Give an example of a commonly used biomaterial that can be found in medicine or everyday life.Student Name: Nathalie BrennanStudent Number: 17231880Date: 16/01/2018Biomaterial: Polydimethylsiloxanes (PDMS)Course Code: 1MV1 2017-2018Module Code: BES513 – Materials science and biomaterials. A biomaterial is a biological or synthetic substance or material which has been engineered to take a form which can be introduced into body tissue as part of an implanted medical device or used to replace an organ, bodily function, etc. or control the course of any therapeutic or diagnostic procedure. Biomaterials are generally used in medical applications e.g. cell culture, assays etc. but they can also be used in everyday life. 1Biomaterials which are commonly used in medicine and in everyday life are Silicones, also known as poly siloxanes. Silicones are polymers consisting of silicon – oxygen covalent bonds however other structures such as silicate resins may also be classed as silicone. The length of the Si – O side chains in silicones can be adjusted, the function of the side groups and the level of cross-linking between the molecular chains can be altered meaning silicones can be synthesised into a variety of materials, each with unique chemical properties and performance characteristics. 2Poly methyl siloxane (PDMS also known as Dimethicone, Dimethylpolysiloxane or E900) is the most widely used silicon-based organic polymer, it is clear, inert, non-toxic, and non-flammable. Poly (dimethyl siloxane) (PDMS) is a bio-stable synthetic polymer used for biomedical applications such as drug delivery vehicles and blood-contacting biomaterials. In its standard state, the reference point used to calculate material properties under different conditions in chemistry, which for PDMS is 25 °C, PDMS has a density of 965 kg m?3 and a sheer modulus of 100 kPa. Its appearance is colourless, clear liquid. It has a boiling point of 101 °C and a melting point of -50 °C.PDMS’s Durability, Oxygen permeability, Elasticity, Blood compatibility, Resistance to degradation, stability, lack of toxicity, and excellent biocompatibility of PDMS makes these materials well suited for use in personal care, pharmaceutical, and medical device applications. 4Polydimethylsiloxanes (PDMS), or silicones, are a class of synthetic polymers with repeating units of silicon and oxygen. Various functional groups, usually methyl, are attached to that backbone. Silicone polymers can be easily transformed into linear or cross-linking materials without the use of any toxic plasticizers. The resulting materials are elastic at body temperature.The simultaneous presence of different groups attached to the silicon-oxygen backbone gives silicones a range of viscous and mechanical properties, which allow their use as fluids, emulsions, compounds, resins, and elastomers in numerous applications. Thus, silicone is a versatile polymer, although its use is often limited by its relatively poor mechanical strength. However, this limitation can be reduced by reinforcing with a silica filler, or by chemical modification of the backbone.PDMS has a unique semi-organic structure – CH3Si(CH3)2OnSi(CH3)3 where “n” is the number of repeating SiO(CH3)2 monomers units, these repeating units can range from zero to several thousand. 5 It consists of a polar inorganic backbone of long, strong Si-O bonds and open Si-O-Si angle as seen in figure one. Figure 2: Molecular structure of PDMS 6There is a low barrier to rotation and a low rotation energy. It consists of non-polar organic substituents (methyl) consisting of shorter Si-C bonds, there are weak intermolecular forces and there is opportunity to substitute for other functional groups not just methyl groups. 6 Figure 3: Molecular structure of PDMS.5The long and strong Si-O bonds combined with a wide bond angle and a low barrier to rotation give poly methyl siloxane (PDMS) its desired flexibility and internal mobility along with its large free volume. All of which enable the functional groups to align efficiently to the interface and reduces the competition among functional groups and lowers the functionality requirements. As a result, the PDMS polymers are being used in applications where rigid organic polymers would require higher concentrations of most expensive functional groups. 6PDMS can be chemically synthesised in industry from dimethyl dichlorosilane and water. A polymerisation reaction occurs which produces hydrochloric acid and PDMS: The siloxane backbone’s high bond energy of ~445 kJ/mol along with its methyl (CH3) functional groups combine to make PDMS a very chemically stable material that can resist extreme temperatures, weathering, aging, oxidation, moisture, many chemical agents as well as ultraviolet radiation. 