Rheology of the vitreous gel: Effects of macromolecule organization on the viscoelastic properties

Roadmap for Clinical Translation of Mobile Microrobotics

Medical microrobotics is an emerging field to revolutionize clinical applications in diagnostics and therapeutics of various diseases. On the other hand, the mobile microrobotics field has important obstacles to pass before clinical translation. This article focuses on these challenges and provides a roadmap of medical microrobots to enable their clinical use. From the concept of a "magic bullet" to the physicochemical interactions of microrobots in complex biological environments in medical applications, there are several translational steps to consider. Clinical translation of mobile microrobots is only possible with a close collaboration between clinical experts and microrobotics researchers to address the technical challenges in microfabrication, safety, and imaging. The clinical application potential can be materialized by designing microrobots that can solve the current main challenges, such as actuation limitations, material stability, and imaging constraints. The strengths and weaknesses of the current progress in the microrobotics field are discussed and a roadmap for their clinical applications in the near future is outlined.

A new look at an old problem: 3D modeling of accommodation reveals how age-related biomechanical changes contribute to dysfunction in presbyopia

Presbyopia is an age-related ocular disorder where accommodative ability declines so that an individual's focusing range is insufficient to provide visual clarity for near and distance vision tasks without corrective measures. With age, the eye exhibits changes in biomechanical properties of many components involved in accommodation, including the lens, sclera, and ciliary muscle. Changes occur at different rates, affecting accommodative biomechanics differently, but individual contributions to presbyopia are unknown. We used a finite element model (FEM) of the accommodative mechanism to simulate age-related changes in lens stiffness, scleral stiffness, and ciliary contraction to predict differences in accommodative function. The FEM predicts how ciliary muscle action leads to lens displacement by initializing a tensioned unaccommodated lens (Phase 0) then simulating ciliary muscle contraction in accommodation (Phase 1). Model inputs were calibrated to replicate experimentally measured lens and ciliary muscle in 30-year-old eyes. Predictions of accommodative lens deformation were verified with additional imaging studies. Model variations were created with altered lens component stiffnesses, scleral stiffness, or ciliary muscle section activations, representing fifteen-year incremental age-related changes. Model variations predict significant changes in accommodative function with age-related biomechanical property changes. Lens changes only significantly altered lens thickening with advanced age (46% decrease at 75 years old) while sclera changes produced progressive dysfunction with increasing age (23%, 36%, 49% decrease at 45, 60, and 75 years old). Ciliary muscle changes effected lens position modulation. Model predictions identified potential mechanisms of presbyopia that likely work in combination to reduce accommodative function and could indicate effectiveness of treatment strategies and their dependency on patient age or relative ocular mechanical properties.

An Inverse Method to Determine Mechanical Parameters of Porcine Vitreous Bodies Based on the Indentation Test

The vitreous body keeps the lens and retina in place and protects these tissues from physical insults. Existing studies have reported that the mechanical properties of vitreous body varied after liquefaction, suggesting mechanical properties could be effective parameters to identify vitreous liquefaction process. Thus, in this work, we aimed to propose a method to determine the mechanical properties of vitreous bodies. Fresh porcine eyes were divided into three groups, including the untreated group, the 24 h liquefaction group and the 48 h liquefaction group, which was injected collagenase and then kept for 24 h or 48 h. The indentation tests were carried out on the vitreous body in its natural location while the posterior segment of the eye was fixed in the container. A finite element model of a specimen undertaking indentation was constructed to simulate the indentation test with surface tension of vitreous body considered. Using the inverse method, the mechanical parameters of the vitreous body and the surface tension coefficient were determined. For the same parameter, values were highest in the untreated group, followed by the 24 h liquefaction group and the lowest in the 48 h liquefaction group. For C₁₀ in the neo-Hookean model, the significant differences were found between the untreated group and liquefaction groups. This work quantified vitreous body mechanical properties successfully using inverse method, which provides a new method for identifying vitreous liquefactions related studies.

The Roles of Vitreous Biomechanics in Ocular Disease, Biomolecule Transport, and Pharmacokinetics

PURPOSE

The biomechanical properties of the vitreous humor and replication of these properties to develop substitutes for the vitreous humor have rapidly become topics of interest over the last two decades. In particular, the behavior of the vitreous humor as a viscoelastic tissue has been investigated to identify its role in a variety of processes related to biotransport, aging, and age-related pathologies of the vitreoretinal interface.

METHODS

A thorough search and review of peer-reviewed publications discussing the biomechanical properties of the vitreous humor in both human and animal specimens was conducted. Findings on the effects of biomechanics on vitreoretinal pathologies and vitreous biotransport were analyzed and discussed.

RESULTS

The pig and rabbit vitreous have been found to be most mechanically similar to the human vitreous. Age-related liquefaction of the vitreous creates two mechanically unique phases, with an overall effect of softening the vitreous. However, the techniques used to acquire this mechanical data are limited by the in vitro testing methods used, and the vitreous humor has been hypothesized to behave differently in vivo due in part to its swelling properties. The impact of liquefaction and subsequent detachment of the vitreous humor from the posterior retinal surface is implicated in a variety of tractional pathologies of the retina and macula. Liquefaction also causes significant changes in the biotransport properties of the eye, allowing for significantly faster movement of molecules compared to the healthy vitreous. Recent developments in computational and ex vivo models of the vitreous humor have helped with understanding its behavior and developing materials capable of replacing it.

CONCLUSIONS

A better understanding of the biomechanical properties of the vitreous humor and how these relate to its structure will potentially aid in improving clinical metrics for vitreous liquefaction, design of biomimetic vitreous substitutes, and predicting pharmacokinetics for intravitreal drug delivery.

Numerical simulation of viscoelastic fluid–structure interaction benchmarks and their application to the human eye

We present a numerical solution method for time-dependent viscoelastic fluid–structure interaction employing the arbitrary Lagrangian Eulerian framework. The derived monolithic variational formulation is discretized in time using the shifted Crank–Nicolson scheme and in space using the finite element method. For the linearisation we employ Newton’s method with exact Jacobians. The viscoelastic fluid is modelled either using the Oldroyd-B or a Burgers-type model. The elastic structures are non-linear hyperelastic materials. We validate the implementation on benchmark problems and numerically analyse the convergence for global mesh refinement and adaptive mesh refinement using the dual-weighted residual method. Furthermore we numerically analyse the influence of the viscoelasticity of the fluid on typical goal functionals like the drag, the lift and the displacement. The derived numerical solution method is applied to ophthalmology where we analyse the interaction of the viscoelastic vitreous with its surrounding elastic structures.

