Combined AFM-SEM for mechanical testing of fibrous biological materials
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Combined AFM-SEM for mechanical testing of fibrous biological materials Fei Hang1, Dun Lu1 and Asa H. Barber1 1 Centre for Materials Research & School of Engineering and Materials Science, Queen Mary University of London, Mile End Road, London E1 4NS, UK. ABSTRACT A technique combining both atomic force microscopy (AFM) and scanning electron microscopy (SEM) is used to evaluate the mechanical properties of individual collagen fibrils from the fractured surface of antler. SEM is used to locate individual mineralized collagen fibrils and allow visualization of the attachment of these fibrils to the end of an AFM probe. Tensile testing of individual collagen fibril to failure was performed using the AFM with resultant stressstrain curves obtained. Tensile strengths of up to 0.18GPa are found for some individual collagen fibrils, indicating the presence of mineral in improving mechanical performance. Consideration of the SEM operating parameters indicates that the amount of time the sample is within the SEM vacuum can affect the resultant mechanical behavior of individual fibrils. INTRODUCTION Bone is a complex composite material which has at least some degree of structural optimization for a mechanical function. The structural hierarchy in bone extends from length scales of millimeters down to the nanoscale [1]. The nanometer length scale is important as collagen fibrils, the building blocks of the bone itself organized from tropocollagen macromolecules into a discrete fibrillar geometry, are the predominant structural feature which contribute to the overall mechanical performance of the bone. The mechanical properties of collagen fibrils are therefore important in understanding the relationship between deformation at this fibrillar level and larger length scales. The characterization of mechanical properties in collagen fibrils is typically carried out using atomic force microscopy (AFM). AFM is particularly beneficial as it provides both high resolution imaging and can apply forces using the AFM probe situated at the end of a flexible bending beam, known as the cantilever. An example of the high force resolution of AFM was demonstrated during mechanical testing of individual tropocollagen macromolecules [2]. Contact of the AFM probe with individual tropocollagen molecules separated out on a substrate followed by translation of the AFM probe away from the substrate caused deformation of the tropocollagen molecule until failure, either within the molecule or at the tropocollagen-AFM probe contact, at forces of around 300pN. Direct testing of individual collagen fibrils was notably attempted by Graham et al using in vitro-assembled human type I collagen fibril [3]. Mechanical testing was performed in a similar manner to individual tropocollagen experiments, with the AFM probe contacting and the pulling an individual collagen fibril from a connecting substrate. Results showed that the elastic modulus of collagen fibril was unusually low with a maximum value of 32MPa, expected to be due to sliding of the col
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