Experimental Testing To Computational Modeling Of Polymeric Nanofibers-An Investigation Involving A Single-Strand Nanofiber To A Nanofibrous Scaffold

With the rapid development of tissue engineering,nanofibrous scaffolds have been gaining popularity for use as an extra-cellular matrix for cell seeding.Since the fibers used in these scaffolds are nano-scaled,an accurate quantification of their strength characteristics becomes necessary in order to design a scaffold that meets the specifc strength requirements for handling different cell types.Also,it is necessary to quantify the strength characteristics of the scaffold itself as the nanofiber arrangement in terms of fiber orientation and porosity can be a significant factor.Thus,this thesis is concerned with an investigation of the single-strand nanofiber,as well as,the nanofibrous scaffold.For the modeling of the single-strand nanofiber,we have provided experimental data to verify the predictions.As for the scaffold,we presented limited experimental verifications since there are very few reported experiments on the mechanical testing of the scaffold in the literature.In our experimental measurements with single-strand polycaprolactone(PCL) nanofibers,we conducted uniaxial tensile and three-point bending tests using the Nano Universal Tensile Machine and diametral measurements using the atomic force microscopy.The tensile test data shows that the elastic modulus of PCL nanofibers is statistically invariant with changing fiber diameters.Our results appear to contradict one set of reported test data for the same fiber type,where it depicts a size dependent trend in the elastic modulus-diameter plot.For the three-point bending tests both our data and reported experimental results are consistent in that the elastic modulus clearly exhibits a significant inverse size-dependent trend with the fiber diameter.Our modeling effort also begins with the single-strand nanofiber as without a prior knowledge of this basic building block,it is not possible to quantify the mechanical properties of the scaffold.We have developed several computational models for studying a single-strand nanofiber under the 2 types of loading;tensile and bending.Recognizing the non-local interactions between discrete elements of the polymeric material,we employed the strain gradient elasticity(SGE) theory to model the size dependent behavior.To handle the reported tensile size dependency,we expanded on the traditional SGE theory by incorporating 2nd-order strain gradients and termed the resulting approach as higher-order strain gradient elasticity(HSGE).Our theoretical work indicates that there are 2 kinds of size dependent response; one that pertains to a decreasing fiber length(while still maintaining the fiber aspect ratio) and the other to a decreasing fiber diameter.We referred the former as L-SD (length size-dependency) and the latter as D-SD(diameter size-dependency).The L-SD is observed in our SGE tensile predictions while the D-SD is seen in our HSGE predictions.Since all the reported experiment data(including ours) are designed to measure only D-SD,the agreement between the model predictions and experimental results is very good.In this thesis,we have also presented a modified surface effect (SE) approach to study size-dependency in polymeric nanofibers and showed that its predictions also matched the measured data but not as closely as our HSGE model.In the bending size dependency work,we developed computational models using both SGE and SE theories for 3 types of boundary conditions:cantilever,fixed-fixed and simply supported.Application of SGE to the bending deformation of nanofibers produces both L-SD and D-SD,while the application of SE leads to D-SD only.Both D-SD models are able to track the bending size-dependent behavior;however the SGE model appears to produce a better fit with the measured data for polymeric nanofibers.With the knowledge gained from the modeling of single strand nanofibers,we now consider the micromechanical modeling of a nanofibrous scaffold.The nanofibers are assumed to be linear-elastic straight rods of constant length.However, in the scaffold the nanofibers are not viewed as distinct elements;instead,it is the individual fiber segment between any 2 fiber crossings that is considered.Further,the length of a fiber segment is assumed to obey a Poisson distribution of fiber-to-fiber contacts.In our 2-D scaffold model we studied 2 kinds of fiber distribution - random and aligned,and obtained its elastic modulus that included an exponential decay term to capture the percolation effect for both these 2 cases.For the randomly distributed nanofibers,the elastic modulus predictions agree with results from a finite element analysis to a reasonably good degree of accuracy.For the aligned nanofibers the scaffold is orthotropic in nature and thus,the elastic modulus has 2 components-parallel and perpendicular to the fiber orientation.A comparison of the 2 moduli with published experimental data shows that although the model is able to capture the data trend the agreement still has minor errors.We also incorporated size dependency into the micromechanical model of the scaffold.Specifically,we introduced HSGE to capture tensile D-SD and SGE for bending D-SD.We presented several plots of the scaffold's elastic modulus versus the fiber density and fiber diameter for the randomly distributed fibers with and without size-dependency.Likewise,we did the same for the aligned fibers but with an additional parameter - the fiber orientation.To summarize,we note that size dependency in polymeric nanofibers is a complex phenomenon.Our research on single strand nanofibers indicates that it can manifest either as a diameter-based and/or a length-based size dependency.The phenomenon in a scaffold is even more complex as it can be affected by additional parameters such as fiber density and fiber orientation.