Drop-of-sample rheometry of biological fluids by noncontact acoustic tweezing spectroscopy

Knowledge of rheological properties, such as viscosity and elasticity, is necessary for efficient material processing and transportation as well as biological analysis. Existing rheometers operate with large sample volume and induce sample contact with container or device walls, which are inadequate for rheological analysis of sensitive fluids limited in availability. In this work, we introduce acoustic tweezing spectroscopy (ATS), a novel noncontact rheological technique that operates with a single 4-6 μl drop of fluid sample. In ATS, a sample drop is acoustically levitated and then exposed to a modulated acoustic signal to induce its forced oscillation. The time-dependent sample viscosity and elasticity are measured from the resulting drop response. The ATS measurements of polymeric solutions (dextran, xanthan gum, gelatin) agree well with previously reported data. The ATS predicts that the shear viscosity of blood plasma increases from 1.5 cP at 1.5 min of coagulation onset to 3.35 cP at 9 min, while its shear elastic modulus grows from a negligible value to 10.7 Pa between 3.5 min and 6.5 min. Coagulation increases whole blood viscosity from 5.4 cP to 20.7 cP and elasticity from 0.1 Pa to 19.2 Pa at 15 min. In summary, ATS provides the opportunity for sensitive small-volume rheological analysis in biomedical research and medical, pharmaceutical, and chemical industries.

Dynamic Pressure Measurements During Vitrectomy in a Model of the Eye

Purpose

To accurately evaluate pressure changes during vitrectomy in a rigid model of the vitreous chamber and to test the efficiency of the EVA phacovitrectomy system (Dutch Ophthalmic Research Center) in terms of compensation of intraocular pressure variations.

Methods

We tested 23-, 25-, and 27-gauge double-blade vitreous cutters in both vented global pressure control and automatic infusion compensation (AIC) modes in a vitreous chamber model, mimicking the real surgical procedure. Balanced salt solution and artificial vitreous, similar to the real vitreous body, were used. We tested both standard-flow (SF) and high-flow (HF) infusion systems, varying the infusion pressure between 20 and 40 mm Hg. In each experiment, flow rate was also measured.

Results

Pressure drop was rapidly and efficiently compensated when 23- and 25-gauge cutters were used in AIC mode, with infusion pressures ranging between 30 and 55 mm Hg. The 27-gauge cutter was less efficient in compensating pressure variations. Pressure fluctuations related to the high-frequency motion of the cutter blade were small compared to the overall pressure variations. The use of the HF infusion system resulted in larger flow rates and lower pressure changes compared to the SF infusion system.

Conclusions

Despite the rigid material of the model, the present pressure measurements are in line with previous studies performed on porcine eye. The use of AIC mode compensates intraoperative pressure drops efficiently, with both 23- and 25-gauge cutters. The HF infusion system is more efficient than the SF infusion system.

Translational Relevance

The AIC infusion mode efficiently compensates intraoperative pressure drops, in both 23- and 25-gauge experimental vitrectomy. The HF infusion system resulted in larger flow rate and lower pressure changes.

Determining vitreous viscosity using fluorescence recovery after photobleaching

PURPOSE

Vitreous humor is a complex biofluid whose composition determines its structure and function. Vitreous viscosity will affect the delivery, distribution, and half-life of intraocular drugs, and key physiological molecules. The central pig vitreous is thought to closely match human vitreous viscosity. Diffusion is inversely related to viscosity, and diffusion is of fundamental importance for all biochemical reactions. Fluorescence Recovery After Photobleaching (FRAP) may provide a novel means of measuring intravitreal diffusion that could be applied to drugs and physiological macromolecules. It would also provide information about vitreous viscosity, which is relevant to drug elimination, and delivery.

METHODS

Vitreous viscosity and intravitreal macromolecular diffusion of fluorescently labelled macromolecules were investigated in porcine eyes using fluorescence recovery after photobleaching (FRAP). Fluorescein isothiocyanate conjugated (FITC) dextrans and ficolls of varying molecular weights (MWs), and FITC-bovine serum albumin (BSA) were employed using FRAP bleach areas of different diameters.

RESULTS

The mean (±standard deviation) viscosity of porcine vitreous using dextran, ficoll and BSA were 3.54 ± 1.40, 2.86 ± 1.13 and 4.54 ± 0.13 cP respectively, with an average of 3.65 ± 0.60 cP.

CONCLUSIONS

FRAP is a feasible and practical optical method to quantify the diffusion of macromolecules through vitreous.

Broadband-excitation-based mechanical spectroscopy of highly viscous tissue-mimicking phantoms

Standard rheometers assess mechanical properties of viscoelastic samples up to 100 Hz, which often hinders the assessment of the local-scale dynamics. We demonstrate that high-frequency analysis can be achieved by inducing broadband waves and monitoring their media-dependent propagation using optical coherence tomography. Here, we present a new broadband wave analysis based on two-dimensional Fourier transformation. We validated this method by comparing the mechanical parameters to monochromatic excitation and a standard oscillatory test data. Our method allows for high-frequency mechanical spectroscopy, which could be used to investigate the local-scale dynamics of different biological tissues and the influence of diseases on their microstructure.

Vitreous function and intervention of it with vitrectomy and other modalities

The vitreous body, the largest intraocular component, plays a key role in eye development, refraction, cell barrier function, oxygen metabolism and the pathogenesis of assorted diseases. Age, refraction and systemic diseases can cause vitreous metabolic abnormalities. With the continuous development of vitrectomy techniques and equipment, vitreous injections and vitrectomies have increased over the recent decades. However, the normal oxygen tension gradient in the vitreous helps to protect the lens and anterior chamber angle from oxidative stress damage, whereas the increased vitreous oxygen tension around lens and the trabecular meshwork after vitrectomy may lead to postoperative nuclear cataract and a high incidence of open angle glaucoma. As a conventional procedure, scleral buckling holds several advantages over vitrectomy in selected cases. This review raises concerns regarding the function of the vitreous, and encourages conducting vitreous interventions prudently.

