Publications

Abstract The pia arachnoid complex (PAC) is a cerebrospinal fluid-filled tissue conglomerate that surrounds the brain and spinal cord. Pia mater adheres directly to the surface of the brain while the arachnoid mater adheres to the inferior side of the dura mater. Collagen fibers, known as subarachnoid trabeculae (SAT) fibers, and microvascular structure lie intermediately to the pia and arachnoid meninges. Due to its structural role, alterations to the biomechanical properties of the PAC may change surface stress loading in traumatic brain injury (TBI) caused by sub-concussive hits. The aim of this study was to quantify the mechanical and morphological properties of ovine PAC. Ovine brain samples (n = 10) were removed from the skull and tissue was harvested within 30 minutes post-mortem. To access the PAC, ovine skulls were split medially from the occipital region down the nasal bone on the superior and inferior aspects of the skull. A template was used to remove arachnoid samples from the left and right sides of the frontal occipital regions of the brain. 10 ex-vivo samples were tested with uniaxial tension at 2 mm/s, average strain rate of 0.59 s − 1 , until failure at < 5 hours post extraction. The force and displacement data were acquired at 100 Hz using LabVIEW. PAC tissue collagen fiber microstructure was characterized using second-harmonic generation (SHG) imaging on a subset of n = 4 stained tissue samples. Samples were stained with DAPI and anti-von Willebrand Factor to visualize cell nuclei and epithelial cells, respectively. The Mooney-Rivlin model for average stress-strain curve fit was used to model PAC material properties. The elastic modulus, ultimate stress, and ultimate strain were found to be 7.7 ± 3.0 MPa, 2.7 ± 0.76 MPa, and 0.60 ± 0.13, respectively. No statistical significance was found across brain dissection locations in terms of biomechanical properties. SHG images were post-processed to obtain average SAT fiber intersection density, concentration, porosity, tortuosity, segment length, orientation, radial counts, and diameter as 0.23%, 26.14%, 73.86%, 1.07 ± 0.28, 17.33 ± 15.25 µm, 84.66 ± 49.18º, 8.15%, 3.46 ± 1.62 µm, respectively. For the sizes, strain, and strain rates tested, our results suggest that ovine PAC mechanical behavior is isotropic, and that the Mooney-Rivlin model is an appropriate curve-fitting constitutive equation for obtaining material parameters of PAC tissues.

Thoracic aortic stent grafts (TASG) are commonly used to repair aortic anomalies or diseases in a minimally-invasive fashion. One complication of TASG is collapse, whereby blood is unable to completely flow through the graft as intended. Most TASG collapses occur in the setting of endovascular repair of traumatic thoracic aortic transection, where a typically smaller diameter aorta of pediatric patients is repaired with a relatively larger diameter endograft designed to treat aneurysmal disease [1]. It is hypothesized that the poor apposition of the leading edge of the proximal stent graft to the lesser curvature of the aortic arch can result increased hemodynamic force at the leading side of the graft can reach sufficient magnitude to cause collapse and gradual occlusion of the tubular graft. The incidence rates of stent graft collapse in endovascular aortic transection repairs have been reported to range from 0.03% to 10% [2].

Magnetically directable materials containing iron oxide nanoparticles (IONPs) have been utilized for a variety of medical applications, including localized drug delivery. Regenerated silk fibroin (RSF) has also been used in numerous regenerative medicine and drug delivery applications, given its biocompatibility and tunable properties. In this work, we explored the hypothesis that chemically conjugating IONPs to RSF to anchor the IONPs to silk microparticles would provide better magnetic guidance than nonconjugated IONPs untethered to silk microparticles. IONPs were fabricated using a coprecipitation method and conjugated with glutathione (GSH) prior to mixing with RSF. IONPs incorporated into RSF were mixed with potassium phosphate buffer to fabricate microparticles. IONPs with and without GSH were characterized for particle size, shape, morphology, GSH conjugation efficiency, and composition. Silk iron microparticles (SIMPs) were also characterized for particle size, shape, and composition and tested for stability, degradation properties, magnetic movability, and cytotoxicity. IONPs demonstrated a uniform size distribution and spherical morphology. Conjugation of IONPs with GSH was verified through changes in the X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FTIR) spectra. IONPs and RSF were able to be chemically conjugated and fabricated into SIMPs, which demonstrated a spherical and porous morphology. FTIR revealed an increased β-sheet content in SIMPs, suggesting that the IONPs may be inducing conformational changes in the silk fibroin. SIMPs showed stability up to 4 weeks in ultrapure water and rapid enzymatic degradation within 24 h. SIMPs were able to be moved magnetically through solution and through a hydrogel and were not cytotoxic.

The probability of developing an abdominal aortic aneurysm (AAA) is greatly increased in individuals with a family history of this disease. The purpose of this work was to determine whether there are fundamental anomalies in the AAA wall integrity in these patients by comparing their AAA wall failure strength with that from patients without a known family history of the disease. A standard t-test was used to compare the group means. The mixed model multiple linear regression technique was also used to test if there is a difference in wall strength after adjusting age, gender, and AAA size. Both tests showed that there is a significant difference between the failure strength of the wall in these two groups. We conclude that there might be fundamental differences in AAA wall integrity between patients with and without a family history of AAA.

Type A aortic dissection (AoD) of an ascending thoracic aortic aneurysm (ATAA) is a life-threatening cardiovascular emergency with a high potential for death. Despite improved surgical techniques, the morbidity risk for emergent surgery remains 24% worldwide according to data from the International Registry of Acute Aortic Dissection [1].

We have shown that the intraluminal thrombus (ILT) commonly found within abdominal aortic aneurysm (AAA) significantly attenuates diffusion of oxygen to the aortic wall as its thickness increases. We hypothesize that, as a result, the strength of the AAA wall is inversely related to ILT thickness. We tested the strength of 6 pairs of freshly excised AAA wall specimens from 6 different patients. Uniaxial tensile testing revealed that AAA wall adjacent to thin ILT or no ILT is significantly stronger than that adjacent to thick ILT (p=0.03).

The manual seeding of cells onto a biodegradable scaffold by pipetting is an effective method of cell seeding. However, the widespread use and ultimate clinical utility of this technique is limited by operator variability. This study was conducted to evaluate an operator-independent vacuum-seeding method for use in an upcoming clinical trial. Using bone marrow-derived mononuclear cells, we achieved seeding comparable to manually seeded scaffolds in terms of cellular attachment, distribution, and viability in vacuum-seeded grafts at vacuum pressures of -25 to -50 mmHg. In conclusion, we describe an operator-independent seeding method for use in the clinical setting.

Interleukin 33 (IL-33), an inflammatory and mechanically responsive cytokine, is an important component of a TLR4-dependent innate immune process in mucosal epithelium. Although TLR4 also plays a role in sensing biomechanical stretch, a pathway of stretch-induced TLR4-dependent IL-33 biosynthesis has not been revealed. In the current study, we show that short term (6 h) cyclic stretch (CS) of cultured murine respiratory epithelial cells (MLE-12) increased intracellular IL-33 expression in a TLR4 dependent fashion. There was no detectable IL-33 in conditioned media in this interval. CS, however, increased release of the notable alarmin, HMGB1, and a neutralizing antibody (2G7) to HMGB1 completely abolished the CS mediated increase in IL-33. rHMGB1 increased IL-33 synthesis and this was partially abrogated by silencing TLR4 suggesting additional receptors for HMGB1 are involved in its regulation of IL-33. Collectively, these data reveal a HMGB1/TLR4/IL-33 pathway in the response of respiratory epithelium to mechanical stretch.