David Shicheng Li

Cell therapy for spinal cord injuries offers the possibility of replacing lost cells after trauma to the central nervous system (CNS). In preclinical studies, synthetic hydrogels are often co-delivered to the injury site to support survival and integration of the transplanted cells. These hydrogels ideally mimic the mechanical and biochemical features of a healthy CNS extracellular matrix while also providing the possibility of localized drug delivery to promote healing. In this work, we synthesize peptide-functionalized polymers that contain both a peptide sequence for incorporation into self-assembled peptide hydrogels along with bioactive peptides that inhibit scar formation. We demonstrate that peptide hydrogels formulated with the peptide-functionalized polymers possess similar mechanical properties (soft and shear-thinning) as peptide-only hydrogels. Small angle neutron scattering analysis reveals that polymer-containing hydrogels possess larger inhomogeneous domains but small-scale features such as mesh size remain the same as peptide-only hydrogels. We further confirm that the integrated hydrogels containing bioactive peptides exhibit thrombin inhibition activity, which has previously shown to reduce scar formation in vivo. Finally, while the survival of encapsulated cells was poor, cells cultured on the hydrogels exhibited good viability. Overall, the described composite hydrogels formed from self-assembling peptides and peptide-modified polymers are promising, user-friendly materials for CNS applications in regeneration.

Ultrasound is one of the most commonly used methods for synthesizing and processing emulsion systems. In this study, the kinetics of acoustically induced emulsion oil exchange was examined using contrast variation time-resolved small angle neutron scattering (CV-SANS). A custom-built sample environment was used to deliver acoustic forces while simultaneously performing CV-SANS experiments. It was observed that the oil exchange rate was significantly accelerated when sonicating at high acoustic pressures, where violent cavitation events can induce droplet coalescence and breakup. No significant oil exchange occurred at acoustic pressures below the cavitation threshold. It was also observed that the oil exchange kinetics was deterred when emulsions were stabilized by surfactants. In addition, oil exchange rates varied non-linearly with the concentration of surfactants present and exchange was slowest when the emulsions were stabilized by an intermediate concentration of surfactant. It is hypothesized that emulsion size, electrostatic repulsion and Gibbs elasticity of the oil-water interface play significant roles in the observed trends. The observed trends in oil exchange rates versus surfactant concentration coincides well with theoretical models for the fluctuation of the elasticity of the interface. Acoustically induced oil exchange was most inefficient when the interfacial elasticity was at its maximum value.

Optical coherence elastography (OCE) uses optical coherence tomography (OCT) to detect local tissue displacement following an applied force, enabling the quantification of biomechanical properties within a region of interest. OCE can potentially identify pathologies opaque to classical OCT using local maps of tissue mechanical properties (namely Young’s modulus), which can vary greatly between healthy and disease states. Detecting spatial heterogeneities in a mechanical structure can help monitor and treat disease progression in numerous fields, including ophthalmology,^{1 – 5} dermatology,⁶ cardiology,⁷ gastroenterology,⁸ and oncology.^{9 – 11} While it is a general belief that dynamic OCE is capable of high-resolution (on the order of the imaging modality, OCT) mapping of tissue mechanical properties, the spatial resolution in wave-based optical elastography has not yet been characterized.

We describe surface wave propagation in soft elastic media at speeds exceeding the bulk shear wave speed. By linking these waves to the elastodynamic Green's function, we derive a simple relationship to quantify the elasticity of a soft medium from the speed of this supershear evanescent wave (SEW). We experimentally probe SEW propagation in tissue-mimicking phantoms, human cornea ex vivo, and skin in vivo using a high-speed optical coherence elastography system. Measurements confirm the predicted relationship between SEW and bulk shear wave speeds, agreeing well with both theoretical and numerical models. These results suggest that SEW measurements may be a robust method to quantify elasticity in soft media, particularly in complex, bounded materials where dispersive Rayleigh-Lamb modes complicate measurements.