5Many researchers have noted that PDMS has advantageous characteristics such as its low surface tension, variety of configurations, large free volume, low glass transition temperature, low boiling points, low environmental hazard, excellent weather resistance. optical transparency, stretch ability, gas permeability, electrical insulation, low thermal conductivity, and low water permeability. Additionally, it is relatively easy to fabricate cell-based micro systems using PDMS and soft lithography, and the material is known to be biocompatible. 5Biocompatibility is the ability for a material or substance to perform with an appropriate host response in specific situations. However, there is not a single test that determines whether a material is biocompatible or not. Appropriate host responses” include lack of blood clotting, resistance to bacterial colonization and normal healing. Ever since the 1960’s, PDMS has been widely used of medical devices and there have been studies on it’s toxicity, stability and tissue responses. 7In an experiment performed by the Indian Institute of Technology, PDMS substrates were prepared by varying the base (pre-polymer) and curing agent (cross-linker) ratios (w/w) in 5:1, 10:1, 15:1 and 20:1. The structural and mechanical properties of different PDMS substrates were characterized using • surface roughness measurement• contact angle measurements• elastic modulus testing of PDMS films• ATR- FTIR (Attenuated Total Reflectance Fourier Transform Infrared spectroscopy) analysis.Figure four shows their results. The roughness, thickness and contact angle increases and Young’s modulus decreases with the decrease of PDMS cross linking agent. The contact angle measurements showed minimum contact angle of 97.83 for pristine PDMS 5:1 and a maximum of 102.545 for pristine PDMS 20:1. Upon treatment with oxygen plasma, surface energy of the PDMS samples increased significantly resulting in a very low contact angle of a minimum of about 12.2 for PDMS 10:1. FTIR results also showed a decrease in -CH3 groups and an increase of -OH groups in plasma treated samples as compared to pristine PDMS. Their overall conclusion showed the all PDMS compositions support cell adhesion and proliferation. 14 Figure 4: Results from Indian Institute of Technology 13It was originally thought that PDMS was biologically inert, however recent studies have shown that there are varying levels of IgG (immunoglobulin G) antibodies which had reacted with PDMS in humans. A blind study was conducted where the serum from patients with and without breast implants was examined to determine the specific anti-silicone IgG antibody levels. Test results showed that patients with implants demonstrated statistically significant elevation in anti-silicone antibodies compared with the unimplanted control groups. The highest anti-silicone antibody levels were measured in implanted women with either frank implant ruptures or leakage of their silicone gel implants. 8Although it is known to be biocompatible, it has also been reported that PDMS inhibits cell attachment; cells only attached on the patterning of cell adhesion factor in the PDMS device, and the attachment of the cells onto a bare PDMS surface was suppressed. a simple and relevant method to improve the biocompatibility of PDMS is proposed; the effects of various treatments using ethanol, water, and boiling-water were evaluated. The results show that the boiling-water treatment is the most appropriate, time saving, and simple method to improve the biocompatibility of PDMS. 9PDMS acts like a viscous liquid at high temperatures or at long flow times, similar to honey, however at lower temperatures or short flow times PDMS it acts like an elastic solid similar to rubber i.e. if PDMS is left on a surface overnight (long flow time) it will flow to cover the surface and mold to any surface imperfections, the same PDMS if rolled into a ball and thrown against a wall will bounce off the wall like a rubber ball. 10 The viscoelastic properties of PDMS can be measured more accurately using dynamic mechanical analysis which determines the material’s flow characteristics over a range of temperatures, flow rates and deformations. Due to PDMS’s chemical stability, it is often used as a calibration fluid for dynamic mechanical analysis.Due to silicones semi-organic structure – organic-inorganic nature, polarity and other unique physical and chemical properties, silicones can influence surface properties. This influence can be stabilizing or destabilizing, depending on the application. 