Week 9

     This week during the class meeting time, we discussed our current state of the project with Dr. Gordon and what we should do to move forward. Our Deliverable 5 has been postponed until the end of the semester, as we need to redesign or add onto our current design in order to address the issue with the legs. (Our current design requires the legs to move, but we have learned that astronauts do not move their legs very often while in a zero gravity environment, rendering our current design largely ineffective.)
     In addition, Dr. Gordon wants us to research further into the amount of load needed to sustain healthy bone. In order to determine this, we decided to also look at physical and mechanical properties of bone, such as the cross-sectional area, modulus of elasticity, etc. We spent several hours researching this, but did not have much luck. Most of the articles we found either did not give the information, or were not accessible. Dr. Gordon suggested that we might be able to find CAD files of bones, from which we could find the cross-sectional area. Below is an example of a femur we found on the NIH's 3D Print Exchange Website (http://3dprint.nih.gov/discover/3dpx-000168). The model was created from a CT scan, meaning that it is probably very accurate (even though the sample size in this case is only 1). A model of the tibia and fibula bones are shown as well.

STL File of a Human Femur found on the NIH 3D Print Exchange.

STL File of a Human Tibia and Fibula found on the NIH 3D Print Exchange.

     During the week, we met as a team to try and figure out what we needed to research still, and also to discuss options for targeting the bone. This was not a formal brainstorming session, but the following options were suggested:

Possible options for targeting the legs.

     Of these suggestions, we deemed that motors might be the most practical to use. The following sketch shows an early idea of how they might be implemented in conjunction with the current design.

A sketch of the leg showing a motor being used to move the leg.

     We also found some research articles that suggested that either a large load could be applied less frequently, or a smaller load could be applied more frequently in order to maintain healthy bone. The amount of strain needed to be applied and the frequency will determine the type of motor(s) we purchase, should we choose to go that route, but we need to find and finalize the modulus of elasticity first.
      The following papers were found when researching this topic:

"The Longitudinal Young’s Modulus of Cortical Bone in the Midshaft of Human Femur and its Correlation with CT Scanning Data"

Abstract: This study was concerned with establishing the regional variations in the magnitude of the longitudinal Young’s modulus of the cortical bone in the femoral midshaft and with investigating whether a relationship existed between the Young’s modulus of bone and the CT number. Were such a relationship to exist this would provide a noninvasive method of assessing the quality of bone in the regions of fixation of implants to bone. The data would be of considerable aid to designers of implant stems to withstand the stresses arising at its interface with the bone. Five pairs of fresh frozen human femora were used. Several beam-shaped small specimens were methodically harvested from each pair and were used to measure the longitudinal modulus adopting the three-point bending test, which was carried out with a specially constructed and validated apparatus. CT scans of the bone were obtained, prior to harvesting the specimens, and the CT number was measured at locations corresponding with the specimen sites. The results indicate that in the femoral midshaft the cortical bone has an average Young’s modulus value of 18600 ± 1900 MPa. This agrees well with data obtained by other researchers using different experimental methods. Statistical analyses revealed no regional variations in the value of the longitudinal modulus of the bone. No correlation was found between the bone modulus and the CT number. Thus a noninvasive method for establishing the bone properties still remains a challenge.
Link: http://link.springer.com/article/10.1007/s00223-002-2123-1

"Elastic modulus and hardness of cortical and trabecular bone lamellae measured by nanoindentation in the human femur"

