Laser-aided manufacturing may hold the solution to one of the most pressing health problems affecting over 10 million people annually – the need for knee replacements. Dr. Soundarapandian proposes a cost-effective, novel implant as well as a manufacturing technique that overcomes the pitfalls associated with the current joint implants.
A marathon runner has persistent pain in his knee that leaves him unable to walk. He sees an orthopaedic specialist at a hospital and undergoes a knee replacement surgery, which takes all of one day. He can now perform any activity he did before and wins that marathon he was training for. A happy ending – the story advertised by every bone and joint specialty hospital. What they don’t talk about, though, is that it takes several weeks to design and manufacture the implant that’s tailored to replace his knee precisely. Moreover, these implants have an average lifetime of only about 10 years, and will have to be replaced by another surgery. That it takes him several weeks of physiotherapy to regain his full range of motion and even then he shall experience chronic pain is an issue that is conveniently ignored.
Total Joint Replacement is seen as the biggest success story of orthopaedic surgery. It has helped hundreds of thousands of people regain or maintain their functional independence and live fuller, more active lives. However, the surgery is prohibitively expensive, rendering it out of reach for an estimated 10 million people who suffer hip, knee and shoulder joint failure every year. Dr. Soundarapandian and his team from the Department of Mechanical Engineering are determined to change this.
They envision a cost-effective instrument integrated with a diagnostic tool that designs and manufactures the body part to be replaced. A patient with joint failure walking into a hospital will go through the rigmarole of diagnosis to surgery in a few hours. As a first step towards this dream, they are working on eliminating the shortcomings of existing bone implants by synthesising a magnesium implant with a calcium-phosphate (CaP) coating by a laser-based coating technique.
The high loads that orthopaedic implants need to support restrict the selection of feasible materials. Today, stainless steel, cobalt, chromium and titanium alloys have been successfully used to fabricate implants because of their strength, comparatively low stiffness, light weight and relative inertness. However, these implants release toxic metallic ions. Moreover, analysis of metal or metal alloy devices provides convincing evidence that implant failure is because of a mismatch of the mechanical and the chemical properties of the implants with the bone at the bone-implant interface. This mismatch leads to the formation of a fibrous layer of tissue at the interface, giving rise to small gaps which cause movement at the interface. Ultimately, this causes a failure of the implant and requires subsequent surgeries to replace the loose implant. One approach to alleviate this problem has been the use of CaP coatings applied to the implant surface. This enables researchers to consider materials with attractive properties that have earlier been rejected for their lack of biocompatibility.
The CaP mineral hydroxylapatite (HAP), Ca5(PO4)3OH, has attracted considerable attention because of its close resemblance to the chemical and mineral components of teeth and bone. As a result, HAP is biocompatible with bone. Instead of forming a fibrous tissue layer at the implant-bone interface like normal biomedical alloys, implants with HAP coating have been shown to form a thin layer bonding with the bone and even promoting bone growth. Plasma spraying is the most popular and the only Food and Drug Administration (FDA) approved method for applying CaP coatings to implant surfaces. This process involves the high-velocity spraying of molten HAP powder onto an implant surface. Upon impact with the substrate, the material rapidly cools and forms a dense coating with a morphology consisting of layers of HAP impact splats. Coatings synthesised by this method form a dense, adherent layer of CaP on metal substrates.
While plasma spraying is a well-understood process, the control of variables is quite complicated. The extremely high temperatures (10,0000C to 12,0000C) used in the plasma spray process can vastly affect the properties of the final coating and result in potentially serious problems such as the coating of complex implant devices containing internal cavities. More serious is the potential for the formation of amorphous CaP phases with a Ca/P ratio between 1.67 and 1.5 in the film rather than stoichiometric HAP which has a Ca/P ratio of 1.67. There is also concern over alteration of the coating structure. In addition, spraying plasma to coat within the pores of porous metal materials proves difficult because it is a line-of-sight process.
