A Face-Shear Mode Piezoelectric Array Sensor for Elasticity and Force Measurement

We present the development of a 6 × 6 piezoelectric array sensor for measuring elasticity and force. The proposed sensor employs an impedance measurement technique, sensing the acoustic load impedance of a target by measuring the electrical impedance shift of face-shear mode PMN-PT (lead magnesium niobate-lead titanate) single crystal elements. Among various modes of PMN-PT single crystals, the face-shear mode was selected due to its especially high sensitivity to acoustic loads. To verify the elasticity sensing performance, gelatin samples with different elastic moduli were prepared and tested. For the force measurement test, different magnitudes of force were loaded to the sensing layer whose acoustic impedance was varied with applied forces. From the experimental results, the fabricated sensor showed an elastic stiffness sensitivity of 23.52 Ohm/MPa with a resolution of 4.25 kPa and contact force sensitivity of 19.27 Ohm/N with a resolution of 5.19 mN. In addition, the mapping experiment of elasticity and force using the sensor array was successfully demonstrated.

Relationship of Structural Characteristics to Biomechanical Profile in Normal, Keratoconic, and Crosslinked Eyes

PURPOSE

To evaluate the correlation of corneal biomechanical parameters with structural characteristics in normal, keratoconic, and collagen crosslinked eyes.

METHODS

A prospective observational study that included 50 normal, 100 keratoconic, and 25 crosslinked eyes. All eyes were imaged using a Scheimpflug camera and an ocular response analyzer. The main outcome measures analyzed were central corneal thickness (CCT), corneal volume (CV), maximal keratometry (Kmax), corneal hysteresis (CH), and corneal resistance factor (CRF).

RESULTS

Significant differences were noted among all 3 groups of eyes for CCT, CV, Kmax, CH, and CRF values (P < 0.05 by analysis of variance). CH and CRF correlated negatively (CH, r = -0.40; CRF, r = -0.44; both P < 0.0001) with the Pentacam topographic keratoconus classification. Both CH and CRF correlated positively with CCT and CV for the normal, keratoconic, and crosslinked eyes. In contrast, significant negative correlations were observed between CH, CRF, and Kmax in the keratoconic eyes (CH, r = -0.43; CRF, r = -0.53; both P < 0.0001), whereas no association was noted for the normal and crosslinked eyes.

CONCLUSIONS

CH and CRF are influenced by the corneal structure, with higher values noted in corneas with greater thickness and volume. Corneal biomechanical parameters progressively decrease as the severity of keratoconus increases. After collagen crosslinking, the relationship of the corneal curvature to the biomechanical profile is similar to normal eyes.

Individual-specific tonometry on porcine eyes

Intraocular pressure (IOP) monitoring is important in the diagnosis and management of glaucoma. The measurement of IOP is affected by corneal properties, but the effect of corneal stiffness on IOP measurement is unaccounted for in pressure measurement instruments such as the Goldmann Applanation Tonometer (GAT). A new instrumented non-invasive indentation tonometry that can measure IOPIST, a corneal stiffness-corrected intraocular pressure is developed. The inter-individual corneal variations of 12 porcine eyes ex vivo were independently characterized; and their true intraocular pressure, IOPT's, were set using a manometer before indentation using the new indentation tonometry. Analyses of the load-displacement data showed that porcine corneal stiffness varied more than five times from 0.045 to 0.253N/mm. Analysis showed that, without individual stiffness correction, inter-individual variation of IOPGAT can vary up to 8mmHg from IOPT at 15mmHg; the error becomes larger at high IOPT. In comparison when corneal stiffness is accounted for, IOPIST has a significantly smaller error of 1.82±1.70mmHg for IOPT between 12 and 40mmHg than IOPGAT. The results showed that the new tonometry successfully accounted for inter-individual variations in IOP measurement.

Characterization of corneal tangent modulus in vivo

PURPOSE

Intraocular pressure (IOP) measured using Goldmann Applanation Tonometry (GAT) changes with individual's corneal properties, but the method to measure the in vivo corneal material properties to account for individual variation in GAT IOP is not available. In this study, a new method to measure the IOP-dependent corneal tangent modulus in vivo is developed to address this research gap.

METHODS

Instrumented indentation and analysis were developed to measure the corneal tangent modulus. The validity of the method and procedure was verified using model silicone eye pressurized to different IOP. In addition, 15 porcine eyes and 3 rabbit eyes were tested using the corneal indentation at different set intraocular pressure and different indentation rates.