A simple indentation technique for identifying localized liquefaction of the vitreous body

Identification of vitreous liquefaction (VL) is important for investigating some eye pathologies related to changes in the mechanical properties of the vitreous, such as posterior vitreous detachment or retinal detachment. However, because of the extremely high-water content, characterization of the structural and mechanical properties of the vitreous remains a challenge. In the current study, a simple indentation test was performed on the fresh vitreous, partially liquefied vitreous and water, using a mechanical testing machine with a high-precision load cell. Pressure loads on the indenter changing with time for different samples were obtained to characterize their mechanical properties. VL identification was achieved based on a comparison of relative differences in some typical mechanical parameters (e.g. the pressure at the moment of the insertion (P₀), the pressure at the steady mechanical state (P_∞) and the time when the pressure on the indenter declined to one half the initial value (t_1/2)) between 1) partially liquefied vitreous and fresh vitreous samples and 2) partially liquefied vitreous and water samples. The results indicated that P₀ of the partially liquefied vitreous was almost half that of fresh vitreous (p < 0.05) and approximately 1.5 times that of water (p < 0.05). t_1/2 of the partially liquefied vitreous was approximately 1.4 times higher than that of fresh vitreous (p < 0.05) and approximately 80% of that of water (p < 0.05). It is concluded that P₀ and t_1/2 can be used as indicators to identify localized liquefaction of the vitreous.

A bioinspired synthetic soft hydrogel for the treatment of dry eye

Natural soft hydrogels are unique elastic soft materials utilized by living organisms for protecting delicate tissues. Under a theoretical framework derived from the Blob model, we chemically crosslinked high molecular weight hyaluronic acid at a concentration close to its overlap concentration (c*), and created synthetic soft hydrogels that exhibited unique rheological properties similar to a natural soft hydrogel: being dominantly elastic under low shear stress while being viscous when the stress is above a small threshold. We explored a potential application of the hyaluronic acid-based soft hydrogel as a long-acting ocular surface lubricant and evaluated its therapeutic effects for dry eye. The soft hydrogel was found to be biocompatible after topical instillation on experimental animals' and companion dogs' eyes. In a canine clinical study, twice-a-day ocular instillation of the soft hydrogel in combination with cyclosporine for 1 month improved the clinical signs in more than 65% of dog patients previously unresponsive to cyclosporine treatment.

Theragnostic nanomotors: Successes and upcoming challenges

The idea of "fantastic voyagers" carrying out medical tasks within the human body has existed as part of popular culture for many decades. The concept revolved around a miniaturized robot that can travel inside the human body and perform complicated functions such as surgery, navigation of otherwise inaccessible biological environments, and delivery of therapeutics. Since the last decade, significant developments have occurred in this arena that are yet to enter mainstream biomedical practises. Here, we define the challenges to make this fiction into reality. We begin by chalking the journey from pills, nanoparticles, and then to micro-nanomotors. The review describes the principles, physicochemical contexts, and advantages that micro-nanomotors provide. The article then describes micro-nanomotors' obstacles such as maneuverability, in vivo imaging, toxicity, and biodistribution. This article is categorized under: Diagnostic Tools > In Vivo Nanodiagnostics and Imaging Nanotechnology Approaches to Biology > Nanoscale Systems in Biology.

The Effect of Low-Dose Atropine on Alpha Ganglion Cell Signaling in the Mouse Retina

Low-dose atropine helps to control myopia progression with few side effects. However, the impact of atropine, a non-selective muscarinic Acetylcholine (ACh) receptor antagonist, on retinal ganglion cells (RGCs) remains unclear. After immersing the cornea and adjacent conjunctiva of enucleated eyes in 0.05% (approximately 800 μM) atropine solution for 30 min, the atropine concentration reached in the retina was below 2 μM. After direct superfusion of the retina with 1 μM atropine (considering that the clinical application of 0.05% atropine eye drops will be diluted over time due to tear flow for 30 min), no noticeable changes in the morphology of ON and OFF alpha RGCs (αRGCs) were observed. Atropine affected the light-evoked responses of ON and OFF αRGCs in a dose- and time-dependent fashion. Direct application of less than 100 μM atropine on the retina did not affect light-evoked responses. The time latency of light-induced responses of ON or OFF αRGCs did not change after the application of 0.05–100 μM atropine for 5 min. However, 50 μM atropine extended the threshold of joint inter-spike interval (ISI) distribution of the RGCs. These results indicated that low-dose atropine (<0.5 μM; equal to 1% atropine topical application) did not interfere with spike frequency, the pattern of synchronized firing between OFF αRGCs, or the threshold of joint ISI distribution of αRGCs. The application of atropine unmasked inhibition to induce ON responses from certain OFF RGCs, possibly via the GABAergic pathway, potentially affecting visual information processing.

The effects of aging on the mechanical properties of the vitreous

The vitreous body is a viscoelastic gel-like network that fills the space between the lens and the retina in the eye. With aging, the vitreous undergoes a liquefaction process in which liquid pockets form in the gel network, thereby motivating the detachment of the vitreous from the retina in a process known as posterior vitreous detachment (PVD). The PVD process may lead to the formation of floaters and even result in partial or complete loss of vision. Experiments show that the liquefaction and the PVD processes alter the mechanical properties of the vitreous. In this work, we propose a microscopically motivated model that characterizes the changes in the mechanical properties of the vitreous due to aging. To this end, we distinguish between four vitreous states: a homogeneous vitreous, a liquefied vitreous, a vitreous that undergoes partial PVD, and a vitreous with full PVD. The model predicts the time-dependent and the steady-state response of the vitreous in each of the four states. The proposed framework is validated through a comparison with various experimental findings and captures the softening of the vitreous due to aging. We illustrate the importance of the age at which the PVD process begins and of the rate of the detachment process. In addition, we introduce a quantifiable parameter that describes the stage of PVD in the eye. Lastly, we employ our model to investigate the possibility of restoring the mechanical properties of a vitreous that has undergone PVD through the addition of reinforcing fibers to the gel. This work provides insight into the consequences of the age-related changes in the microstructure of the eye and serves as a motivation for new therapeutic measures.

