Segmental biomechanics of the normal and glaucomatous human aqueous outflow pathway

A Biocompatible, Magnetic-Responsive Shape Memory Silicone Composite for Active Flow Controlling Valve

A magnetically responsive shape memory silicone composite is developed for fabricating an active valve in biomedical fluidic systems. The composite consists of magnetic neodymium-iron-boron (NdFeB) microparticles (MPs) and poly(glycerol-dodecanoate) (PGD) MPs in silicone. PGD has tunable transition temperatures (Tg) of 42-50 °C, empowering shape programmability to the composite, while the NdFeB MPs make the composite responsive to a magnetic field gradient. The composite can be reprogrammed into a compact form for ease delivery and subsequently recovered to its original shape stimulated by external heat that is generated by an oscillation magnetic field (Bh) applied to the NdFeB MPs. Because of the dramatically reduced stiffness above Tg, the valve shape can be actuated to a different one by exerted torques induced by a simultaneously applied actuation magnetic field (Ba). By tuning the magnetic density and direction of Ba, the shape changing extent, e.g., bending angles, which determine the fluid flow resistance, can be modulated. This works provides a proof-of-concept for a programmable, remotely controlled valve for fluid regulation. The adaptability, biocompatibility, and capability for minimally invasive delivery highlight the potential of the valve for applications in drug delivery, embolization, and flow management in blood vessels and eyes.

Segmental Uveoscleral Outflow and its Relationship With Trabecular Outflow in Monkey Eyes

Purpose

Segmental trabecular outflow has been observed in various species, and we recently reported segmental uveoscleral outflow in mouse eyes. However, whether this pattern exists in other species remains unclear. This study aimed to investigate segmental uveoscleral outflow and its correlation with trabecular outflow in monkey eyes.

Methods

Five healthy eyes of aged cynomolgus macaques were examined. After anesthesia, a fixed volume of tracer was injected into the anterior chamber and allowed to diffuse for 45 minutes before fixation. The eyes were dissected into 12 radial segments, and images were captured using a confocal microscope. Segments were randomly selected for histological study. Tracer intensity and stromal thickness were measured.

Results

Four distinct tracer patterns were observed: (1) low flow in both pathways, (2) high flow (HF) in both, (3) HF in trabecular outflow, and (4) HF in uveoscleral outflow. As trabecular outflow contributed 75% of the total outflow, the "HF in uveoscleral outflow" pattern was the least frequent. Segmental flow patterns were observed in both outflow pathways, including components along the uveoscleral outflow pathway: supraciliary and suprachoroidal spaces, spaces between muscle bundles, and ciliary stroma. A positive correlation was found between tracer intensity along the uveoscleral outflow pathway and stromal thickness.

Conclusions

Uveoscleral outflow is segmental and uncorrelated with trabecular outflow in monkey eyes. It primarily occurs in the ciliary stroma, where it positively correlates with stromal thickness. Future studies in human eyes may inform the optimal placement of drainage devices and drug delivery systems targeting the uveoscleral outflow pathway.

High-resolution modeling of aqueous humor dynamics in the conventional outflow pathway of a normal human donor eye

BACKGROUND AND OBJECTIVE

The conventional aqueous outflow pathway, which includes the trabecular meshwork (TM), juxtacanalicular tissue (JCT), and inner wall endothelium of Schlemm's canal (SC) and its basement membrane, plays a significant role in regulating intraocular pressure (IOP) by controlling aqueous humor outflow resistance. Despite its significance, the biomechanical and hydrodynamic properties of this region remain inadequately understood. Fluid-structure interaction (FSI) and computational fluid dynamics (CFD) modeling using high-resolution microstructural images of the outflow pathway provides a comprehensive method to estimate these properties under varying conditions, offering valuable understandings beyond the capabilities of current imaging techniques.

METHODS

In this study, we utilized high-resolution 3D serial block-face scanning electron microscopy (SBF-SEM) to image the TM/JCT/SC complex of a normal human donor eye perfusion-fixed at an IOP of 7 mm Hg. We developed a detailed 3D finite element (FE) model of the pathway using SBF-SEM images to simulate the biomechanical environment. The model included the TM/JCT/SC complex (structure) with interspersed aqueous humor (fluid). We employed a 3D, inverse FE algorithm to calculate the unloaded geometry of the TM/JCT/SC complex and utilized FSI to simulate the pressurization of the complex from 0 to 15 mm Hg.

RESULTS

Our simulations revealed that the resultant velocity distribution in the aqueous humor across the TM/JCT/SC complex is heterogeneous. The JCT and its deepest regions, specifically the basement membrane of the inner wall of SC, exhibited a volumetric average velocity of ∼0.011 mm/s, which is higher than the TM regions, with a volumetric average velocity of ∼0.007 mm/s. Shear stress analysis indicated that the maximum shear stress, based on our FE code criteria, was 0.5 Pa starting from 10 µm into the TM from the anterior chamber and increased to 0.95 Pa in the JCT and its adjacent SC inner wall basement membrane. Also, the tensile stress and strain distributions showed significant variations, with the first principal stress reaching up to 57 Pa (compressive volumetric average) and the first principal strain reaching up to 3.5 % in areas of high mechanical loading. The resultant stresses, strains, and velocities exhibited relatively similar average values across the TM, JCT, and SC regions, primarily due to the uniform elastic moduli assigned to these components. Our computational fluid dynamics (CFD) analysis revealed that while the velocity of the aqueous humor remained consistent, the maximum shear stress was reduced by a factor of thirty.

CONCLUSION

The uneven distribution of shear stress and velocity within the TM/JCT/SC complex highlights the complex biomechanical environment that regulates aqueous humor outflow.

