as water vapour transport through a membrane is concerned, two phenomena: nucleation and clustering on the hydrophobic surface contribute to the diffusion process  .
In this case nucleation can probably be neglected since the availability of any hydrophilic sites for water molecules nucleation is extremely low, given that polymer and lipid are hydrophobic and the likelihood of hydrophilic impurities, eg in the form of ionic salts is practically zero.
Clustering on the other hand, can occur when mutual penetrant-penetrant interactions at a surface are stronger than those of the penetrant-polymer, which certainly appears the case for the hydrophobic polydimethyl siloxane (PDMS)  . Hence, water clusters are likely to have been created on the present hydrophobic surface. However, from a modelling study  , the chances of significant penetration of water clusters from surface to bulk PDMS, as well as cluster formation within the PDMS, is highly improbable.
Therefore, it would seem reasonable to conclude that transport involves non-aggregated water transport through the membrane.
Figure 5. Effect of IPM concentration on water vapour transport through modified silicone membranes during 7 days.
Liquid water transport through modified silicones
Compared with a reference aluminium foil film the silicones allowed substantial water transport. The nominal amount seen for the aluminium indicated the experimental error during measurement due to transient CuSO4 exposure to ambient air during apparatus assembly. Improved water resistance can be observed (Figure 6) with increasing of IPM concentration up to 1% w/v and then a further increase of IPM concentration had no apparent effect on water transmission through membranes, however, any effect is small in any case. Again, a statistical analysis performed using one-way ANNOVA coupled with Dunnett's Multiple Comparison Test to evaluate differences between water permeability of materials (GraphPad Prism Version 5.01) shows the results to be significant with a confidence taken at 0.05. However, results were not statistically significant at a confidence 0.1 and 0.01. Overall, data can be interpreted as showing that IPM modified materials do reduce liquid water permeability when compared with unmodified silicone to a degree that reaches statistical significance  .
These results are quite the reverse of the water vapour trends; this may be because with liquid water interfacial clustering, known to be prevalent at such membranes  , included initial water uptake. Thus, on a silicone membrane liquid water is exposed to the repelling action of IPM molecules across all of the available surface area. However, whilst IPM at the membrane surface might repel water aggregates in the liquid state, it does not sufficiently suspend the flux of monomeric water vapour.
Figure 6. Liquid water transport through IPM modified silicone membranes - IPM concentration dependence.
Introduction of any additional overlaying silicone layer significantly improved the scratch resistance of the substrate as reflected in observed lower scratch width (Table 2). It is also worth noting that modification of the MED-4211 silicone with IPM had the greatest effect on scratch resistance. Improved scratch resistance accompanies, with some advantage, the elasticity increase of a modified material20. A more elastic material would also conform more readily to natural tissue mechanics and follow the irregularity of any device surface. Another advantage of increased elasticity is that such materials are less vulnerable to mechanical damage during the implantation process itself, e.g. scratching by surgical instruments or during handling.
Average width [mm]
Table 2. Scratch test results of various double silicone layers measured as an average value of created fissures
Results of the adhesion strength determination between silicone base MED 4211 material and either unmodified, or 1% w/v IPM modified, additional silicone layers are shown in Figure 7. Modification for both kinds of silicone results in a lower strength of adhesion but when a double coating is used (silicone first layer) adhesion was at least as good as that with a control single silicone adherent layer, the standard coating. Therefore, a double layer has advantages, and if delamination does occur it will take place at the silicon-silicone interface rather than between two silicone layers.
Figure 7. Adhesion test results of various double silicone layers measured as minimum force required to start a single layer pealing process.
A quantitative assessment of cell growth in each well as a whole is provided by the AlamarBlue dye reduction test. The test produces a fluorescence signal of arbitrary units that is proportional to the number of viable cells in each well. The mean AlamarBlue fluorescence from six replicate tests for each material and the time points is shown in Figure 8. All the test materials and the biocompatible material control displayed less cell growth at 48 hours incubation than the cell-only control, and increasing amounts of IPM appeared to be associated with increasing amounts of cell growth up to 5% IPM.
Figure 8. Cell Growth as measured by AlamarBlue fluorescence for the two parts of the experiment.Points show the mean Â± sem for 12 (controls) or 6 (test samples) replicate incubations.
The comparative effect of these materials on cell growth is best assessed if growth is normalised to that in the negative toxicity (no material present) control. From this, the mean background fluorescence without cells (coefficient of variation < 5%) was subtracted from all other measurements and cell growth expressed as a proportion of the matching cell-only control in the subsequent analysis.
The comparative cell growth at 96 hours is shown in Figure 9, and effectively summarises the findings. An indicative statistical analysis was performed for the 96 hour incubations using one-way ANNOVA coupled with Bonferroni's Multiple Comparison Test  to evaluate differences between the materials (GraphPad Prism Version 4.02). Significance was taken at p=<0.05. The cell only control showed significantly greater cell growth than all other incubations except the MED 4211 with 5% IPM. However, there was no statistical difference between the biocompatible material control and all test incubations other than the MED 4211 with 10% IPM, which had significantly less growth than all other incubations.
In conclusion, the results show the unmodified MED 4211 used in this study, as prepared, had a substantially lower biocompatibility than the biocompatible control, but the incorporation of IPM resulted in a dose-dependent increase in cell growth on the material up to a concentration of 5%. Inclusion of 10% IPM resulted in much less cell growth than on the unmodified material. Thus, it appears that IPM up to 5% can increase the short-term biocompatibility of this silicone rubber to a significant degree.
Figure 9. Relative cell growth at 96 hours. Mean Â± 95% CI (n=12 for controls and 6 for test materials).
A potentially useful candidate material was established as possible packaging material and laminate for passive microelectronic device surfaces. Preliminary coating of a range of microelectronic devices with IPM modified silicone showed promising results for its functionality and in vitro compatibility35. Furthermore, some reduction in water permeability for the modified material, though not evident to a major degree, offers an improved protection barrier against water ingress that may be of cumulative importance over extended periods of implantation. In combination with the better biocompatibity, the material of would be of potential use for in vivo devices.