Experimental evaluation of the viscoelasticity of porcine vitreous

This study aims to estimate the material properties of the porcine vitreous while testing it in close to its natural physiological conditions. Eighteen porcine eyes were tested within 48 h post-mortem. A custom-built computer-controlled test rig was designed to support, load and monitor the behaviour of eye globes while being subjected to dynamic rotation cycles mimicking saccade eye movement. Specimens were glued to the base of a container, surrounded by gelatin, frozen and cut in half to expose the vitreous. After thawing, the container was subjected to concentric dynamic rotations of up to 5°, 10° or 15°, while taking 50 MP photos of the specimen every 2 ms. The images were analysed by a digital image correlation algorithm to trace the movement of marked points on the vitreous surface with different radii from the centre of the posterior chamber. The initial camera image was used in building a finite-element model of the test set-up, which was used in an inverse analysis exercise to estimate the material properties of the vitreous. Angular displacements of the monitored points were up to 3.3°, 4.1° and 3.9° in response to eye rotations of 5°, 10° and 15°, respectively. With the experimental relationships between eye rotation and angular displacements used as target behaviour, the inverse analysis exercise estimated the initial shear modulus, the long-term shear modulus and the viscoelastic decay constant of the porcine vitreous as 2.10 ± 0.15 Pa, 0.50 ± 0.04 Pa and 1.20 ± 0.09 s^-1, respectively. Consideration of the viscoelasticity of the vitreous was essential to represent its experimental behaviour. Testing the vitreous in close to its normal physiological conditions produced estimations of the initial shear modulus and long-term shear modulus that were, respectively, smaller and larger than reported values (Zimberlin et al. 2010 Soft Matter 6, 3632-3635. (doi:10.1039/b925407b), Liu et al. 2013 J. Biomech. 46, 1321-7. (doi:10.1016/j.jbiomech.2013.02.006), Rossi et al. 2011 Invest. Ophthalmol. Vis. Sci. 52, 3994-4002. (doi:10.1167/iovs.10-6477)).

The action of ciliary muscle contraction on accommodation of the lens explored with a 3D model

The eye's accommodative mechanism changes optical power for near vision. In accommodation, ciliary muscle excursion relieves lens tension, allowing it to return to its more convex shape. Lens deformation alters its refractive properties, but the mechanics of ciliary muscle actions are difficult to intuit due to the complex architecture of the tissues involved. The muscle itself comprises three sections of dissimilarly oriented cells. These cells contract, transmitting forces through the zonule fibers and extralenticular structures. This study aims to create a finite element model (FEM) to predict how the action of the ciliary muscle sections leads to lens displacement. The FEM incorporates initialization of the disaccommodated lens state and ciliary muscle contraction, with three muscle sections capable of independent activation, to drive accommodative movement. Model inputs were calibrated to replicate experimentally measured disaccommodated lens and accommodated ciliary muscle shape changes. Additional imaging studies were used to validate model predictions of accommodative lens deformation. Models were analyzed to quantify mechanical actions of ciliary muscle sections in lens deformation and position modulation. Analyses revealed that ciliary muscle sections act synergistically: the circular section contributes most to increasing lens thickness, while longitudinal and radial sections can oppose this action. Conversely, longitudinal and radial sections act to translate the lens anteriorly with opposition from the circular section. This FEM demonstrates the complex interplay of the three sections of ciliary muscle in deforming and translating the lens during accommodation, providing a useful framework for future investigations of accommodative dysfunction that occurs with age in presbyopia.

Experimental assessment of the performance of vitreous cutters with fluids with different rheological properties

PURPOSE

To assess the influence of rheological properties of an artificial vitreous (AV) on the performance of double-blade (DB) and single-blade (SB) guillotine vitreous cutters, with 23-, 25-, and 27-gauge (G) probes.

METHODS

We evaluate the aspiration flow rate, using an optical method, based on image processing. Experiments are conducted using ten viscoelastic vitreous phantoms, with different properties that are measured with rheological tests.

RESULTS

Aspiration rate strongly varies with fluid properties. Regardless of cutter geometry and operational conditions, the flow rate significantly decreases as vitreous viscosity and elasticity increase.

CONCLUSIONS

All tested vitreous probes are very sensitive to changes in fluid rheology. SB cutters produce smaller flow rates compared with DB ones of the same caliber; however, they are less sensitive to fluid properties at low aspiration pressures. The use of vitreous substitutes for test performance guarantees comparability between flow rate results achieved with different vitrectomy systems operating in different media. This outcome is further confirmed by the low values of estimated flow rate relative errors.

Macro- and Microscale Properties of the Vitreous Humor to Inform Substitute Design and Intravitreal Biotransport

Research on the vitreous humor and development of hydrogel vitreous substitutes have gained a rapid increase in interest within the past two decades. However, the properties of the vitreous humor and vitreous substitutes have yet to be consolidated. In this paper, the mechanical properties of the vitreous humor and hydrogel vitreous substitutes were systematically reviewed. The number of publications on the vitreous humor and vitreous substitutes over the years, as well as their respective testing conditions and testing techniques were analyzed. The mechanical properties of the human vitreous were found to be most similar to the vitreous of pigs and rabbits. The storage and loss moduli of the hydrogel vitreous substitutes developed were found to be orders of magnitude higher in comparison to the native human vitreous. However, the reported modulus for human vitreous, which was most commonly tested in vitro, has been hypothesized to be different in vivo. Future studies should focus on testing the mechanical properties of the vitreous in situ or in vivo. In addition to its mechanical properties, the vitreous humor has other biotransport mechanisms and biochemical functions that establish a redox balance and maintain an oxygen gradient inside the vitreous chamber to protect intraocular tissues from oxidative damage. Biomimetic hydrogel vitreous substitutes have the potential to provide ophthalmologists with additional avenues for treating and controlling vitreoretinal diseases while preventing complications after vitrectomy. Due to the proximity and interconnectedness of the vitreous humor to other ocular tissues, particularly the lens and the retina, more interest has been placed on understanding the properties of the vitreous humor in recent years. A better understanding of the properties of the vitreous humor will aid in improving the design of biomimetic vitreous substitutes and enhancing intravitreal biotransport.

Flow dynamics of vitreous humour during saccadic eye movements

In this work, we reveal the flow dynamics of Vitreous Humour (VH) gel and liquid phases during saccadic movements of the eye, considering the biofluids viscoelastic character as well as realistic eye chamber geometry and taking into account the saccade profile. We quantify the differences in the flow dynamics of VH gel and liquid phases using viscoelastic rheological models that are able to model the VH shear rheology, considering different amplitudes of saccadic movements (10^∘, 20^∘, 30^∘ and 40^∘). For this purpose, the computational fluid dynamics (CFD) open source software OpenFOAM® was used. The results portray a distinct flow behaviour for the VH gel and liquid phases, with inertial effects being more significant for the VH liquid phase. Moreover, the Wall Shear Stress (WSS) values produced by the VH gel phase are more than twice of those generated by the VH liquid phase. Results also show that for different amplitudes of eye movement both the velocity magnitude in the vitreous cavity and the shear stresses on the cavity walls rise with increasing saccadic movement displacement.