Impact of aging on anterior segment morphology and aqueous humor dynamics in human Eyes: Advanced imaging and computational techniques

Objective

Aging results in significant structural and functional changes in the anterior segment of the eye, influencing intraocular pressure (IOP) and overall ocular health. Although aging is a well-established risk factor for primary open-angle glaucoma, a leading cause of irreversible blindness, the specific mechanisms through which aging drives morphological changes in anterior segment tissues and affects aqueous humor dynamics remain incompletely understood.

Methods

In this study, we employed cutting-edge light sheet fluorescence microscopy (LSFM) to capture high-resolution, volumetric images of cleared human donor eyes' anterior segment tissues. This advanced imaging enabled a comprehensive morphological analysis of key parameters, including central and peripheral corneal thickness (CCT and PCT), iris thickness, anterior chamber area (ACA), and ciliary body area (CBA). By integrating these morphological parameters with computational fluid dynamics (CFD) models, we analyzed aqueous humor dynamics across n = 6 female human donor eyes, spanning a wide age range of 5 to 94 years (all of Caucasian descent).

Results

The CCT and PCT demonstrated thinning with age, accompanied by a reduction in ACA. In contrast, the CBA remained relatively stable across all age groups. Computational fluid dynamics analysis showed a decline in aqueous humor velocity and wall shear stress, with younger eyes exhibiting higher velocities and shear stress, compared to older eyes.

Conclusion

These findings emphasize the value of integrating LSFM and CFD approaches to provide a detailed understanding of how aging impacts the anterior segment and its fluid dynamics. This study contributes to the understanding of age-related ocular changes, highlighting the importance of considering these changes in the diagnosis and management of age-related eye diseases.

Effects of 0.024% latanoprostene bunod on intraocular pressure and pupil diameter in normal cats and cats with congenital glaucoma

OBJECTIVE

To evaluate the effects of latanoprostene bunod on intraocular pressure (IOP) and pupil diameter (PD) in normal cats and cats with feline congenital glaucoma (FCG).

ANIMALS STUDIED

Five normal and 5 FCG cats.

PROCEDURES

This masked, controlled crossover study comprised a 1-day Pre-treatment phase followed by two 10-day Treatment phases, each followed by a 10-day Recovery phase. During treatment, all cats received twice daily 0.005% latanoprost (LAT) or 0.024% latanoprostene bunod (LBN) in a randomized eye. Following Recovery, the same eye was treated with the opposite drug. Contralateral eyes served as saline-treated controls. Intraocular pressure and PD measurements were performed three times daily during all study phases. Data were analyzed via constrained longitudinal data analysis models.

RESULTS

Neither drug significantly reduced IOP in normal cats. In FCG cats, statistically significant reductions in mean (95% CI) IOP were observed relative to controls 4 h after LAT and LBN treatment (-5.5 mmHg [-8.4, -2.5], p < .001, -7.2 mmHg [-10.2, -4.3], p < .001, respectively). These differences represented 28.4% and 37.9% IOP reductions, respectively. Mean IOP reduction after 4 h was significantly greater with LBN treatment compared to LAT (-1.8 mmHg [-3.2, -0.4], p = .012). However, these IOP reductions were not considered clinically significant. Both drugs similarly reduced PD in normal and FCG cats.

CONCLUSIONS

Transient IOP reduction was observed after topical administration of LAT and LBN in FCG cats; and mean IOP difference was statistically significantly greater in LBN-treated eyes. However, the apparent enhanced hypotensive effect of LBN is not clinically significant.

Comparative analysis of traction forces in normal and glaucomatous trabecular meshwork cells within a 3D, active fluid-structure interaction culture environment

This study presents a 3D in vitro cell culture model, meticulously 3D printed to replicate the conventional aqueous outflow pathway anatomical structure, facilitating the study of trabecular meshwork (TM) cellular responses under glaucomatous conditions. Glaucoma affects TM cell functionality, leading to extracellular matrix (ECM) stiffening, enhanced cell-ECM adhesion, and obstructed aqueous humor outflow. Our model, reconstructed from polyacrylamide gel with elastic moduli of 1.5 and 21.7 kPa, is based on serial block-face scanning electron microscopy images of the outflow pathway. It allows for quantifying 3D, depth-dependent, dynamic traction forces exerted by both normal and glaucomatous TM cells within an active fluid-structure interaction (FSI) environment. In our experimental design, we designed two scenarios: a control group with TM cells observed over 20 hours without flow (static setting), focusing on intrinsic cellular contractile forces, and a second scenario incorporating active FSI to evaluate its impact on traction forces (dynamic setting). Our observations revealed that active FSI results in higher traction forces (normal: 1.83-fold and glaucoma: 2.24-fold) and shear strains (normal: 1.81-fold and glaucoma: 2.41-fold), with stiffer substrates amplifying this effect. Glaucomatous cells consistently exhibited larger forces than normal cells. Increasing gel stiffness led to enhanced stress fiber formation in TM cells, particularly in glaucomatous cells. Exposure to active FSI dramatically altered actin organization in both normal and glaucomatous TM cells, particularly affecting cortical actin stress fiber arrangement. This model while preliminary offers a new method in understanding TM cell biomechanics and ECM stiffening in glaucoma, highlighting the importance of FSI in these processes. STATEMENT OF SIGNIFICANCE: This pioneering project presents an advanced 3D in vitro model, meticulously replicating the human trabecular meshwork's anatomy for glaucoma research. It enables precise quantification of cellular forces in a dynamic fluid-structure interaction, a leap forward from existing 2D models. This advancement promises significant insights into trabecular meshwork cell biomechanics and the stiffening of the extracellular matrix in glaucoma, offering potential pathways for innovative treatments. This research is positioned at the forefront of ocular disease study, with implications that extend to broader biomedical applications.