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Tribological Assessment on Accelerated Aging Bones in Polymeric Condition

Ramdziah M. Nasir Law C. Gan and Abdul Y. Saad

University of Science, Malaysia, School of Mechanical Engineering, Engineering Campus, Nibong Tebal, 13400, Penang, Malaysia

1.1 Introduction

The global trends of doctor-diagnosed arthritis and osteoporosis show a dramatic increase. According to a statistical survey by the Centers for Disease Control and Prevention (), the percentage of adults suffering this disease has gradually increased for both women and men, especially the aged, as shown in Figures 1.1 and 1.2 [1]. Lack of exercise in daily life, obesity, and family-inherited genetic problems are the main causes of the disease.

Figure 1.1 Sex-specific prevalence of doctor-diagnosed arthritis, National Health interview, from 2007-2009 [1].

Figure 1.2 Projected prevalence of doctor-diagnosed arthritis among adults 2005-2030 [1].

Recently, there has been much effort by researchers toward improving bone defects - for example, the transplant of scaffold for regeneration of new bones. However, most of the research is focused on the strengthening of defective bones by using the replacement method. There is a possibility to research on transplant of stronger bones from one person to another in the near future.

The first stage is to observe the changes in properties of stronger animal bones at different conditions that include temperature and relative humidity (). Research on human bones can help determine the factors that can strengthen the physical properties and enhance the elemental properties of bones. This project will study animal bones such as bovine and goat bones for strength and simulate the similarity between human bones and animal bones in terms of accelerating aging factors and conditions. The work will focus on selecting the most suitable animal model for the study to simulate human bones and to assess how daily activity and load bearing affect the properties of animal bones. Parameters such as temperature, hardness, and tensile forces that affect the properties of animal bones are assessed by comparing the structural changes of two samples of bone before and after exposure to different conditions.

1.2 Bone

In order to determine how the various environmental factors can affect the properties of the selected animal bone model, the basic knowledge of the composition of bones must be revisited. Much work has been done to explore how the various types of bones react in various environments. Bone is a composite that consists of minerals (calcium phosphate or hydroxyapatite) and protein (collagen) in a hierarchical architecture [2]. Approximately 20% of bone consists of water; 35% of the dry bone material consists of collagen, proteins, and glycosaminoglycans [3] and the remaining consists of other hybrids of organic and inorganic substances.

Basically, bone not only provides robust mechanical support for the whole human body but also protects the various organs from external impacts. The mechanical properties of bone are good owing to its strong structural hierarchical architecture in the human body, built with strong and stiff minerals and weak and soft proteins [2]. In the past, Isaksson et al. [4] reported that majority of mammal bones are similar to one another except for the presence of plexiform bone in large mammals such as pigs, cows, and goats [5]. Plexiform bone is primarily found in large rapidly growing animals and offers increased mechanical support for long periods of time. It is reported by Pearce et al. [6] that there are a few animal models that are suitable for use in testing of orthopedic and dental implants prior to clinical use in humans. Frequently used bones include dog, sheep, goat, pig, and rabbit models for the evaluation of bone-implant interactions. There were some minor differences in bone composition between the various species and humans. The pig has a good likeness with human bone but difficulties arise due to its size and ease of handling. So, the dog and goat were chosen as better animal models for testing of bone implant materials. Moreover, Pearce et al. [6] had proved that goat is the chosen species for 8.2% of animal studies published in the Journal of Orthopedic Research between 1992 and 1996. In comparison with sheep, goats tend to have a more inquisitive and interactive nature, which can make long time confinement easier than with sheep. Besides, in the Southeast Asian region, which has high temperature and humidity, goats are more tolerant toward ambient conditions than other species. According to biomechanical consideration of animal models used in tissue engineering, a small difference is apparent in the ashes density between goats and humans. This indicates that there is not much variation between anatomic sites of the same species. Besides, the goat has the same mineral composition as humans. Prothero et al. [7] had studied the attributes of hoofed mammals whereby the tarsal of the foot of the cow can be compared to the human foot. At this point, they had started to observe the structural and functional similarities between cow's leg and the human foot.

According to ASTM E8, strip materials commonly used for tensile testing are in dog bone shape, with wide ends and a narrow middle [8]. According to ASTM [9], conducting the experiment and performing the milling process by using bone samples of "dog bone" shape for the bone-shaped tensile specimen can lower the ultimate force at failure, which can reduce the stress concentration in the grips and concentrate failure in specific area for brittle mass. Besides, Subit et al. [10] had discovered that the gradual taper allows for a smooth transition of load distribution from the edge to the center. So, it can reduce the possibility of stress concentration from the sample edge and focus the failure in the gauge section of the sample. There were a few studies by the researchers on RH testing. Ajadi and Sanusi [11], Huynh et al. [12], and Karr and Outram [13] had studied the effect of RH on oven temperature of solar cabinet dryer. There were a few studies on tensile tests on bones [14-22].

In general, [22-25] have conducted nano-indentation on bovine specimen surface, with a surface contact force of 30?mN and a constant loading rate of 2000?µN?s-1. The hardness depression was held for a period of 5?s at the maximum load to eliminate creep behavior, before unloading at the rate of 2000?µN?s-1. Rho et al. [26] also have performed nano-indentation using nano-indenter II, which is a fully automated hardness testing system. The load and displacements of the indenter used were 0.3?µN and 0.16?nm respectively. Sharp Berkovich diamond indenters, a three-sided pyramid with the same area-to-depth ratio as the Vickers indenter, were also used. They reassessed the elastic modulus and hardness of secondary osteonal and interstitial bone through the cortex of human femora of various ages using the nano-indenter [27]. To minimize the effects of viscoelasticity and creep on proper measurement, a long constant load hold period was introduced before final loading. The second constant load hold period was introduced near the end of the test, at 10% of the peak load, in order to establish the rate of thermal expansion or contraction of the testing apparatus to correct the displacement data for thermal drift. The nano-indentation was conducted with a maximum load of 8?mN, with a loading rate of 400?µN?s-1, which can produce a hardness impression with a depth of 700?nm. Three target areas from near the center of the specimen along the radial direction were selected in each endosteal, middle, and periosteal site of the specimens. Van had also determined the hardness of the cortical bone in goats at different preservation durations by conducting the Vicker test. In the experiment, a Vicker automatic hardness measurement device (Buehler 1600-6400; Buehler, Lake Bluff, IL) was used to perform six measurements on each specimen [28]. The indentation was made using a diamond indenter and an indent weight of 50?g. Then, the Ominimet MHT software was used to analyze the indentation.

1.3 Methodology

The experimental methodology was divided into four phases. They include planning the parameters selection for the experiment, design of the experiment, observation, and analysis phases. Design of experiment () plays an important role in optimization of the temperature parameter in accelerated temperature testing. In order to determine the desired information, both the number of factors and levels of parameters must be predetermined.

1.3.1 Phase I: Planning

Before carrying out any experiment, the parameters for each experiment were studied and discussed. The information gained from journals on the experiments done by previous researchers, suggestions from the supervisor and laboratory technicians, and references from standards were all recorded for DOE settings.

After analyzing the information, decision making on the parameter settings for the experiments was carried out. In accelerated temperature testing, the parameters used were...