Creep compliance rheology with a probe-like cylindrical geometry.

BACKGROUND: Rheology experiments have been performed on the vitreous humor, a soft gel that rests inside of the eye, to study its viscoelastic behavior and underlying macromolecular structure. A significant challenge for experimentalists is preserving the macromolecular structure when removing vitreous from in vivo conditions. OBJECTIVE: We have developed a novel probe-like rheometer geometry that allows us to perform shear rheology experiments on the vitreous humor in situ. The aim of this study is to assess the feasibility of the probe geometry. METHODS: Creep compliance responses of silicone oils, Xanthan gum solutions, and bovine and porcine vitreous humor were measured using the probe geometry and compared to measurements performed with standard geometries. RESULTS: Viscosities calculated from the creep responses of silicone oils closely match between the probe and standard geometry. Viscosities and creep compliance values of Xanthan gum measurements achieve order of magnitude agreement between the probe and standard geometry. Significant differences are detected with the probe between bovine and porcine vitreous (p<0.001). CONCLUSIONS: These results suggest the probe may feasibly measure viscosities of Newtonian fluids, and correctly detect differences in the creep response of complex fluids with varying viscoelastic behaviors.

Comparison of occlusion break responses and vacuum rise times of phacoemulsification systems.

BACKGROUND: Occlusion break surge during phacoemulsification cataract surgery can lead to potential surgical complications. The purpose of this study was to quantify occlusion break surge and vacuum rise time of current phacoemulsification systems used in cataract surgery. METHODS: Occlusion break surge at vacuum pressures between 200 and 600 mmHg was assessed with the Infiniti® Vision System, the WhiteStar Signature® Phacoemulsification System, and the Centurion® Vision System using gravity-fed fluidics. Centurion Active FluidicsTM were also tested at multiple intraoperative pressure target settings. Vacuum rise time was evaluated for Infiniti, WhiteStar Signature, Centurion, and Stellaris® Vision Enhancement systems. Rise time to vacuum limits of 400 and 600 mmHg was assessed at flow rates of 30 and 60 cc/minute. Occlusion break surge was analyzed by 2-way analysis of variance. RESULTS: The Centurion system exhibited substantially less occlusion break surge than the other systems tested. Surge area with Centurion Active Fluidics was similar to gravity fluidics at an equivalent bottle height. At all Centurion Active Fluidics intraoperative pressure target settings tested, surge was smaller than with Infiniti and WhiteStar Signature. Infiniti had the fastest vacuum rise time and Stellaris had the slowest. No system tested reached the 600-mmHg vacuum limit. CONCLUSIONS: In this laboratory study, Centurion had the least occlusion break surge and similar vacuum rise times compared with the other systems tested. Reducing occlusion break surge may increase safety of phacoemulsification cataract surgery.

Effect of cut rates on fluidic behavior of chopped vitreous.

PURPOSE: To analyze the viscoelastic properties of the chopped vitreous at different cut rates to better understand complex fluidic behavior of chopped vitreous during vitrectomy. METHODS: Twenty- and 25-gauge cutters were used to cut 107 porcine eyes at different cut rates of 500, 1000, 1500, 2000, and 2500 cuts per minute with a fixed vacuum pressure of 500 mmHg. Each sample was immediately tested using a shear rheometer to obtain its rheologic properties. RESULTS: Chopped vitreous demonstrated significantly lower viscosity (0.039 ± 0.01 Pa·s) than intact vitreous (908.1 ± 210.8 Pa·s). However, cut rate did not have any significant impact on viscosity. In addition, chopped vitreous presented elastic behavior. It was shown that the compliance, the inverse of stiffness, of chopped vitreous is much higher than that of intact vitreous (1.83 ± 0.31 Pa for intact vitreous and 85.3 ± 14.4 Pa for chopped vitreous) and varies in a nonlinear fashion when cut at different cut rates. CONCLUSION: Cut rate affects the rheologic properties of the chopped vitreous and, therefore, its flow inside the vitrectomy system. It is essential to account for both viscosity and elasticity of chopped vitreous to understand flow behavior during vitrectomy.

Rheology of the vitreous gel: effects of macromolecule organization on the viscoelastic properties.

The macromolecular organization of vitreous gel is responsible for its viscoelastic properties. Knowledge of this correlation enables us to relate the physical properties of vitreous to its pathology, as well as optimize surgical procedures such as vitrectomy. Herein, we studied the rheological properties (e.g. dynamic deformation, shear stress-strain flow, and creep compliance) of porcine vitreous humor using a stressed-control shear rheometer. All experiments were performed in a closed environment with the temperature set to that of the human body (i.e. 37°C) to mimic in-vivo conditions. We modeled the creep deformation using the two-element retardation spectrum model. By associating each element of the model to an individual biopolymeric system in the vitreous gel, a distinct response to the applied stress was observed from each component. We hypothesized that the first viscoelastic response with the short time scale (~1 s) is associated with the collagen structure, while the second viscoelastic response with longer time scale (~100 s) is related to the microfibrilis and hyaluronan network. Consequently, we were able to differentiate the role of each main component from the overall viscoelastic properties.

Incorporation of active DNA/cationic polymer polyplexes into hydrogel scaffolds.

The effective and sustained delivery of DNA and siRNAs locally would increase the applicability of gene therapy in tissue regeneration and cancer therapy. One promising approach is to use hydrogel scaffolds to encapsulate and deliver nucleotides in the form of nanoparticles to the disease sites. However, this approach is currently limited by the inability to load concentrated and active gene delivery nanoparticles into the hydrogels due to the severe nanoparticle aggregation during the loading process. Here, we present a process to load concentrated and un-aggregated non-viral gene delivery nanoparticles, using DNA/polyethylene imine (PEI) polyplexes as an example, into neutral polyethylene glycol (PEG), negatively charged hyaluronic acid (HA) and protein fibrin hydrogels crosslinked through various chemistries. The encapsulated polyplexes are highly active both in vitro and in vivo. We believe this process will significantly advance the applications of hydrogel scaffold mediated non-viral gene delivery in tissue regeneration and cancer therapy.