Design and Development of Load Bearing Composite Scaffolds for Bone Implants

Abstract

Biocomposites mimicking hard tissue in the process of bone repair and regeneration have been in development for several decades. Contributions of several researchers in selection of materials, development of preparation routes, conduct of post surgery response studies and clinical trials for continual developments are commendable. Among several disciplines involved in this process, engineers’ role has been identified vital in materials and their processing. Natural bone made up of Collagen and reinforcement ceramic phase, has dense structure at the middle of the bone and porous structure at the ends. Damaged or diseased bone needs regeneration with implant reinforcements for load sharing during healing process. Essential requirement of an implant is to share the load and to stimulate the bone growth by proper mechanics. Structural point of view, mechanical properties of an ideal implant shall match native bone properties. Known the properties of Hydroxyapatite (HA), Ca10 (PO4)6 (OH)2 in bone applications for its bio active, bio conductive and bio inductive nature, it was chosen as bone material. Hydroxyapatite (HA) being ceramic, it suffers from low strength, low toughness and low tensile properties. In order to overcome the structural weaknesses of HA, Polyetheretherketone (PEEK-polymer) matrix was chosen to match the properties of Collagen. The polymer part of composite, PEEK, by brand name VESTKEEP 2000FP, a non-medical grade polymer is a justified selection due to its inherent superior properties such as- semi-crystalline, high strength, temperature resistant, chemically inert, bio inert, bio conductive, radiolucent, clinically proven and approved by Food and Drug Administration (FDA)-U S A. Hydroxyapatite, the reinforcing ceramic powder was extracted from chicken egg shells. Thoroughly washed egg shells were heated through predefined thermal cycles for the formation of CaO. Obtained CaO after thermal treatment has been chemically treated with tri-calcium phosphate (TCP) in wet condition at temperatures 10000C and above for the formation of HA. After ball milling and sieve separation, the obtained HA powder was characterized at International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI), Hyderabad For better bone native tissue interaction, HA was extracted from chicken eggshells. Fragile nature of HA had limited its content to a maximum of 40% in total biocomposite. Biocomposite with PEEK and egg shell extracted HA was synthesized and processed for i) dense composite through compression molding to mimic cortical bone and ii) porous structure through polymeric sponge method to mimic Cancellous bone. Patient specific scaffold to repair damaged bone with precise geometry was successfully demonstrated in present work. CT/ MRI scan data in DICOM format was taken from KD Hospitals, Ambala (Haryana state), India. The obtained 2D images were converted in to 3D surface modeled at Central Tool Room (CTRL), Luthiana (Punjab) using 3D Doctor Software. 3D surface data was imported through IGES exchange protocol to PRO/E WF 3. Volume model of human pelvis was generated and exported in stl format to VANGUARD HS Selective Laser Sintering (SLS) machine. Scaled human pelvis was printed in Acrylonitrile-Butadiene-Styrene (ABS) material at CTRL, India. Taguchi’s orthogonal approach was used in design of experiments of dense specimens for effective and quicker experimentation. After several trials in arriving at the specimen formation, three influencing factors (or processing parameters) were identified viz., the constituent composition of PEEK and HA, the sintering temperature or maximum temperature and the rate of heating. Three fine levels of each processing parameters were arrived at after comprehending the properties of biocomposite components from literature and delimiting studies on dense specimens. Two different sets of samples were prepared, the first one with PEEK/ HA (natural or eggshell extracted HA) and the second one with PEEK/ HA (synthetic or commercial, purchased from Clarion pharmaceuticals, New Delhi). Dense specimens mimicking cortical bone structure were produced in powder metallurgy route. Mechanical properties of dense specimens viz., flexure strength and Young’s modulus of biocomposite were established through three point bend test as per ASTM D790 at Central Institute of Plastics Engineering& Technology, Hyderabad. The test results were revealing superior interaction of natural HA with PEEK. Porous specimens to mimic Cancellous bone were prepared through Polymeric sponge method. A nylon mesh of 10 pores per inch was impregnated in Alumina slurry for preparation of template. Alumina template was used for preparation PEEK/HA porous specimens with porosities 41%, 45% and 51%. First set of porous specimens as sintered were tested in unconstrained compression test to establish the mechanical properties. Second set of porous specimens exposed to artificial sea water for 25 days at 370C were tested in unconstrained compression to establish degradation of mechanical properties invitro. PEEK/HA composite was concluded to be hydrophobic as the degradation effect was minimal. Thermal stability study was conducted on biocomposite to pave guide lines for processing of the material to ensure bone in-situ applications Higher contents of HA in PEEK was found to be delaying the degradation of the composite. Weight loss of composite was found to be negligible even at 5000C. This test was establishing safe processing temperature of PEEK/HA could be as high as 3500C. Finally an effort was made to extrapolate the porous scaffold’s mechanical behavior using DEFORM 3D software. Three sets of porosity (75%, 82% & 89%) in two different constituent percentages (PEEK/HA 70/30 and 80/20) were analyzed for load - deformation behaviour. The total work entitled ‘Design and development of load bearing composite scaffolds for bone implants’ has been concluded as the dense and porous specimens were found prospective at load bearing sites with appropriate selection.