Investigation on Mechanical Properties of Biological Materials

Abstract

Fundamental principles guiding the mechanical behaviors of biological materials are of paramount importance for gaining in-depth knowledge about the biological materials as well as designing novel bioinspired materials. In this dissertation, we aim to investigate two types of biological materials, human hair and rat liver.

During liver preservation, viability test is an important however complex procedure. In this process, temperature is observed to play important role in affecting liver status, but no quantitative relation between these two elements has been constructed yet. In this dissertation, we aimed to study this relation by investigating mechanical behavior of rat liver within certain temperature range. For sake of introducing minimal damage to liver, nanoindentation was selected to obtain elastic modulus of liver sample while its vicinity temperature was in the range of 8 to 22 ºC. This range was selected as it was crucial for graft preservation of liver transplantation. Results demonstrated that in the temperature range of 8 to 22 ºC, elastic modulus of rat liver positively correlates with temperature and had little dependency on its heating-cooling history. These founding implicated that within safe preservation time, variation of vicinity temperature in the range of 8 to 22 ºC did not induce permanent damage to rat liver. All conclusions may serve as a mechanophenotype for viability of liver graft.

Human hair is a sophisticated composite containing cuticle, cortex and medulla. Cuticle is a thin multi-layered structure which circularly wraps and protects cortex and medulla. Being the major part, cortex is assembled by keratin bundles and protein matrix. Medulla is the very central structure of hair. This unique structure endows human hair outstanding mechanical properties. Diameter of hair fiber was selected as a structural factor and its effect on mechanical properties of hair was investigated. As a standard method of evaluating mechanical properties of fibers, uniaxial tensile tests were conducted to obtain tensile properties of hair fiber. Besides, fracture patterns and failure mechanism of hair fibers after tensile tests were also explored. It was demonstrated that human hair had a typical elastic-plastic constitutive behavior. Diameter showed significant effects on tensile properties of human hair. Tensile fracture patterns were also affected by diameter. Therefore, diameter effect is highly related to hair structure and knowledge in this part has potential to be utilized in designing bioinspired tougher fibers. What’s more, treatment effect on hair surface was also investigated. Uniaxial tensile tests were performed on treated hair for specifying contribution of cuticle and cortex to tensile properties of entire hair. Tensile results indicated that treatment had little effect on tensile properties of hair and cortex was the major contributor. Besides, morphology and mechanical properties of treated hair surface were characterized with aid of Atomic Force Microscopy (AFM). Different to tensile behaviors, treatment effect in surface properties of human hair was remarkable. AFM results demonstrated that hair dye could severely deteriorate surface performance of human hair, while shampoo and conditioner had little effect. Furthermore, gender effect was discovered to manifest both on tensile and surface properties of human hair. Both tensile and surface behaviors showed that female hair was tougher and more easily affected by treatment than male hair. Physiological difference might play important role in this part and in-depth mechanism would be further explored.

In conclusion, rat liver and human hair were investigated as two representatives of biological materials. Both of these two materials have multi-layered structure and this kind of structure contributes quite a lot to their mechanical properties. However, their architecture and mechanical behaviors are distinctly different. Rat liver represents hierarchical soft tissue. It has sophisticated mechanical behavior which is hard to be described by any constitutive law. To minimize the damage, this work is scoped within linear elastic range. Differently, human hair is a biological composite material whose constitutive behavior can be described by typical elastic-plastic law.

Date of Award	25 Jul 2016
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Xinrui NIU (Supervisor)