5 Some silicones are applied directly to the surface or interface in the form of sealants, films, or coatings. Others work as integral additives that diffuse or travel to the interface when needed. The surface tension of PDMS is 20.4 mN/m, this is a relatively low surface tension and therefore PDMS easily wets most surfaces. Its methyl functional groups can align to create water-repellent films. The critical surface tension of wetting is 24 mN/m which is slightly higher than its own surface tension mean PDMS can flow over itself which enables it to outperform hydrocarbons in forming extremely thin (mono molecular) self-levelling films. 5The surface chemistry of PDMS can also be altered by plasma oxidation, which is the addition of Silanol (SiOH) groups to the materials surface. After the treatment with Silanol, the PDMS’s surface is hydrophilic which will allow for water to wet it. The oxidised surface can be further influenced by the addition of tri-chlorosilanes. 11 After a prolonged period of time, the surface hydrophobicity will return regardless of whether the surrounding medium is vacuum, air, or water; the oxidized surface is stable in air for about 30 minutes. Approaches such as plasma treatments and laser treatments have been used to modify the silicone surface in order to improve the surface wettability and cell-adhesive properties, however these techniques were unable to produce stable cell-adhesive properties due to surface reorganisation. 13 Cell – adhesive proteins like laminin, collagen and or fibronectin have been coated to PDMS surface in order to produce long – lived cell – adhesive properties. These proteins induce immune responses, disease transmission and poor stability as there is a proteolytic action which can restrict PDMS’s use in medical applications. Stable cell – adhesive peptide conjugation has been used to modify the PDMS surface PDMS can be polymerised and cross-linked with tetrakis (dimethyl siloxane) for example. PDMS can then have a hydrophobic surface after the polymerisation and cross-linking procedures. 4 This leaves the PDMS with a metallic and shiny looking surface while the substrate is clear. It’s surface energy makes it difficult for polar solvents such as water to wet the PDMS surface, this can lead to the potential adsorption of hydrophobic contaminants. Solid PDMS will not allow solvents to infiltrate and swell the material, whether the surface is oxidised or not. Therefore, PDMS structures can be used along with water and alcohol solvents without material deformation. However most organic solvents will diffuse into the material and cause it to swell making them incompatible with PDMS devices. 12Applications for PDMS include: Oxygenator membranes, Shunts, Prosthesis, Breast Implants, Joints, Tracheal reconstruction, bladder reconstruction, Maxillofacial reconstruction, Heart pacemaker leads, Heart valves, Burn dressings, Catheters, Drainage tubing materials. 13Surfactants and Anti-foaming agents PDMS is a common surfactant, a compound that can lower the surface tension of two liquids, a gas and a liquid, or between a liquid and a solid. Surfactants can be detergents, wetting agents, emulsifiers, foaming agents etc. Hydraulic FluidsHydraulic fluid is a medium by which power is transferred in hydraulic machinery. PDMS is an excellent insulator and unlike carbon it is non – flammable. Soft LithographyPDMS is commonly used as a resin stamp in soft lithography, making it a common material in micro fluid chips. Many researchers use PDMS and soft lithography as it is easy and quick to use. CosmeticsPDMS can be used in various cosmetic and consumer products, for example it is used in head lice treatments as well as in skin moisturisers with their purpose being to protect the skin. Some cosmetic formulations use di-methicone and related siloxane polymers in concentrations of use up to 15%.HairPDMS compounds are also used in conditioners.FoodsPDMS can be added to oils used for cooking. They work as anti-foaming agents to help prevent oil splatter whilst cooking. Domestic PDMS is present in day to day toys, for example it’s concentration in “Silly Putty” is 4 %, the presence of PDMS in “Silly Putty” accounts for it’s characteristic viscoelastic properties. MedicineSilicones are used in medicines because it is thermally stable, chemically stable, provides electrical insulation and has a high gas permeability. It has low surface tension of 20.4 mN/m, Critical surface tension of wetting 24nM/m and is hydrophobic. Activated di-methicone, a mixture of polydimethylsiloxanes and silicon dioxide (sometimes called simethicone), is often used in over-the-counter drugs as an antifoaming agent. PDMS has also been used in the pharmaceutical industry as a potential tablet coating for potential zero- order release. In the pharmaceutical industry, coating is often used to change and enhance the performance of drug delivery systems, especially solid dosage forms. PDMS and its derivatives have been extensively used in the pharmaceutical area, such as in controlled drug delivery systems due to their bio-stability, non-carcinogenicity, nontoxicity, biocompatibility, and good mechanical properties. 