RESULTS

The results from silicone eye showed that the measured tangent modulus is in good agreement with the standard silicone rubber modulus. The results on the porcine eyes and rabbit eyes showed that the method can be used to measure corneal tangent modulus in vivo in the human range of intraocular pressure from 10 to 40 mmHg.

CONCLUSIONS

An indentation method to measure the corneal tangent modulus in vivo was developed, and the IOP dependence of the corneal tangent modulus was characterized. The developed indentation method provides a new means to measure the in vivo corneal tangent modulus to account for individual and pressure variations in measurement of intraocular pressure.

Deformation of yellow spot area by compulsory increase of eye pressure

The deformation characteristics of yellow spot of retina under a compulsory eye pressure increase is discussed for both normal and glaucoma eyes. We impart the force corresponding to 50[mmHg] of eye pressure to eyelid through a contact probe with force gauge. We analyze the deformation of yellow spot area for three different layers. An interesting observation is that yellow spot suffered by glaucoma has thinner layer and smaller deformation than that of normal subjects, especially the characteristics is enhanced in both the surface and the bottom layers.

Detection of biomechanical changes after corneal cross-linking using Ocular Response Analyzer software

PURPOSE

To investigate biomechanical changes after corneal cross-linking (CXL) with riboflavin/ultraviolet-A (UVA) in keratoconus using the recently developed Ocular Response Analyzer (ORA, Reichert Technologies) software.

METHODS

Through use of the new ORA software (version 2.04), 37 new parameters derived from the best measurement signal with the highest wavescore of 4 measurements from 50 eyes of 46 patients with keratoconus were obtained before and 1 year after CXL. The parameters of 96 eyes from 96 age-matched, healthy individuals with a spherical equivalent refraction <3.00 diopters served as controls.

RESULTS

Corneal hysteresis (CH) and corneal resistance factor (CRF) before CXL were 7.38±1.42 mmHg and 6.16±1.42 mmHg, respectively, compared to 7.37±1.26 mmHg (P=.971) and 6.16±1.50 mmHg after CXL (P=.997), respectively. Based on these 37 new parameters, the area under peak 2 (p2area) showed a statistically significant increase from 1262.3±623.1 before CXL to 1704.3±732.3 1 year after CXL (35%; P=.001). The related value for the p2area of the healthy control group was 3374.9±1099.9. A significant negative correlation was observed between the p2area and the difference in CH-CRF values (r=-0.29, P=.001).

CONCLUSIONS

The area under peak 2 appears to be a more sensitive parameter to detect biomechanical changes after CXL than CH or CRF alone. After CXL, keratoconic corneas display altered biomechanical properties, which remain different to those observed in healthy corneas.

Point-type non-contact stiffness sensing of soft tissue with coupling effect

This paper discusses the point-type non-contact active sensing for evaluating the stiffness of a soft tissue with coupling effect. Tfhe sensing system imparts a point force to a tissue and measures the displacement at the point of application of force. The benefit is that such point based information enables us to estimate two stiffness parameters constructing the 3D spring network tissue model. Through the experiment for a human skin, we show that the deformation shapes between human skin surface and the model with the estimated parameters are nicely coincident.

[Biomechanical condition of the cornea as a new indicator for pathological and structural changes]

AIM

Several methods permit the measurement of geometric parameters of the cornea, but until now biomechanical conditions of the cornea have been ignored (e.g. in refractive corneal surgery). Besides the geometric condition, biomechanical properties of the cornea have been shown to influence applanation measurement of intra-ocular pressure (IOP) and epidemiological studies have identified corneal thickness as an independent risk factor for the development and progression of glaucoma. The aim of this investigation was to characterize the biomechanical properties of the cornea using the ocular response analyzer (ORA).

METHODS

The ocular response analyzer (ORA) is a new method available for non-contact measurement of the biomechanical properties of the cornea. We evaluated the reproducibility of measurements, the difference between static and dynamic factors and the impact of independent factors (e.g. IOP, age, CCT, swelling of the cornea) on 2,500 measurements of corneal hysteresis (CH) and corneal resistance factor (CRF).

RESULTS

In a large sample size we observed changes in CH and CRF after refractive surgery procedures (LASIK, UV-A cross-linking, keratoplasty) and in other corneal disorders (keratoconus, corneal dystrophies).

CONCLUSIONS

CRF and CH changes may reflect structural changes of the cornea. Thus, the ORA provides valuable information for a better understanding and characterization of the biomechanical condition of the cornea, especially with regard to diseases such as keratoconus and glaucoma.