Date of Award

2021

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Graduate Group

Physics & Astronomy

First Advisor

Alison M. Sweeney

Second Advisor

Paul A. Janmey

Abstract

Most everyone has heard of a marine sponge, probably not many have heard of a magnetoviscoelasticsolid. Both are complex materials in the sense that they are heterogeneous and responsive to stimuli like applied flows and fields. The natural way to describe these materials is rheology: the study of how things deform and flow and how those macroscopic, mechanically responsive properties can be attributed to mesoscopic and microscopic processes. In the case of sponges, those processes are likely biologically important (not to mention fascinating), and thus the goal of Ch. 2–3 of this thesis is to construct the beginnings of a rheological picture of living sponges. Ch. 1 provides relevant biological information about the pore bearers and also theoretical physics background. Ch. 2 asks the question of what elastic properties do sponges of different growth forms have. Small uniaxial deformation experiments on a set of common species of spherical, cylindrical, and more disordered growth forms, are presented. Sponge tissue mechanics are shown to be diverse, particularly in the behavior of the shear modulus as a function of applied compression. Correlations between degree of tissue anisotropy and growth morphology are discussed from which are drawn broad, physiologically relevant conclusions about sponges. Ch. 3 asks if it can be shown that sponges are somehow tuned to the local hydrodynamics of their environment, leading to the high plasticity observed as no two sponges are alike. A natural experimental extension of Ch. 2 is to make the deformations time-dependent. If sponge tissue mechanics are highly time-dependent, this immediately suggests tuning, or a memory of recent straining. Oscillatory shear strains of different frequencies at both small and larger amplitudes are made on the same set of sponges as in Ch. 2. Further diversity is uncovered and conclusions made about the dynamical properties of sponges – particularly those of more disordered growth forms and tissue microstructures and their potential to tune to local flow. Ch. 2–3 resulted in the manuscript “rheology of marine sponges reveals diverse dynamics and anisotropic mechanics.” Ch. 4 is a short chapter born from years of thinking about sponges as a physicist. A simple phenomenological transport view of passive flow in sponges is presented and back-ofthe- envelope estimates of hydraulic properties of some sponges made. Implications of this non-equilibrium steady state description of flow in sponges are explored. In the case of magnetoviscoelastic solids, the magic of the magnetic field is utilized for new substrates in mechanobiology. The goal of Ch. 5–6 of this thesis is to outline novel techniques for magnetically controlling the mechanics of substrates populated with cells – ultimately for learning about 3D cellular mechanotransduction. These chapters are the result of collaborations in the Center for Engineering Mechanobiology at UPenn, the Biofluid Mechanics Laboratory at Rowan University, and the Laboratory of Magnetic Soft Materials at the University of Latvia. The magnetic-field induced stiffening effect in carbonyl iron microparticle embedded polydimethylsiloxane (PDMS) based elastomers is characterized in Ch. 5 and the tunability of these ultrasoft substrates for applications in mechanobiology are explored. Collaborators for this chapter include Andy Clark (Ph.D. candidate, Physics at Bryn Mawr College) and Alex Bennett (Bioengineering postdoc, UPenn), and the other coauthors on the published work: (Clark et al., 2021). Ch. 6 extends the general field-stiffening effect of magnetic microparticles embedded in a polymer network to the more physiologically relevant collagen and fibrin based hydrogels. The main part of this work was done by Kiet Tran (Ph.D. candidate, Biomedical Engineering at Rowan University): the confirmation of rapid cellular response to three-dimensional changes in substrate mechanics affected by an applied magnetic field (Tran et al., 2021). Andrejs Cebers (Professor, Theoretical Physics, University of Latvia) constructed and I communicated and validated a continuum magnetoelastic stiffening model with the data for both the biopolymer gels as well as the PDMS-based elastomer of Ch. 5. Ch. 7 is an extremely short, poetic conclusion to the science presented. Rheology is a natural bridge between biology and physics. Complex materials, such as living marine sponges and cellularized magnetoviscoelastic solids are here powerfully probed by a rheometer. The contributions presented in this thesis push the boundaries of how to describe biological materials and their behaviors with physics.

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