Date of Award

1-2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biomedical and Chemical Engineering and Sciences

First Advisor

Boris Akhremitchev

Second Advisor

Joel Olson

Third Advisor

Kurt Winkelmann

Fourth Advisor

Vipuil Kishore

Abstract

Conventional methods of mechanical testing cannot measure properties of soft materials at the nanoscale. In fields such as tissue engineering, it is important to distinguish the bulk elastic modulus from the surface elastic modulus and to characterize the spatial distribution of material with non-uniform stiffness. One of the important new methods of testing is the force mapping using the atomic force microscope. The existing force mapping approaches often suffer from omitting important effects that might result in artifacts. The most important effects include taking into account sample’s adhesion and viscoelasticity and considering more realistic probe shapes. In this work, we have applied a method that take into consideration probe shape and sample adhesion and developed elastic modulus mapping. This inclusive approach can be applied to samples with adhesion that exhibit indentation large enough that requires indenter shape models more sophisticated than the traditional paraboloid shape model. Sample viscoelasticity can be revealed by comparing parameters obtained in opposite scanning directions. The application developed methodology is illustrated with the study of composite biomaterial scaffolds that are used to support the differentiating cells. Samples are composed of four groups: uncrosslinked electrochemically aligned collagen (ELAC), uncrosslinked Bioglass incorporated ELAC (BG-ELAC), genipin crosslinked-ELAC and crosslinked electrochemically compacted collagen (ECC, unaligned). The force mapping on BG-ELAC sample did not exhibit any area with high elastic modulus (measured modulus was around 0.6 MPa), indicating that there is no Bioglass particle protrusion observed at the BG-ELAC surface. Elastic modulus of ELAC molecules appears softer (~ 0.1 MPa) for samples without Bioglass added. Adding genipin crosslinker to collagen threads made the ELAC and ECC samples stiffer even more than uncrosslinked BG-ELAC with characteristic values of elastic modulus approximately 0.97 MPa and 1.28 MPa for crosslinked-ELAC and crosslinked ECC respectively. More extensive studies are necessary to fully investigate effect of crosslinking ratio and adding Bioglass at different concentrations on the elastic modulus values of collagen samples. Methodology reported here is a suitable tool for such studies and can be applied to other soft and potentially heterogeneous biomaterial samples.

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