Abstract: "The mechanical properties of bone tissue are determined by composition as well as structural, microstructural and nanostructural organization. The aim of this study was to quantify the elastic properties of bone at the lamellar level and compare these properties among osteonal, interstitial and trabecular microstructures from the diaphysis and the neck of the human femur. A nanoindentation technique with a custom irrigation system was used for simultaneously measuring force and displacement of a diamond tip pressed 500 nm into the moist bone tissue. An isotropic elastic modulus was calculated from the unloading curve with an assumed Poisson ratio of 0.3, while hardness was defined as the maximal force divided by the corresponding contact area. The elastic moduli ranged from 6.9±4.3 GPa in trabecular tissue from the femoral neck of a 74 yr old female up to 25.0±4.3 GPa in interstitial tissue from the diaphyseal cortex of a 69 yr old female. The mean elastic modulus was found to be significantly influenced by the type of lamella (p<10⁻⁶) and by donor (p<10⁻⁶). The interaction between the type of lamella and the donor was also highly significant (p<10⁻⁶). Hardness followed a similar distribution as elastic modulus among types of lamellae and donor, but with lower statistical contrast. It is concluded that the nanostructure of bone tissue must differ substantially among lamellar types, anatomical sites and individuals and suggests that tissue heterogeneity is of potential importance in bone fragility and adaptation."

Link: http://www.sciencedirect.com/science/article/pii/S0021929099001116

"The elastic properties of trabecular and cortical bone tissues are similar: results from two microscopic measurement techniques"

Abstract: "Acoustic microscopy (30–60 μm resolution) and nanoindentation (1–5 μm resolution) are techniques that can be used to evaluate the elastic properties of human bone at a microstructural level. The goals of the current study were (1) to measure and compare the Young’s moduli of trabecular and cortical bone tissues from a common human donor, and (2) to compare the Young’s moduli of bone tissue measured using acoustic microscopy to those measured using nanoindentation. The Young’s modulus of cortical bone in the longitudinal direction was about 40% greater than (p<0.01) the Young’s modulus in the transverse direction. The Young’s modulus of trabecular bone tissue was slightly higher than the transverse Young’s modulus of cortical bone, but substantially lower than the longitudinal Young’s modulus of cortical bone. These findings were consistent for both measurement methods and suggest that elasticity of trabecular tissue is within the range of that of cortical bone tissue. The calculation of Young’s modulus using nanoindentation assumes that the material is elastically isotropic. The current results, i.e., the average anisotropy ratio () for cortical bone determined by nanoindentation was similar to that determined by the acoustic microscope, suggest that this assumption does not limit nanoindentation as a technique for measurement of Young’s modulus in anisotropic bone."

Link: http://www.sciencedirect.com/science/article/pii/S0021929098001778

"THE MATERIAL PROPERTIES OF HUMAN TIBIA CORTICAL BONE IN TENSION AND COMPRESSION: IMPLACATIONS FOR THE TIBIA INDEX"

Abstract: "The risk of sustaining tibia fractures as a result of a frontal crash is commonly assessed by applying measurements taken from anthropometric test devices to the Tibia Index. The Tibia Index is an injury tolerance criterion for combined bending and axial loading experienced at the midshaft of the leg. However, the failure properties of human tibia compact bone have only been determined under static loading. Therefore, the purpose of this study was to develop the tensile and compressive material properties for human tibia cortical bone coupons when subjected to three loading rates: static, quasistatic, and dynamic. This study presents machined cortical bone coupon tests from 6 loading configurations using four male fresh frozen human tibias. A servo-hydraulic Material Testing System (MTS) was used to apply tension and compression loads to failure at approximately 0.05 s-1, 0.5 s-1, and 5.0 s-1 to cortical bone coupons oriented along the long axis of the tibia. Although minor, axial tension specimens showed a decrease in the failure strain and an increase the modulus with increasing strain rate. There were no significant trends found for axial compression samples, with respect to the modulus or failure strain. Although the results showed that the average failure stress increased with increasing loading rate for axial tension and compression, the differences were not found to be significant. The average failure stress for the static, quasi-static, and dynamic tests were 150.6 MPa, 159.8 MPa, and 192.3 MPa for axial tension specimens and 177.2 MPa, 208.9 MPa, and 214.1 MPa for axial compression specimens. When the results of the current study are considered in conjunction with the previous work the average compressive strength to tensile strength ratio was found to range from 1.08 to 1.36."
Link: http://www-nrd.nhtsa.dot.gov/pdf/esv/esv20/07-0470-O.pdf

     This week team members also updated their ethics papers as the final draft is due at the beginning of Week 10.