The history of biocompatibility began with an implanted pacemaker in 1958 designed by Wilson Greatbatch. The device was typically used for patients that had only a 50% chance of survival without intervention. It had many flaws- for example, bodily fluids could permeate the protective casing and destroy the electronics, and the batteries were made of zinc and mercury and needed to be replaced every two years. Despite these flaws, the biocompatibility of this device was sufficient to help one patient live up to thirty more years, and as the first implantable medical device, it saved many lives.
In the 1960s Greatbatch and others in the industry were working to make improvements to the implanted pacemaker, specifically with different types of materials for the electrode of the pacemaker, as the biocompatibility of the original myocardial wires was unstable in the long-term. Medtronic’s Hunter-Roth electrode, made of two stainless-steel pins inside of a silicon-rubber base, was successful. Another version by Dr. Chardack, who had worked with Greatbatch on the first implantable pacemaker, was a myocardial electrode with a platinum/iridium spring coil that was used clinically for several years.
The previous zinc/mercury batteries were replaced in the 1970s with lithium iodine batteries by Greatbatch, which could last ten years rather than two. This improvement to the implanted pacemaker improved the biocompatibility as well, because there would be less frequent surgeries and the pacemaker could now be hermetically sealed, so the electronics would be better protected from bodily fluids.
The 1980s saw even greater improvements to the biocompatibility of the implanted pacemaker and other medical devices. With a focus on the shift from bioinert materials to bioactive materials, “second-generation biomaterials” were frequently used. For example, bioactive glasses and ceramics were used clinically for both dental and orthopedic applications. Because of the use of bioactive materials, when used as a bone implant the material triggers a tissue response called osteoconduction, in which bone could grow over the surface of the implanted material. For a material of low biocompatibility, osteoconduction typically occurs rarely to not at all. Due to the process of osteoconduction, as the bone grows along the material coating, a mechanically strong interface is formed. Another advance in types of biomaterials would be the development of resorbable biomaterials or a controlled chemical breakdown. With this type of biomaterial, the foreign material is eventually replaced by regenerating tissue, and there should be “no discernable difference between the implant site and the host tissue.” For example, by 1984 resorbable polymers as sutures were frequently used clinically.
The current state of biocompatibility is with the previously described “second-generation materials” and how they ideally would function in the body; however, with any implanted material, there is an immune system response because the material is foreign to the body. The response depends on a variety of factors, but it is typically a form of inflammation as a result of the release of neutrophils and cytokines. The neutrophils release various enzymes that break down any biodegradable materials. With either the removal of the implant or the complete breakdown of the material, inflammation should end. If the implant is not removed or fully broken down, chronic inflammation begins.
The immune system’s response is based on the surface interactions between the biomaterial and the biological system, therefore, by modifying surface properties, the compatibility of the implant can be improved. Some methods of surface modification include thermal spray, electrophoresis, pulsed laser deposition, and more. Many coatings for metallic implants are made of hydroxyapatite (or HA), a bioactive material that can accelerate bone formation which would improve implant fixation. Thermal spraying is an industrial coating process in which melted or heated materials are sprayed onto a surface at a high velocity. This coating on an implanted device would have antibacterial and bone bonding abilities[footnoteRef:6], thus increasing the biocompatibility of the device by decreasing the likelihood of a severe immune response. A flaw with this method is that some of the bioactive properties of HA are lost during thermal spraying, as the coatings can suffer from high porosity levels. Electrophoresis is a method that separates molecules according to their size and electrical charge, which can be used to coat a metallic implant (titanium due to its biocompatibility) with hydroxyapatite. The advantage of this method is that there is easy control over thickness and shape of the coatings, it is a low-cost flexible technique, and as its coating process is does not rely solely on sight, it can be used to deposit the material on complex surfaces. Pulsed laser deposition (PLD) is a physical vapor deposition technique where a high-power pulsed laser beam is focused inside a vacuum chamber to strike a target of the material that is to be deposited. PLD is useful for complex coatings due to its “stoichiometric transfer, excited oxidizing species, and simplicity in initial setup and in the investigation of arbitrary oxide compounds.” Other ways to minimize the immune system response would be to apply anti-inflammatory agents at the site of implantation, or enhancement of neovascularization to improve a sensor’s lifetime.[footnoteRef:10] By using any of these methods, the biocompatibility of an implanted device is improved as the immune response should be minimized.
The future of ways to optimize biocompatibility by minimizing the immune system response to an implanted device or sensor lies with designing materials that are both bioactive and resorbable. This could be beneficial with tissue engineering, as a material that is bioactive and resorbable can be used to repair diseased or damaged tissue. Furthermore, the development of a bioactive and resorbable material would have various applications with the regenerative process and would make surgery for implanted devices and sensors minimally invasive. Improvements in this field would vastly improve the quality of life for people, especially those of the aging population.
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