Mapping tissue mechanical properties, or elastography, has become an important medical imaging modality. There is a large body of work using different imaging systems, such as MRI and ultrasound, to track internal displacements and strains resulting from either external or internal mechanical loads to infer tissue mechanical properties.^{1 – 3}

Phase-change contrast agents are rapidly developing as an alternative to microbubbles for ultrasound imaging and therapy. These agents are synthesized and delivered as liquid droplets and vaporized locally to produce image contrast. They can be used like conventional microbubbles but with the added benefit of reduced size and improved stability. Droplet-based agents can be synthesized with diameters on the order of 100 nm, making them an ideal candidate for extravascular imaging or therapy. However, their synthesis requires low boiling point perfluorocarbons (PFCs) to achieve activation (i.e., vaporization) thresholds within FDA approved limits. Minimizing spontaneous vaporization while producing liquid droplets using conventional methods with low boiling point PFCs can be challenging. In this study, a new method to produce PFC nanodroplets using spontaneous nucleation is demonstrated using PFCs with boiling points ranging from −37 to 56 °C. Sometimes referred to as the ouzo method, the process relies on saturating a cosolvent with the PFC before adding a poor solvent to reduce solvent quality, forcing droplets to spontaneously nucleate. This approach can produce droplets ranging from under 100 nm to over 1 μm in diameter. Ternary plots showing solvent and PFC concentrations leading to droplet nucleation are presented. Additionally, acoustic activation thresholds and size distributions with varying PFC and solvent conditions are measured and discussed. Finally, ultrasound contrast imaging is demonstrated using ouzo droplets in an animal model.

Ultrasonic devices are common tools in laboratory and industrial settings to produce cavitation events for cleaning, emulsification, cell lysis and other materials applications. Effects of sonication at the macroscopic scale can be visible while effects at the molecular and nano-scales are not easily probed and, therefore, not fully understood. We present a new small angle scattering sample environment designed specifically to study structural changes occurring in various types of dispersions at the nano-scale due to ultrasonic acoustic waves. The sample environment features two face-to-face high-intensity focused ultrasound transducers coaxially aligned and normal to the neutron/X-ray beam propagation direction. A third broadband transducer is fixed beneath the scattering volume to acoustically monitor for cavitation events. By correlating acoustic data to scattering data, measured structural changes can be correlated to changes in parameters such as frequency, acoustic pressure, or cavitation pressure threshold. Several example applications of colloidal systems effectively influenced by ultrasound fields are also presented to demonstrate the capabilities of the device and to motivate future work on in situ scattering analysis of ultrasound materials processing methods.

Ultrasound acoustic waves are demonstrated to assemble poly-3-hexylthiophene (P3HT) chains into nanofibers after they are fully dissolved in what are commonly considered to be ‘good’ solvents. In the absence of ultrasound, the polymer remains fully dissolved and does not self-assemble for weeks. UV-vis spectroscopy, ultra-small angle X-ray scattering (USAXS) and small angle neutron scattering (SANS) are used to characterize the induced assembly process and to quantify the fraction of polymer that forms nanofibers. It is determined that the solvent type, insonation time, and aging periods are all important factors affecting the structure and final concentration of fibers. The effect of changing polymer regio-regularity, alkyl chain length, and side chain to thiophene ratio are also explored. High intensity focused ultrasound (HIFU) fields of variable intensity are utilized to reveal the physical mechanisms leading to nanofiber formation, which is strongly correlated to cavitation events in the solvent. This in situ HIFU cell, which is designed for simultaneous scattering analysis, is also used to probe for structural changes occurring over multiple length scales using USAXS and SANS. The proposed acoustic assembly mechanism suggests that, even when dispersed in ‘good’ solvents such as bromobenzene, dichlorobenzene and chloroform, P3HT chains are still not in a thermodynamically stable state. Instead, they are stabilized by local energy barriers that slow down and effectively prevent crystallization. Ultrasound fields are found to provide enough mechanical energy to overcome these barriers, triggering the formation of small crystalline nuclei that subsequently seed the growth of larger nanofibers.