Fluid Dynamic Assessment of Hypersonic and Guillotine Vitrectomy Probes in Viscoelastic Vitreous Substitutes

Purpose

To assess the fluidics of 23-gauge (G) large-port (L) and tear drop-port (TD) hypersonic vitrectomy probes (HVPs) compared with guillotine vitrectomy probes (GVPs) of various calibers (23G, 25G, and 27G) and geometries (single and double blades). Also, to identify the working parameters that provide the best balance between acceleration and flow rate, and, for HVPs, to measure temperature variations in the fluid.

Methods

We used particle image velocimetry to measure flow fields in balanced salt solution and viscoelastic artificial vitreous. We analyzed acceleration, kinetic energy, and volumetric flux. The parameters considered were vacuum pressure, ultrasound stroke, and cut rate. Temperature measurements were taken using an infrared thermal camera.

Results

The flow rate was significantly higher for HVPs than GVPs. With both probes, flow rate and acceleration increased with vacuum pressure. Flow rate depended weakly on the ultrasound stroke or cut rate. In HVPs, the acceleration peaked at a stroke of 30 µm, whereas in GVPs it peaked at a cutting rate of 4000 to 5000 cuts per minute (cpm). The HPV/TD combination generated higher flow rates and lower accelerations than did HPV/L. The increase in temperature was small.

Conclusions

Under the present experimental setup and medium, HVPs offered better fluidics compared with GVPs in terms of flow and acceleration; however, the flow structure for HVPs is more complicated and unsteady. The HPV/TD combination produced larger flows than did the HPV/L combination and slightly smaller accelerations. HPVs generated a small temperature increase.

Translational Relevance

In the tested artificial vitreous, HVPs were found to be more efficient in terms of generating lower acceleration for a given flow rate. The slight increase in temperature observed with HVPs is unlikely to be clinically significant.

Vitreous and Vision Degrading Myodesopsia

Macromolecules comprise only 2% of vitreous, yet are responsible for its gel state, transparency, and physiologic function(s) within the eye. Myopia and aging alter collagen and hyaluronan association causing concurrent gel liquefaction and fibrous degeneration. The resulting vitreous opacities and collapse of the vitreous body during posterior vitreous detachment are the most common causes for the visual phenomenon of vitreous floaters. Previously considered innocuous, the vitreous opacities that cause floaters sometimes impact vision by profoundly degrading contrast sensitivity function and impairing quality-of-life. While many people adapt to vitreous floaters, clinically significant cases can be diagnosed with Vision Degrading Myodesopsia based upon echographic assessment of vitreous structure and by measuring contrast sensitivity function. Perhaps due to the ubiquity of floaters, the medical profession has to date largely ignored the plight of those with Vision Degrading Myodesopsia. Improved diagnostics will enable better disease staging and more accurate identification of severe cases that merit therapy. YAG laser treatments may occasionally be slightly effective, but vitrectomy is currently the definitive cure. Future developments will usher in more informative diagnostic approaches as well as safer and more effective therapeutic strategies. Improved laser treatments, new pharmacotherapies, and possibly non-invasive optical corrections are exciting new approaches to pursue. Ultimately, enhanced understanding of the underlying pathogenesis of Vision Degrading Myodesopsia should result in prevention, the ultimate goal of modern Medicine.

MEASUREMENT OF THE HYDRAULIC CONDUCTIVITY OF THE VITREOUS HUMOR

The hydraulic conductivity of the vitreous humor has been measured for the bovine eye. The experiment was carried out by placing it within upright cylindrical chamber, open at both ends, and letting its liquid content drain out of the bottom opening by gravity, through a 20μm nylon mesh filter. Additional negative pressure was provided at the exit by a hanging drainage tube. The diminishing vitreous volume was measured in terms of the height in the chamber and recorded as a function of time. The reduction in the vitreous liquid content also caused the hydraulic conductivity to reduce and this parameter was quantified on the basis of previously-developed theories of fibrous porous media that have been very well established. A theoretical model with a fully analytical expression for the vitreous volume undergoing drainage was developed and used as a least-squares best fit to deliver the initial hydraulic conductivity value of K 0/μ=(7.8 ± 3.1) × 10-12 m2 (Pa-s). The measurements were made with the hyaloid membrane intact and therefore represents an effective conductivity for the entire system, including possible variations within the vitreous.

Establishment of a multilayered 3D cellular model of the retinal-blood barrier

Retinal disorders are leading causes of blindness. Still, treatment strategies are limited and the challenging anatomical barriers of the eye limit the evaluation and development of new therapeutics. Among these layers of barriers is the blood-retinal barrier, which separates the retina from the choroid by the Bruch's membrane. This work aimed to establish a 3D cellular model that recapitulates barrier properties of the BRB and diffusion through the vitreous, the main barriers encountered upon intravitreal injection. Several parameters were evaluated namely co-culture time of ARPE-19 and HUVECs and different biomaterial compositions of hydrogels to better mimic the human vitreous. The developed vitreous mimic has viscoelastic properties similar to human vitreous. Co-culture of human retinal and endothelial cells showed increased transepithelial resistance with longer co-culture times concomitant with reduced permeability to FITC-dextran 40 kDa. The proposed models lay the foundation of a platform for faster assessment of a large number of samples and without the use of animals.

Laser-induced plasmon-mediated treatment of retinoblastoma in viscous vitreous phantom

Retinoblastoma (RB) is a rare form of cancer of the retina most prevalent in young children. We successfully show that laser-induced cell disruption, mediated by gold plasmonic nanoparticle (NP), is a potential and efficient therapy to kill the cancerous cells. The proof of concept is demonstrated in vitro on cultured Y79 RB cancer cells with a nanosecond laser at 527 nm, for both attached cells at the bottom of a Petri dish and for floating, clustered cells in a viscous vitreous phantom comprised of hyaluronan. We report a cellular death of 82% after irradiation in classic culture medium and a cellular death of 98% in vitreous phantom, for similar number of NPs in each sample. It is found that the NPs efficiently penetrate the floating Y79 clusters cells in the vitreous phantom, leading to a cellular death of over 85% even within the centre of the aggregates. The proposed treatment technique is based on a similar nanosecond laser used to eliminate floaters in the vitreous, but with much lower (100-1000 times) fluences of 20 J cm^-2 .

