Natural materials have long inspired the design and fabrication of advanced materials. Characterized by intricate structures featuring hierarchical porosity, spatial heterogeneities, and architectural gradients, these materials offer unique combinations of stiffness, strength, and toughness at low density. Inspired by nature, this thesis explores the development of advanced materials through two distinct avenues.In the initial investigation, the adhesive capabilities of marine mussels are examined on various geometry-controlled surfaces. Through mechanical tests, the adhesion strength of mussel structures is assessed, revealing consistent detachment forces . This suggests their robustness to different substrate geometries. Subsequent scanning electron microscopy (SEM) analysis reveals substrate-dependent damages within microstructures, indicating that mussels are capable of changing damage mechanisms to maintain characteristic pull-off force, suggesting their adaptability to geometric conditions. Building upon the insights gained from natural materials, subsequent work focuses on the development of advanced materials using additive manufacturing (AM) techniques. Specifically, digital light processing (DLP) of photocurable resins combined with thermally expandable microspheres is employed to create polymer composite foams that are lightweight yet stiff. A two-step material processing approach enables precise control over porosity with tunable mechanical properties of the materials. Through mechanical analysis under compression, we elucidate how foaming alters the mechanics of the composite material, including changes in modulus, Poisson’s ratio, energy dissipation, and fatigue performance. In the final chapter, we investigate thermally induced transformations in the properties of the polymer foams. We observe that 3D-printed objects undergo extra-curing upon heating above the expansion temperature of the microspheres, indicating that the materials bear latent transformation capabilities. Through comprehensive thermal analysis and mechanical characterization, we identify exothermic reactions at elevated temperatures, along with shifts in the glass transition temperature. This confirms that the processed material still contains unreacted residual monomers, which can lead to on-demand thermal polymerization. An understanding of thermal polymerization kinetics is established, enabling the further tailoring of mechanical properties of the foams post-cure. This work provides insights into the processing-structure-property relationships of additively manufactured composite foams, offering feasible pathways for the design of materials with practical applications across various engineering fields.

This work is a culmination of our efforts in understanding cellular mechanics at the scale of single cells and small tissues. We developed methods to quantify cell-generated traction forces using cell-sized, synthetic, functionalized hydrogel microspheres. Cell-sized solid microspheres can provide information regarding cell-generated normal and shear forces while allowing natural cell-cell interactions and facilitating a convex cell-hydrogel interface. Therefore, they are a better mimic than the current methods for understanding the natural cell-cell interactions in a physiologically relevant geometry. In the analysis of the microsphere deformations, we use a boundary spectral method based on spherical harmonics decomposition of the traction field on the spherical gel surface. Using the techniques developed here, we measure the boundary traction profiles that mammalian cells exert on the synthetic microspherical hydrogel bodies. In this report, we briefly review the state of the art in cellular force quantification methods and discuss the contributions of our work to the field and its strengths as well as its limitations.

Microtubules are an essential component of the cytoskeleton that provide structural integrity and facilitate important functions in cells. In this work, I will explore several aspects of microtubule mechanics and function using a wide range of biophysical techniques. I am particularly interested in the effects of stabilizing agents, including small molecule inhibitors of microtubule dynamics (such as taxol or epothilone), and microtubule-binding proteins, such as tau. First, using advanced spectral analytical techniques, we studied the effects of tau proteins on the mechanical properties of single microtubule (MT) filaments under controlled in vitro conditions. We found that the short forms of 3-repeat (3R) and 4-repeat (4R) wild-type (WT) tau proteins reduce the stiffness of taxol-assembled MTs compared to a no tau control, despite the fact that the microtubule diameter is known to increase when these tau proteins bind. In contrast, single point tau mutants P301L, R406W and ΔN296, which are all linked to devastating neurological diseases, do not have significant effects on microtubule stiffness compared to a no tau control, raising interesting questions about possible mechanical origins of tauopathy diseases.

Next, we examined the effects of tau proteins on in vitro transport properties of kinesin-bound cargoes on microtubules. In comparison to the no tau control, kinesin-bound quantum dot cargoes had lower velocity and shorter run lengths in the presence of tau when moving on taxol-assembled microtubules. Most of the single point tau mutants that we tested showed similar slowing of cargo translocation, with speed reductions similar to those observed with WT tau. One exception was the 4R short R406W mutant tau, in which the velocities of kinesin-bound cargoes were significantly lower than in WT case. The disease mechanism for mutant has been particularly puzzling, so our results may suggest a more prominent role in disruption of axonal transport than was previously appreciated.

Next, we investigated the effects of microtubule-stabilizing chemicals epothilone-A and -B on 1) microtubule mechanics, 2) the transport properties of kinesins and 3) the ability of WT tau proteins modify the stiffness of microtubules assembled with epothilone. We found that microtubules assembled in the presence of epothilone-A or -B were less stiff compared to taxol-assembled MTs, and the addition of WT tau further reduced the stiffness. However, the differences in stiffness/persistence length between epothilone-assembled microtubules and taxol-assembled microtubules diminished after the addition of WT tau. Kinesin translocation speed was sensitive to the type of stabilizing chemical, and the changes in velocity in the presence of tau were also depended on the assembly condition. Epothiolone and taxol compunds are both frequently used in cancer chemotherapies, and our results shed light on the possible molecular mechanisms of neurological side effects (such as debilitating nerve pain) when these agents are used.

