Joseph T. Walsh

Shear Strength of Composite Bonded to Er:YAG Laser-prepared Dentin

An Er:YAG laser coupled with a cooling stream of water effectively removes dental hard tissues. However, before such a system can be deemed clinically viable, some safety and efficacy issues must be addressed. We compared the bonding of composite to dentin following the preparation of the dentinal surface with either an Er:YAG laser (A = 2.94 pm) or a standard dental bur and with and without a subsequent acid-etching treatment. The crowns of extracted human molars were removed, revealing the underlying dentin. We removed an additional thickness of material with either a dental handpiece or an Er:YAG laser (350 mj/pulse at 6 Hz) by raster-scanning the samples under a fixed handpiece or laser. Comparable surface roughnesses were obtained. Several samples from each group received an acid-conditioning treatment. A cylinder of composite was bonded onto the prepared surfaces. The dentin-composite bond was then shear-stressed to failure on a universal testing apparatus. The results indicate that laser-irradiated samples had improved bond strengths compared with acid-etched and handpiece controls. SEM photographs of the surfaces show exposed tubules following the laser treatment; tubules could also be exposed with acid etching. We conclude that Er:YAG laser preparation of dentin leaves a suitable surface for strong bonding of an applied composite material.

Erbium laser ablation of dental hard tissue : Effect of water cooling

Several lasers have been explored for hard dental tissue applications; used alone they have resulted in potentially harmful temperature increases in the pulp chamber.

Quantitative measurements of linear birefringence during heating of native collagen.

Linear birefringence is an anisotropic property of rat tail tendon, which is largely composed of collagen. Our goal is to show that the dynamic range and sensitivity of the linear birefringence loss of collagen during heating are sufficient for kinetic modeling of the reaction.

Comparison of polarized-light propagation in biological tissue and phantoms.

We demonstrate significant differences in the propagation of polarized light through biological tissue compared with two common tissue phantoms. Depolarization of linearly and circularly polarized light was measured versus propagation distance by use of two independent measurement techniques. The measurements were performed on adipose and myocardial tissues and on tissue phantoms that consisted of polystyrene microsphere suspensions and Intralipid. The results indicate that, in contrast with results obtained in tissue phantoms, linearly polarized light survives through longer propagation distances than circularly polarized light in biological tissue.

Optical Parameter Variability in Laser Nerve Stimulation: A Study of Pulse Duration, Repetition Rate, and Wavelength

Pulsed lasers can evoke neural activity from motor as well as sensory neurons in vivo. Lasers allow more selective spatial resolution of stimulation than the conventional electrical stimulation. To date, few studies have examined pulsed, mid-infrared laser stimulation of nerves and very little of the available optical parameter space has been studied. In this study, a pulsed diode laser, with wavelength between 1.844-1.873 mum, was used to elicit compound action potentials (CAPs) from the auditory system of the gerbil. We found that pulse durations as short as 35 mus elicit a CAP from the cochlea. In addition, repetition rates up to 13Hz can continually stimulate cochlear spiral ganglion cells for extended periods of time. Varying the wavelength and, therefore, the optical penetration depth, allowed different populations of neurons to be stimulated. The technology of optical stimulation could significantly improve cochlear implants, which are hampered by a lack of spatial selectivity

Transcutaneous Glucose Sensing by Surface-Enhanced Spatially Offset Raman Spectroscopy in a Rat Model

This letter presents the first quantitative, in vivo, transcutaneous glucose measurements using surface enhanced Raman spectroscopy (SERS). Silver film over nanosphere (AgFON) surfaces were functionalized with a mixed self-assembled monolayer (SAM) and implanted subcutaneously in a Sprague-Dawley rat. The glucose concentration was monitored in the interstitial fluid. SER spectra were collected from the sensor chip through the skin using spatially offset Raman spectroscopy (SORS). The combination of SERS and SORS is a powerful new approach to the challenging problem of in vivo metabolite and drug sensing.

Laser Stimulation of Auditory Neurons: Effect of Shorter Pulse Duration and Penetration Depth

We have pioneered what we believe is a novel method of stimulating cochlear neurons, using pulsed infrared radiation, based on the hypothesis that optical radiation can provide more spatially selective stimulation of the cochlea than electric current. Very little of the available optical parameter space has been used for optical stimulation of neurons. Here, we use a pulsed diode laser (1.94 microm) to stimulate auditory neurons of the gerbil. Radiant exposures measured at CAP threshold are similar for pulse durations of 5, 10, 30, and 100 micros, but greater for 300-micros-long pulses. There is evidence that water absorption of optical radiation is a significant factor in optical stimulation. Heat-transfer-based analysis of the data indicates that potential structures involved in optical stimulation of cochlear neurons have a dimension on the order of approximately 10 microm. The implications of these data could direct further research and design of an optical cochlear implant.

Comparative study of polarized light propagation in biologic tissues

We report the depolarization of light scattered by a variety of birefringent and nonbirefringent tissues. We used Stokes polarimetry to investigate how scatterer structures in each tissue contribute to the depolarization of linearly versus circularly polarized light propagating through that tissue. Experiments were performed on porcine blood, fat, tendon, artery, and myocardium. The results indicate that the two incident polarization states are depolarized differently depending on the structure of the sample. As seen in sphere suspensions, for tissues containing dilute Mie scatterers, circularly polarized light is maintained preferentially over linearly polarized light. For more dense tissues, however, the reverse is true. The results illustrate situations where polarized light will provide an improvement over unpolarized light imaging, information that is crucial to optimizing existing polarimetric imaging techniques.

Optical stimulation of the facial nerve : A new monitoring technique?

Objectives/ Hypothesis: One sequela of skull base surgery is iatrogenic damage to cranial nerves, which can be prevented if the nerve is identified. Devices that stimulate nerves with electric current assist in nerve identification. Contemporary devices have two main limitations: 1) the physical contact of the stimulating electrode and (2) the spread of the current through the tissue. In contrast to electrical stimulation, pulsed infrared optical radiation can be used to safely and selectively stimulate neural tissue and might be valuable for screening.

Polarized light propagation through tissue phantoms containing densely packed scatterers.

We demonstrate that polarized light is maintained differently in densely packed versus dilute suspensions of polystyrene microspheres. The degrees of linear and circular polarization were measured versus scatterer concentration in aqueous suspensions of 0.48-, 0.99-, 2.092-, and 9.14-mum-diameter polystyrene microspheres. The results indicate that, for dilute suspensions of microspheres where independent scattering is assumed, the degrees of linear and circular polarization decrease as the scatterer concentration increases. For dense suspensions, however, the degree of polarization begins to increase as the scatterer concentration increases. The preferential propagation of linear over circular polarization states in dense suspensions is similar to results seen in biological tissue.