Session A03: Biophysics and Medical Physics I

Mechanical limitation of bacterial sliding motility mediated by growing cell chains

Contrasting most known bacterial motility mechanisms, a bacterial sliding motility discovered in at least two Gram-positive bacterial families does not depend on designated motors. Instead, the cells maintain end-to-end connections following cell divisions to form long chains and exploit cell growth and division to push the cells forward. To investigate the dynamics of this motility mechanism, we constructed a mechanical model that depicts the interplay of the forces acting on and between the cells comprising the chain. Due to the exponential growth of individual cells, the tips of the chains can, in principle, accelerate to speeds faster than any known single-cell motility mechanism can achieve. However, analysis of the mechanical model shows that the exponential acceleration comes at the cost of an exponential buildup in mechanical stress in the chain, making overly long chains prone to breakage. Additionally, the mechanical model reveals that the dynamics of the chain expansion hinges on a single non-dimensional parameter. Perturbation analysis of the mechanical model further predicts the critical stress leading to chain breakage and its dependence on the non-dimensional parameter. Finally, we developed a simplistic population expansion model that uses the predicted breaking behavior to estimate the physical limit of chain-mediated population expansion. Predictions from the models provide critical insights into how this motility depends on key physical properties of the cell and the substrate. Overall, our models present a generically applicable theoretical framework for cell chain-mediated bacterial sliding motility and provide guidance for future experimental studies on such motility.   

Effect of aerogel’s topography and surface roughness on neurite extension in the presence and absence of applied DC bias.

Aerogels have been studied for their biomedical applications including drug delivery, regenerative medicine, wound healing, and biosensing. Previous work from this group has shown the suitability of aerogels as neuronal scaffolds. Aerogel’s inherent 3-D structure offers an advantage over other biocompatible substrates that lack the dimensionality needed to mimic the in vitro topography of tissues. It has also been shown that cell behavior is strongly influenced by external factors including the surface topography, stiffness, and electric fields. Here, the authors studied these effects by using aerogels as scaffolds for PC-12 cells, a neuronal analogue. Surface roughness ranged from 0 to 3 μm and stiffness 10 kPa – 4 MPa. Applied DC bias was investigated in both conductive and insulating aerogel types. This investigation reveals what the optimal substrate features for neurite extension are and the significance of these findings to optimize materials for nerve repair is discussed.

    

Long Term Effects of Annealing Temperature, Storage Condition, and Antioxidants on Oxidative Behavior of Free Radicals in Medical Grade UHMWPE

Ultra-High Molecular Weight Polyethylene (UHMWPE) is used as a medical joint replacement material. Since 1962, materials improvements have been made and the lifetimes of the components have increased significantly. However, challenges remain, one being the focus of this study: free radicals in the UHMWPE, created mostly during necessary radiation treatments (e.g., sterilization), which induce oxidative degradation when exposed to oxygen. Heat treatments (annealing) and antioxidants (e.g., vitamin E) attempt to minimize oxidation, through reduction of the root cause: the free radicals which enable oxidation in the first place. Related annealing research for biocompatible UHWMPE was started over 20 years ago, including at the University of Memphis Physics Department, as a part of a project initiated in 1998 between the State University of New York at Buffalo (SUNY Buffalo) and the University of Memphis under the sponsorship of the NSF Center for Industry/University Collaborative Research on Biosurfaces. The samples have been aging over 20 years, and antioxidant-containing samples for over 10 years. The present study was aimed to report the change in free radical concentration with temperature, storage condition, and antioxidants. Results do show a change with storage temperature, greater in one type vs. another, and strong oxidation prevention in all inert-stored samples, and that vitamin E plays an important role in quenching the free radicals after irradiation.   

Modelling partitioning and diffusion into polydimethylsiloxane (PDMS) for microfluidic devices

Polydimethylsiloxane (PDMS) is a polymer commonly used in microfluidic devices. While PDMS has many excellent properties, it tends to interact with hydrophobic compounds. Therefore, the nominal concentration of a chemical in an organ-on-chip device is not necessarily the actual concentration. To better estimate the actual concentration, we experimentally determine the parameters of a partition-diffusion model that describes chemicals’ interaction with PDMS. With this model, we can predict how these chemicals will behave in microfluidic devices and correct for concentration loss.