Fabio A. Guarnieri

Characterisation and simulation of an active microvalve for glaucoma.

Glaucoma drainage device (GDD) has the potential to eliminate hypotony but still suffers from poor flow control and fibrosis. The ideal shunt should change its hydraulic resistance to achieve the desired intraocular pressure (IOP). In this study, the characterisation of a preliminary design of a new GDD is presented. This is activated by means of a diaphragm, which is actuated by conducting polymers. The valve can be manufactured employing microelectromechanical system technology by soft lithography. The characterisation process is performed by numerical simulation using the finite element method, considering the coupling between the fluid and the structure (diaphragm) obtaining the hydraulic resistance for several positions of the diaphragm. To analyse the hydraulic system of the microvalve implanted in a human eye, an equivalent circuit model was used. The parameters of the equivalent circuit model were obtained from numerical simulation. The hydraulic resistance of the designed GDD varies in the range of 13.08–0.36 mmHg min/μl compared with 3.38–0.43 mmHg min/μl for the Ahmed valve. The maximum displacement of the diaphragm in the vertical direction is 18.9 μm, and the strain in the plane is 2%. The proposed preliminary design allows to control the IOP by varying the hydraulic resistance in a greater range than the existing passive valves, and the numerical simulation facilitates the characterisation and the improvement of the design before its construction, reducing time and costs.

Intraocular pressure: Goldmann tonometry, computational model, and calibration equation.

Purpose:There exists a concern about the general accuracy of intraocular pressure (IOP) measurements using tonometry, and especially Goldmann applanation tonometer (GAT) because it considers the cornea as an infinite thin shell. In this study, the relationship between the true IOP and tonometric IOP, external curvature radius (ECR), and central corneal thickness (CCT) is explored. Methods:In this study, the calibration of the IOP measurements through GAT for different values of CCT, ECR, and E (Young modulus), is done through computational simulations of the mechanical behavior of the cornea subjected to the applanation process using the finite element method (FEM). Previous to this simulations, experimentations on rabbits were performed to confirm that inaccurate readings are obtained with GAT in certain conditions. This methodology is also followed to establish the range of corneal parameters of patients for which the GAT measure of pressure is reliable. The calibration equation for GAT measurements is developed from a statistical multiple linear regression analysis. Results:Based on a statistical variable analysis of the computational modeling results, a calibration equation is established for the GAT that relates the true IOP with the ECR, CCT, and GAT measurements. Conclusions:Our results show that GAT measures are linearly dependent on the modulus of elasticity of the cornea; nevertheless, if we consider a healthy cornea with a specific modulus of elasticity, it is possible to correct the measure with a linear equation involving CCT and ECR.

Biomechanics of Additive Surgery: Intracorneal Rings

The aim of supervision relies on a thorough understanding of corneal biomechanics in order to predict refractive surgery outcome. The study of changes in stress and elasticity after corneal reshaping by additive or subtractive surgical techniques are very important in order to obtain reliable procedures.

Diamond-Based Thin Film Bulk Acoustic Wave Resonator for Biomedical Applications

Nowadays it is in constant growing the development of thin film bulk acoustic resonators. If the piezoelectric material is going to be implanted in the human body, an important requirement is the biocompatibility of the implant. In this regard, Aluminum Nitride (AlN) has emerged as an attractive alternative for use in biomedical MicroElectroMechanical Systems. Ultrananocrystalline Diamond (UNCD) is a promising material to be used in biomedical applications, due to its extraordinary mulifunctionality; it is exceptional for implantable medical devices requiring stringent biological performance. Since both UNCD and AlN films can be processed via photolithography processes used in microfabrication, the integration of UNCD and AlN films provides the bases for developing a new generation of biocompatible Bio-MEMS/NEMS. Research and development was conducted to produce implantable MEMS devices: Pt/piezoelectric AlN/Pt layer heterostructure was grown and patterned on the UNCD membrane with a Ti adhesion layer. By applying voltages between the top and bottom Pt electrodes layers the piezoelectric AlN layer is energized. The feasibility of the fabrication of biocompatible AlN/diamond-based FBAR structure has been demonstrated.

Introduction: Corneal Biomechanics and Refractive Surgery

This book is related to corneal biomechanics but stresses its importance to Refractive Surgery, an outstanding procedure that changed the way ophthalmologists and patients see ophthalmology.

Biomechanical Instrumentation in Refractive Surgery

The determination of the intraocular pressure by means of instruments is still a developing science. Several physical properties and mechanisms affect this measure due to the corneal shape and its rigidity. Experimental and analytical researches are tools to improve indirect technologies (tonometric devices) for this relevant quantity for ophthalmologic diagnosis. In this chapter the applanation contact tonometry is the main subject (biomechanical modeling, computational modeling, and calibration equations); nevertheless, new tonometric devices tend to avoid contact to diminish influence of probe shape in the contact procedure and also pain in the patient.

Biomechanics of Subtractive Surgery: From ALK to LASIK

Myopic and hyperopic excimer laser in situ keratomileusis (LASIK) have become widely accepted procedures. Although LASIK does not rely on the mechanical response of the cornea to obtain the optical correction, the creation of a flap and ablation of the exposed stromal bed must disturb the state of stress in the tissue below the ablation zone. Little attention appears to have been paid to the mechanical response of the cornea to LASIK [1–3]. For low to moderate correction, the stress change induced by the surgical procedure is probably relatively small, although, to the best of our knowledge, it has not been quantified to date. As interest moves to deeper ablation depths, it is increasingly important to understand the mechanical response of the tissue to its new geometric configuration. Clearly, as ablation depths vary from shallow to very deep, the cornea can be expected to exhibit a corresponding range of deformational responses. The deformational response of the cornea to LASIK can be identified with an instantaneous component associated with the intrinsic elasticity of the tissue and a delayed postoperative component that may be associated with possible regression and that derives from complex and poorly understood mechanisms.

Biomechanics of Incisional Surgery

Cornea is the main responsible of the refraction of the eye. Its structural properties are changed in Refractive Surgery [1], a technique used in ophthalmology to modify the refractive properties of the optical system of the eye. It is used in the correction of several ametropias like myopia, astigmatism, and hypermetropia. Among the different procedures, radial keratotomy, photorefractive keratectomy, and keratomileusis are commonly used in clinical practice. Radial keratotomy [2] is based on diamond knife relaxing incisions along the cornea, with maximum depth, but without perforation. The refractive change is reached by means of the action of the intraocular pressure over the relaxed cornea, by steepening where the thickness is smaller (at the incisions) and by flattening the uncut central zone to correct myopia 1 (Fig. 1.1).