Age-Related Loss of Human Vitreal Viscoelasticity

Purpose

To determine the viscoelasticity of human vitreous bodies and its changes with age in order to benefit the understanding and therapy of vitreoretinal diseases.

Methods

In a postmortem study, 190 human vitreous bodies were extracted from 33- to 92-year-old donors, analyzed with regard to their viscoelastic properties via dynamic mechanical analyses, and compared with bovine and porcine vitreous. Postmortem intervals and donor-related parameters were examined as potential parameters influencing vitreous viscoelasticity. Dynamic moduli of different hyaluronic acid (HA) solutions as well as human vitreous treated with HA injections were determined by frequency sweep tests.

Results

With age the viscoelasticity of human vitreous bodies decreased significantly and independently of postmortem intervals, diabetes, and the donor's sex. The storage modulus G′ and loss modulus G″ correlated strongly with the donor's age with r = −0.789 and r = −0.764, respectively. Bovine and porcine vitreous bodies exhibited dynamic moduli comparable only to the viscoelastic properties of aged human vitreous and are thus limited models for the simulation of the human vitreous. The viscoelasticity of aged human vitreous bodies was found to be increased after intravitreal injections of highly concentrated HA.

Conclusions

The present postmortem study is the first to show a significant age-related reduction in the viscoelasticity of entire human vitreous bodies. Highly concentrated HA injections may serve as a possible therapeutic approach for restoring the viscoelasticity of aged vitreous bodies.

Translational Relevance

These findings improve the understanding and therapy of the vitreous liquefaction with age and the associated vitreoretinal diseases.

Gels of Amyloid Fibers

Protein self-assembly and formation of amyloid fibers is an early event of numerous human diseases. Continuous aggregation of amyloid fibers in vitro produces biogels, which led us to suspect that amyloid plaques and neurofibrillary tangles in Alzheimer’s disease are of biogels in nature. We applied atomic force microscopy, size exclusion chromatography, and differential scanning calorimetry to elucidate the gel’s structure, kinetics of gel formation, and melting point. We found that (1) lysozyme gelation occurs when the protein concentration is above 5 mg/mL; (2) nonfibrous protein concentration decreases and plateaus after three days of gel synthesis reaction; (3) colloidal lysozyme aggregates are detectable by both atomic force microscopy (AFM) and fast protein liquid chromatography (FPLC); (4) the gels are a three-dimensional (3D) network crosslinked by fibers coiling around each other; (5) the gels have a high melting point at around around 110 °C, which is weakly dependent on protein concentration; (6) the gels are conductive under an electric field, and (7) they form faster in the presence than in the absence of salt in the reaction buffer. The potential role of the gels formed by amyloid fibers in amyloidosis, particularly in Alzheimer’s disease was thoroughly discussed, as gels with increased viscosity, are known to restrict bulk flow and then circulation of ions and molecules.

Effects of Collagenase Type II on Vitreous Humor—An In Situ Rheological Study

The purpose of this study is to quantify the impact of enzyme activity on the vitreous humor structure over time to understand the mechanical characteristics of the vitreous humor gel. Changes in the mechanical behavior of the vitreous occur due to many reasons including aging, which may lead to many vitreoretinal diseases. The degeneration process of the vitreous has been studied; however, in situ experimental procedures to validate the existing hypotheses are limited. We examined thirty-eight porcine eyes using in situ rheological creep tests to measure the mechanical properties of the vitreous humor of the eyes prior to, 1 h and 24 h after the intravitreal injection. Eyes in one group were injected with collagenase type II solution and eyes in the control group were injected with phosphate buffered saline solution (PBS) with calcium and magnesium chloride. Prior to the injection, viscosity and creep compliance intercept values between both groups were not statistically different. At 1 h and 24 h after the injection, vitreous properties in the eyes from the first group showed a statistically significant increase in the J intercept values (representing the inverse of elasticity) compared to the control group. In addition, 1 h and 24 h after the injection, vitreous viscosity was lower in the eyes from the first group than in the eyes from the control group. These findings are a foundation for future studies on the effectiveness of intravitreal drugs that modify the mechanical properties of the vitreous humor.

Rheological Properties and Age-Related Changes of the Human Vitreous Humor

The vitreous humor is a fragile, transparent hydrogel situated between the lens and the retina, occupying 80% of the eye's volume. Due to its viscoelastic behavior, the vitreous serves as a mechanical damper for the eye, absorbing impacts, and protecting the lens and retina. The vitreous liquefies with age, which compromises its function as a shock absorber and causes complications including retinal detachment, macular holes, and vitreous hemorrhage. Studies on the viscoelastic properties of the vitreous have been limited. Rheological testing of the vitreous has commonly been done on non-primate mammalian species. Human vitreous rheological properties have been previously reported; however, various measurement techniques were used, resulting in data that differed by orders of magnitude. Shear rheometry is commonly used to characterize soft tissues and hydrogels such as the vitreous humor. However, no human vitreous rheological data have been reported using this technique, preventing direct comparison to other published work. Additionally, no age-related changes in the mechanical properties of the human vitreous humor have been reported. Human vitreous samples (n = 39, aged 62 ± 15 years) were tested using a shear rheometer. Small amplitude oscillatory shear and creep experiments were performed. The linear viscoelastic region of the human vitreous was found to be below 1% strain. The solid phase of the old human vitreous was found to be stiffer than the young human vitreous and the porcine vitreous. The stiffness of the human vitreous gel also appeared to be positively correlated with age. Vitreous dehydration due to a decrease in hyaluronic acid concentration with age was proposed to cause the stiffening of the solid phase of the vitreous gel. Vitreous liquefaction, therefore, might be characterized as a simultaneous increase in liquid volume and localized stiffening of the vitreous gel. The phase separation of the vitreous humor with age has been hypothesized as the cause of many vitreous-related complications. This study provides viscoelastic properties and age-related changes of the human vitreous humor, which will aid in the design of biomimetic vitreous substitutes, enhancement in analyzing intravitreal transport of therapeutics, and understanding the pathological conditions of the vitreous humor.

Fluidics of Single and Double Blade Guillotine Vitrectomy Probes in Balanced Salt Solution and Artificial Vitreous

PURPOSE

To assess the fluidics of double-vitreous cutter blade (DB) compared with single-blade (SB) guillotine with 23-, 25-, and 27-gauge vitrectomy probes. To assess flow characteristics and flow rates in viscous and viscoelastic fluids.