19PDMS has been used as a filler fluid in breast implants and similar cosmetic products and procedures. PDMS is also being used for pacemakers, hydrocephalic shunts wound dressings, contact lenses, for duct repair in biliary surgery.In summary, PDMS has ideal properties such as non-toxicity, biocompatibility, blood compatibility, elasticity, transparency and durability. The flexibility of the PDMS backbone allows exposure of the methyl group interfaces, which have low interaction properties, minimising surface interactions. PDMS is bio-inert which inhibits micro bacterial growth making it a promising biomaterial for biomedical applications. High gas permeability of PDMS allows the diffusion of gases like oxygen and carbon dioxide and makes it more appropriate in medical applications like wound dressing and contact lenses However, the hydrophobic nature of PDMS does not encourage cell adhesion, which is a very critical requirement for wound healing process and angiogenesis.By varying the levels of cross-linking in a sample of PDMS, one can develop a wide variety of biomaterials which can be used in many applications as listed above. Also PDMS is of low cost to manufacture therefor making it an ideal biomaterial for everyday use or to be used in the medical field. Bibliography1. Bhat, S. Biomaterials.2. UWEB :: Research : Biomaterials Tutorial (2018). Uweb.engr.washington.edu. Available at: https://www.uweb.engr.washington.edu/research/tutorials/pdms.html Accessed January 9, 2018.3. polydimethylsiloxane widely used silicon-based organic polymer (2018). Worldofchemicals.com. Available at: http://www.worldofchemicals.com/chemicals/chemical-properties/polydimethylsiloxane.html Accessed January 15, 2018.4. PDMS: A review – Elveflow (2018). Elveflow. Available at: https://www.elveflow.com/microfluidic-tutorials/microfluidic-reviews-and-tutorials/the-poly-di-methyl-siloxane-pdms-and-microfluidics/ Accessed January 10, 2018.5. Fascinating Silicone™ Chemistry – Physical & Chemical Properties of Silicone – Dow Corning (2018). Dowcorning.com. Available at: http://www.dowcorning.com/content/discover/discoverchem/properties.aspx Accessed January 9, 2018.6. Plasma, H. (2018). Improved Adhesion/Bonding due to Plasma Treatment of PDMS. AZoM.com. Available at: https://www.azom.com/article.aspx?ArticleID=14346 Accessed January 9, 2018.7. Ratner, B., Hoffman, A., Schoen, F., and Lemons, J. (2014). Biomaterials Science (Saint Louis: Elsevier Science).8. Bondurant, S., Ernster, V., and Herdman, R. (2000). Safety of Silicone Breast Implants (Washington: National Academies Press).9. Park, Joong & Hwang, Chang & Lee, Sang-Hoon & Lee, Hoon. (2008). Effective Methods to Improve the Biocompatibility of Poly (dimethyl siloxane). BioChip journal. 2.10. Revolvy, L. (2018). “Polydimethylsiloxane” on Revolvy.com. Revolvy.com. Available at: https://www.revolvy.com/main/index.php?s=Polydimethylsiloxane Accessed January 9, 2018.11. Xiao, D., Zhang, H. and Wirth, M. (2002). Chemical Modification of the Surface of Poly(dimethylsiloxane) by Atom-Transfer Radical Polymerization of Acrylamide. Langmuir, 18(25), pp.9971-9976.12. Vinothkumar, T., Arathi, G., Kandaswamy, D., and Dinesh, K. (2011). Influence of different organic solvents on degree of swelling of poly (dimethyl siloxane)-based sealer. Journal of Conservative Dentistry 14, 156.13. Kumbar, S., Laurencin, C., and Deng, M. (2014). Natural and Synthetic Biomedical Polymers (Burlington: Elsevier Science). 14. Tambe, N., and Bhushan, B. (2005). Micro/nanotribological characterization of PDMS and PMMA used for BioMEMS/NEMS applications. Ultramicroscopy 105, 238-247.15. Khorasani, M., Mirzadeh, H., and Sammes, P. (1999). Laser surface modification of polymers to improve biocompatibility: HEMA grafted PDMS, in vitro assay—III. Radiation Physics and Chemistry 55, 685-689.16. Peterson, S., McDonald, A., Gourley, P., and Sasaki, D. (2004). Poly(dimethylsiloxane) thin films as biocompatible coatings for microfluidic devices: Cell culture and flow studies with glial cells. Journal of Biomedical Materials Research 72A, 10-18.17. Kim, S., Moon, J., Kim, J., Jeong, S., and Lee, S. (2011). Flexible, stretchable and implantable PDMS encapsulated cable for implantable medical device. Biomedical Engineering Letters 1, 199-203.18. Black, J. (1992). Biological performance of materials (New York: Marcel Dekker).19. Soroory, H., Mashak, A., and Rahimi, A. (2013). Application of PDMS-based coating in drug delivery systems using PVP as channeling agent. Iranian Polymer Journal 22, 791-797.
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