Lastly, we looked at the effects of tau on intracellular trafficking in COS-7 cells. We found that the microtubule network in the cells were dramatically modified after the introduction of tau via transient transfection, with substantial bundling, aggregation and membrane-association in the presence of tau. However, there were no obvious differences in the microtubule network structures between cells that were transfected with mutant tau and wild-type tau. Interestingly, despite the major modification of the cytoskeletal structures in the presence of tau, the measured velocities of lysosomes in directed transport in cells that were transfected with tau were not significantly different compared to the lysosome velocities in cells with no tau.

Taken together, studies provide important new insights into how tau proteins and other small molecule stabilizers modulate the mechanical and functional properties of cytoskeletal microtubules and how misregulations in tau may be related to the development of neurodegeneration and dementia diseases.

Intracellular transport along microtubules is a process critical to cell health, and

one whose breakdown is associated with neurodegenerative disorders. In this thesis, we

describe new technologies that will enable and improve investigations of multiple types

of intracellular transport on microtubules.

Although the single molecule properties of many motor proteins have been well characterized,

their behavior when transporting biological cargoes as a group of motors is still

not well understood due to measurement challenges, as well as a lack of a model system

for systematically studying collective motor behavior in vitro. In this work, we discuss

the construction of an optical trapping setup capable of applying and measuring forces

with 2 pN accuracy. We report our design of a biomimetic droplet system that reproduces

the relevant surface properties of biological cargoes while allowing the droplets to

be used as optical trapping probes. We also present a new optical trapping calibration

technique that allows experimenters to utilize the nonlinear range of the trapping force

profile, and thereby measure the high forces developed by groups of motor proteins.

Finally, we investigate the distributions of diffusion coeffcients found for Microtubule

Associated Proteins diffusing on microtubules through Monte Carlo simulations, and

examine the subtleties of interpreting and reporting single molecule diffusion data.

The proteinaceous byssal plaque-thread structures created by marine mussels exhibit extraordinary load-bearing capability. Although the nanoscopic protein interactions that support interfacial adhesion are increasingly understood, major mechanistic questions about how mussel plaques maintain toughness on supramolecular scales remain unanswered. This study explores the mechanical properties of whole mussel plaques subjected to repetitive loading cycles, with varied recovery periods. Mechanical measurements were complemented with scanning electron microscopy to investigate strain-induced structural changes after yield. Multicyclic loading of plaques decreases their low-strain stiffness and introduces irreversible, strain-dependent plastic damage within the plaque microstructure. However, strain history does not compromise critical strength or maximum extension compared with plaques monotonically loaded to failure. These results suggest that a multiplicity of force transfer mechanisms between the thread and plaque-substrate interface allow the plaque-thread structure to accommodate a wide range of extensions as it continues to bear load. This improved understanding of the mussel system at the micron- to-millimeter lengthscale offers strategies for including similar fail-safe mechanisms in the design of soft, tough and resilient synthetic structures.

I present the design and characterization of a high force magnetic tweezers device that can apply controlled forces to magnetic beads embedded into soft materials or biological systems, while visualizing the resultant material deformation with microscopy. Using finite element analysis (FEA), I determined the effect of the geometry of the NdFeB magnet array, as well as the geometry of iron yokes designed to focus and shape the magnetic fields. Sixteen shape parameters including the magnet size, positioning and yoke curvature were defined and modeled using open-source magnetic FEA software. Parameter sweeps were performed using custom-written Matlab code. Geometries were optimized for the magnitude of the magnetic field gradient and the length scale over which the magnetic force operated. Once an optimal design was identified, the yoke was fabricated in-house and the FEA validated by mapping the device's magnetic field using a Hall probe. To demonstrate the usefulness of this approach, I produced a magnetic tweezers device designed for use with optical microscopes available in a core imaging facility. The application demanded device portability and the ability to interface with a number of microscopes, thus imposing significant size restrictions on the magnets used. Iterative FEA delivered an optimal magnet-yoke geometry, which could be mounted to a carriage that advances or retracts on command, giving the operator fine control over the applied force. Such automation allows for rapid force switching, and also allows the effects of long periods of cyclical loading to be determined. The carriage design, automation and implementation were produced in collaboration with a summer intern, Timothy Thomas from the INSET program at UCSB. In future work, such an FEA approach could easily be adapted to a range of design goals/restrictions to create an efficient means of testing possible magnet configurations, while streamlining the design and construction of specialized instrumentation for force-sensitive microscopy.

Soft robotics represents a shift in engineering design methodology: one that draws inspiration from biology and strives to move away from complex, rigid assemblies of numerous discrete parts and towards soft assemblies comprised of relatively few parts. Proponents of this shift advocate that soft robots will enable safe operation around humans, increased resistance to mechanical damage, operation in hazardous environments, and other functions that are presently inaccessible via traditional (i.e., rigid) robotics. However, bringing these visions to life requires efficient energy systems, soft controllers, and new material systems. This dissertation is an exploration of the myriad ways that light-material interactions can be leveraged to offer innovations in each of these areas. Light serves as the common thread among each of these investigations as it is an accessible form of energy that can be modulated (e.g., by wavelength or intensity) and transmitted (e.g., lasers) or harvested (e.g., sunlight) over long distances with highly tunable spatial and temporal precision. First, we examine the capabilities of soft heat engines, identify their major sources of losses and offer three key insights that soft roboticists can leverage to minimize them (Chapter II). Second, we investigate the photokinetics of a novel class of photoswitches, which constitute the basis for a thermofluidic switch (Chapter III). Finally, we investigate the mechanical properties of bio-inspired stiff-soft multi-material composites that exhibit impressive toughness (Chapter IV). These endeavors at the intersection of materials, mechanics, chemistry, and thermodynamics put forth foundational knowledge that advances the field of soft robotics and also inspires new avenues for meeting the growing demand for adaptive and autonomous materials and devices.