METHODS

We used Particle Image Velocimetry to measure the flow field close to the tip of each cutter probe and we derived kinematic quantities of interest, such as kinetic energy and acceleration. We performed measurements both on a balanced salt solution (BSS) and on a viscoelastic artificial vitreous (AV).

RESULTS

The flow rate is significantly higher with DB than SB vitrectomy probes, for a given pumping pressure and cutting rate. The fluid flow observed is very different between BSS and AV tests.

CONCLUSIONS

The DB has more efficient fluidics than SB vitrectomy probe in all tested conditions. Fluid acceleration depends on the cutting frequency, especially in the case of measurements in AV. The flow rate strongly depends on the pressure and it is little affected by the cutting frequency, in a range of clinical interest. The 27-G DB produces flow rates similar to the 23- and 25-G SB, with significantly smaller acceleration. The flow induced in the AV is different from that in BSS and oscillates at different frequencies.

TRANSLATIONAL RELEVANCE

DB cutters prove to be more efficient in terms of lower acceleration for a given flow rate. The latter is mainly controlled by aspiration pressure and less by cut rates. The influence of vitreous rheology deserves further investigations.

Derivation of the Variants of the Burgers Model Using a Thermodynamic Approach and Appealing to the Concept of Evolving Natural Configurations

Viscoelastic rate-type fluid models involving the stress and frame-indifferent time derivatives of second order, like those in Burgers’ model, are used to describe the complicated response of fluid like materials that are endowed with a complex microstructure that allows them to possess two different relaxation mechanisms as well as other non-Newtonian characteristics. Such models are used in geomechanics, biomechanics, chemical engineering and material sciences. We show how to develop such rate-type fluid models that include the classical Burgers’ model as well as variants of Burgers’ model, using a thermodynamic approach based on constitutive assumptions for two scalar quantities (namely, how the material stores energy and how the energy is dissipated) and appealing to the concept of natural configuration associated with the placement of the body that evolves as the body deforms.

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.

Injectable uncrosslinked biomimetic hydrogels as candidate scaffolds for neural stem cell delivery

Mammalian central nervous system has a limited ability for self-repair under diseased or injury conditions. Repair strategies focused on exogenously delivering autologous neural stem cells (NSCs) to replace lost neuronal populations and axonal pathways in situ, and promote endogenous repair mechanisms are gaining traction. Successful outcomes are contingent on selecting an appropriate delivery vehicle for injecting cells that promotes cell retention and survival, elicits differentiation to desired lineages, and enhances axonal outgrowth upon integration into the host tissue. Hydrogels made of varying compositions of collagen, laminin, hyaluronic acid (HA), and chondroitin sulfate proteoglycan (CSPG) were developed, with no external crosslinking agents, to mimic the native extracellular matrix composition. The physical (porosity, pore-size, gel integrity, swelling ratio, and enzymatic degradation), mechanical (viscosity, storage and loss moduli, Young's modulus, creep, and stress-relaxation), and biological (cell survival, differentiation, neurite outgrowth, and integrin expression) characteristics of these hydrogels were assessed. These hydrogels exhibited excellent injectability, retained gel integrity, and matched the mechanical moduli of native brain tissue, possibly due to natural collagen fibril polymerization and physical-crosslinking between HA molecules and collagen fibrils. Depending on the composition, these hydrogels promoted cell survival, neural differentiation, and neurite outgrowth, as evident from immunostaining and western blots. These cellular outcomes were facilitated by cellular binding via α₆ , β₁ , and CD44 surface integrins to these hydrogels. Results attest to the utility of uncrosslinked, ECM-mimicking hydrogels to deliver NSCs for tissue engineering and regenerative medicine applications. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 790-805, 2017.

Hydraulic Resistance of Vitreous Cutters: The Impact of Blade Design and Cut Rate

Purpose

To measure the hydraulic resistance (HR) of vitreous cutters equipped with a Regular guillotine Blade (RB) or double edge blade (DEB) at cut rates comprised between 0 and 12,000 cuts per minute (CPM) and compare it with vitreous fragment size. This was an in vitro experimental study; in vivo HR measure and vitreous sampling.

Methods

HR, defined as aspiration pressure/flow rate, was measured in balanced salt solution (BSS; Alcon, Fort Worth, TX) (in vitro) and during pars plana vitrectomy of 20 consecutive patients aged 18 to 65, undergoing macular surgery. HR was recorded at increasing cut rates (500–6000 CPM for the RB and 500–12,000 CPM for the DEB; 5 mL/min flow). Vitreous samples were withdrawn and analyzed with Western and collagen type II and IX immunostaining to evaluate protein size. The main outcome measures were hydraulic resistance (mm Hg/ml/min [±SD]) and optic density for Western blot and immunostaining.

Results

RB and DEB showed identical HR in BSS between 0 and 3000 CPM. Above 3000 CPM, RB HR steadily increased, and was significantly higher than DEB HR. Vitreous HR was also similar for the two blades between 0 and 1500 CPM. Above 1500 CPM, RB offered a significantly higher resistance. Western blot and immunostaining of vitreous samples did not yield a significant difference in size, regardless of blade type and cut rate.

Conclusions

DEB is more efficient, offering a lower HR than RB over 1500 CPM in human vitreous. There is no viscosity reduction as a function of cut-rate between 1500 and 12,000 CPM, as HR does not vary.

Translational Relevance

Future vitreous cutters will benefit of a DEB; optimal cut rate needs to be defined, and the simple increase of cut rate does not provide benefits after a certain limit to be assessed.

Soret and Dufour Effects on MHD Peristaltic Flow of Jeffrey Fluid in a Rotating System with Porous Medium

The objective of present paper is to examine the peristaltic flow of magnetohydrodynamic (MHD) Jeffrey fluid saturating porous space in a channel through rotating frame. Unlike the previous attempts, the flow formulation is based upon modified Darcy's law porous medium effect in Jeffrey fluid situation. In addition the impacts due to Soret and Dufour effects in the radiative peristaltic flow are accounted. Rosseland's approximation has been utilized for the thermal radiative heat flux. Lubrication approach is implemented for the simplification. Resulting problems are solved for the stream function, temperature and concentration. Graphical results are prepared and analyzed for different parameters of interest entering into the problems.

