Nanomechanical Testing of Medical Grade Polymeric Materials: Evaluating Nanoindentation as a Characterization Tool

In the coming decades, there will be a significant growth in primary and revision total joint replacement (TJR) procedures. The increase in TJR poses stress on the healthcare system and the patients afflicted by the need for joint replacement. An important concern arising in the arthroplasty community is the increase demand for TJR in the younger and more active population. Many of the current designs and material formulations are aimed towards an older and sedentary population. As a result there is a need to understand current clinical failures, material microstructure and implant design to address the changing patient demographics – active lifestyle and longer lifespan. Over the past decades there has been an extensive effort to develop new material formulations to improve ongoing challenges in orthopedic bearings; namely resistance to wear, fatigue and fracture, as well as oxidation or corrosion in metals. The majority of TJR use a metal-on-polymer articulation to restored function to damaged or diseased articular cartilage. The gold standard polymeric material used in orthopedics is ultra-high molecular weight polymer (UHMWPE). While there are a variety of UHMWPE formulations that address challenges such as wear, fracture and fatigue, and oxidative degradation, the optimal UHMWPE formulation does not yet exist. Furthermore, identifying a material formulation that is well suited and assessing its microstructural-property relationship is important for understanding the long-term behavior in vivo. A common technique to characterize mechanical properties to understand the mechanical behavior of materials is to perform bulk mechanical testing. However, bulk testing such as tensile and compression are insufficient to capture information related to minute changes in the microstructure. Alternatively, nanoindentation (the focus of my doctoral work) can provide insight into UHMWPE’s nano-scale behavior, including the surface property changes of retrieved implants. Surface properties such as of localized modulus and load-displacement behaviors are important to understanding the changes in properties at the articulating surface and provide insight into tribological behavior. Understanding nanomechanical behavior is important for optimizing wear resistance and tailoring UHMWPE microstructures for long-term performance in orthopedic bearings.In the past few decades, the need for developing bio-inspired materials to address many shortcomings from current medical-grade biomaterials gave rise to PEEK and PEEK composites. As such, there is a need to understand the mechanical behavior to assess suitability for load bearing applications in the body. Thus, equally important to studying the nanomechanical properties of UHMWPE is assessing the nanomechanical properties of Poly-ether-ether-ketone (PEEK) and PEEK composites, a potential alternative to UHMWPE and metallic components. Insight into the nanoscale may offer valuable information about fiber-matrix interactions that may influence long- term integrity of these biomaterials when used in the body. This dissertation is the first study to use nano in PEEK and to bridge the nano-micro-macro scales in UHMWPE and elucidate a unified methodology for use in polymers biomaterials. This body of work shows the utility and validity of using nanoindentation as a characterization technique for characterizing medical-grade polymers. This research highlights the structure- property development and the comparison across the microstructural length scales of UHMWPE. Secondly, characterizing the nanomechanical properties of PEEK and PEEK composites provide insight into the relationship between the heat treatment process, microstructure, and tip diameter. Since nanoindentation is a growing field for characterizing biomaterials, there is a dire need for developing robust nanoindentation protocol that yields reproducible and reliable data. As such, this dissertation develops a framework for (a) addressing challenges and potential errors when performing indentations on soft or hydrated materials and (b) explore best practices for mitigating experimental error. This framework is a primer and an impactful body of work for researchers performing indentations on soft/hydrated biomaterials and polymers. Lastly, this research aims to validate nano-indentation to study UHMWPE-based orthopedic retrievals’ surface properties. The benefit of using nano-indentation is understanding and quantifying the changes in mechanical properties at the articulating surface. Prior research has shown that articulating motion yields a damaged layer on the surface (3-5 μm in-depth), a precursor to wear. Unlike bulk mechanical testing, nanoindentation probes the local surface properties and can measure properties on the damaged layer. By assessing the surface properties, we can learn more about the wear mechanisms and the structure-property evolution owed to in vivo conditions. Looking forward, more research studies on the nanomechanical characterization of polymers may allow researchers to optimally tailor the microstructure for long-term structural applications and gain insight into the mechanics of other bio-inspired systems.

Advisor: Pruitt, Lisa