During his PhD training, Dr. Soundarapandian identified Magnesium (Mg) as a suitable alternative because its mechanical properties are closer to that of natural bone. However, due to the corrosion of Mg in physiological environments, it cannot be used directly. The solution proposed by the team is deploying HAP coatings on Mg implant surfaces using a laser-guided manufacturing technique. This exploits the bio-compatible and bone-bonding properties of the ceramic, while using the superior mechanical properties of Mg implants.
Additive manufacturing caught the professor’s eye during his Master’s in mathematical modelling at Blekinge Institute of Technology, Sweden where he developed the modelling technique to predict the right manufacturing process given the required geometry and material. However, typically, additive manufacturing isn’t intended to accommodate materials with dissimilar properties. Bones are a composite of both organic and inorganic materials. This implies that exactly mimicking a bone would require materials with disparate properties and existing additive manufacturing techniques were thus inappropriate for the task at hand. Further, bones have a porous geometry which must also be mimicked by the implant in addition to being compatible with the bone environment, a property called osteo-integration.
To address all these concerns, the research group has developed a new instrument that can accommodate metals, ceramics and polymers. The novel technique involves a commonly used 3D printing method called Fused Deposition Modelling. The process basically involves a hot air gun controlled by a robot arm that zig-zags back and forth depositing layers of powdered HAP mixed with a polymer mixture that promotes binding to the surface of the metal. This surface is air-dried and then bombarded with a laser at a pre-characterised energy density that minimises the corrosion rate for a given combination of materials. Lasers are very precise and powerful and the process is a non-contact process and hence ideal for bio-implants.
The next step in the process of designing a novel implant is a set of rigorous tests. The first step to ensure bio-compatibility is a set of in vitro tests. In this step, you immerse the implant in artificially developed bio-fluids that mimic the physiological conditions and study them over several days. The implants manufactured by the research group passed this stage and they noticed an interesting effect. Not only was it bio-compatible, meaning it wasn’t harmful to the body, but it also turned out to promote bone growth. This led to another set of tests to study cell behaviour, particularly adhesion to the implant. The experiments performed in association with a collaborator showed increased adhesion of bone cells (osteoblasts) to the implant surface. They subsequently worked on tweaking several factors such as surface chemistry and topology to enhance adhesion. Surface chemistry was altered by using several polymers proven to increase cell adhesion. The surface roughness was also altered to study effects on cell adhesion.
Currently, the team is busy ensuring that the method, the instrument and the process can be used for wildly different materials including several bodily derived materials. One such bodily derived material being considered is Fibrin, a fibrous protein involved in blood clotting. It’s a tough, resilient material that has properties very similar to that of cartilage. They have already extracted fibrin and are currently working on fabricating implants from it. As with any biomedical device, ensuring reproducibility continues to be a major challenge and they’re working on verifying and validating their methods for the same. The next step will be to use these implants in animal models for in vivo studies. Dr. Soundarapandian is currently looking for collaborators to carry out some of the biological tests. He says that the next stage will take 2 to 3 years after which he would be allowed preliminary human trials in his long haul to see his dream come to fruition.
Dr. Soundarapandian did his Ph.D. in Mechanical Engineering (2010) at Southern Methodist University (SMU), Dallas, USA followed by Postdoctoral research at University of North Texas (UNT), Denton, USA. Currently, he is an Assistant Professor of Mechanical Engineering at IIT Madras. His research focuses on synthesis and characterisation of structural and bio-materials, LASE, computational modelling, manufacturing automation, fabrication of next-gen bio-implants, and laser applications in medical industry.
Aparnna Suresh is a final year student of Biotechnology at IIT Madras. While not holed up in the lab dreaming about creating a Jurassic Park, she enjoys quizzing, reading and swimming. Her long term goal is to pursue a career in the academia and her research interests include synthetic and systems biology, and biological computing. She can be reached at firstname.lastname@example.org.
Cover Image Source: Wikimedia Commons