Mobility-Enhancing Coatings for Vitreoretinal Surgical Devices: Hydrophilic and Enzymatic Coatings Investigated by Microrheology

Ophthalmic wireless microrobots are proposed for minimally invasive vitreoretinal surgery. Devices in the vitreous experience nonlinear mobility as a result of the complex mechanical properties of the vitreous and its interaction with the devices. A microdevice that will minimize its interaction with the macromolecules of the vitreous (i.e., mainly hyaluronan (HA) and collagen) can be utilized for ophthalmic surgeries. Although a few studies on the interactions between the vitreous and microdevices exist, there is no literature on the influence of coatings on these interactions. This paper presents how coatings on devices affect mobility in the vitreous. Surgical catheters in the vasculature use hydrophilic polymer coatings that reduce biomolecular absorption and enhance mobility. In this work such polymers, polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), and HA coatings were utilized, and their effects on mobility in the vitreous were characterized. Hydrophilic titanium dioxide (TiO2) coating was also developed and characterized. Collagenase and hyaluronidase enzymes were coated on probes' surfaces with a view to enhancing their mobility by enzymatic digestion of the collagen and HA of the vitreous, respectively. To model the human vitreous, ex vivo porcine vitreous and collagen were used. For studying the effects of hyaluronidase, the vitreous and HA were used. The hydrophilic and enzymatic coatings were characterized by oscillatory magnetic microrheology. The statistical significance of the mean relative displacements (i.e., mobility) of the coated probes with respect to control probes was assessed. All studied hydrophilic coatings improve mobility, except for HA which decreases mobility potentially due to bonding with vitreal macromolecules. TiO2 coating improves mobility in collagen by 28.3% and in the vitreous by 15.4%. PEG and PVP coatings improve mobility in collagen by 19.4 and by 39.6%, respectively, but their improvement in the vitreous is insignificant at a 95% confidence level (CL). HA coating affects mobility by reducing it in collagen by 35.6% (statistically significant) and in the vitreous by 16.8% (insignificant change at 95% CL). The coatings cause similar effects in collagen and in the vitreous. However, the effects are lower in the vitreous, which can be due to a lower concentration of collagen in the vitreous than in the prepared collagen samples. The coatings based on enzymatic activity increase mobility (i.e., >40% after 15 min experiments in the vitreous models) more than the hydrophilic coatings based on physicochemical interactions. However, the enzymes have time-dependent effects, and they dissolve from the probe surface with time. The presented results are useful for researchers and companies developing ophthalmic devices. They also pave the way to understanding how to adjust mobility of a microdevice in a complex fluid by choice of an appropriate coating.

An experimental model of vitreous motion induced by eye rotations

BACKGROUND

During eye rotations the vitreous humour moves with respect to the eye globe. This relative motion has been suggested to possibly have an important role in inducing degradation of the gel structure, which might lead to vitreous liquefaction and/or posterior vitreous detachment. Aim of the present work is to study the characteristics of vitreous motion induced by eye rotations.

METHODS

We use an experimental setup, consisting of a Perspex model of the vitreous chamber that, for simplicity, is taken to have a spherical shape. The model is filled with an artificial vitreous humour, prepared as a solution of agar powder and hyaluronic acid sodium salt in deionised water, which has viscoelastic mechanical properties similar to those of the real vitreous. The model rotates about an axis passing through the centre of the sphere and velocity measurements are taken on the equatorial plane orthogonal to the axis of rotation, using an optical technique.

RESULTS

The results show that fluid viscoelasticity has a strong influence on flow characteristics. In particular, at certain frequencies of oscillation of the eye model, fluid motion can be resonantly excited. This means that fluid velocity within the domain can be significantly larger than that of the wall.

CONCLUSIONS

The frequencies for which resonant excitation occurs are within the range of possible eye rotations frequencies. Therefore, the present results suggest that resonant excitation of vitreous motion is likely to occur in practice. This, in turn, implies that eye rotations produce large stresses on the retina and within the vitreous that may contribute to the disruption of the vitreous gel structure. The present results also have implications for the choice of the ideal properties for vitreous substitute fluids.

Preservation of the structure of enzymatically-degraded bovine vitreous using synthetic proteoglycan mimics

PURPOSE

Vitreous liquefaction and subsequent posterior vitreous detachment can lead to several sight-threatening diseases, including retinal detachment, macular hole and macular traction syndrome, nuclear cataracts, and possibly, open-angle glaucoma. In this study, we tested the ability of three novel synthetic chondroitin sulfate proteoglycan mimics to preserve the structure and physical properties of enzymatically-degraded bovine vitreous.

METHODS

Chondroitin sulfate proteoglycan mimics, designed to bind to type II collagen, hyaluronic acid, or both, were applied to trypsin- or collagenase-treated bovine vitreous in situ and in vitro. Rheology and liquefaction tests were performed to determine the physical properties of the vitreous, while Western blots were used to detect the presence and degradation of soluble collagen II (α1). Deep-etch electron microscopy (DEEM) identified the ultrastructure of mimic-treated and untreated enzyme-degraded bovine vitreous.

RESULTS

Proteoglycan mimics preserved the physical properties of trypsin-degraded bovine vitreous and protected against vitreous liquefaction. Although the collagen-binding mimic maintained the physical properties of collagenase-treated vitreous, liquefaction still occurred. Western blots indicated that the mimic provided only marginal protective ability against soluble collagen degradation. Deep-etch electron microscopy, however, showed increased density and isotropy of microstructural components in mimic-treated vitreous, supporting the initial result that vitreous structure was preserved.

CONCLUSIONS

Proteoglycan mimics preserved bovine vitreous physical properties after enzymatic degradation. These compounds may be useful in delaying or preventing the pathological effects of age-related, or enzymatically-induced, degradation of the vitreous body.

Nanopropellers and their actuation in complex viscoelastic media

Tissue and biological fluids are complex viscoelastic media with a nanoporous macromolecular structure. Here, we demonstrate that helical nanopropellers can be controllably steered through such a biological gel. The screw-propellers have a filament diameter of about 70 nm and are smaller than previously reported nanopropellers as well as any swimming microorganism. We show that the nanoscrews will move through high-viscosity solutions with comparable velocities to that of larger micropropellers, even though they are so small that Brownian forces suppress their actuation in pure water. When actuated in viscoelastic hyaluronan gels, the nanopropellers appear to have a significant advantage, as they are of the same size range as the gel's mesh size. Whereas larger helices will show very low or negligible propulsion in hyaluronan solutions, the nanoscrews actually display significantly enhanced propulsion velocities that exceed the highest measured speeds in Newtonian fluids. The nanopropellers are not only promising for applications in the extracellular environment but small enough to be taken up by cells.