Exogenous mineralization of hard tissues using photo-absorptive minerals and femto-second lasers; the case of dental enamel.

A radical new methodology for the exogenous mineralization of hard tissues is demonstrated in the context of laser-biomaterials interaction. The proposed approach is based on the use of femtosecond pulsed lasers (fs) and Fe3+-doped calcium phosphate minerals (specifically in this work fluorapatite powder containing Fe2O3 nanoparticles (NP)). A layer of the synthetic powder is applied to the surface of eroded bovine enamel and is irradiated with a fs laser (1040 nm wavelength, 1 GHz repetition rate, 150 fs pulse duration and 0.4 W average power). The Fe2O3 NPs absorb the light and may act as thermal antennae, dissipating energy to the vicinal mineral phase. Such a photothermal process triggers the sintering and densification of the surrounding calcium phosphate crystals thereby forming a new, dense layer of typically ∼20 µm in thickness, which is bonded to the underlying surface of the natural enamel. The dispersed iron oxide NPs, ensure the localization of temperature excursion, minimizing collateral thermal damage to the surrounding natural tissue during laser irradiation. Simulated brushing trials (pH cycle and mechanical force) on the synthetic layer show that the sintered material is more acid resistant than the natural mineral of enamel. Furthermore, nano-indentation confirms that the hardness and Young's modulus of the new layers are significantly more closely matched to enamel than current restorative materials used in clinical dentistry. Although the results presented herein are exemplified in the context of bovine enamel restoration, the methodology may be more widely applicable to human enamel and other hard-tissue regenerative engineering. STATEMENT OF SIGNIFICANCE: In this work we provide a new methodology for the mineralisation of dental hard tissues using femtosecond lasers and iron doped biomaterials. In particular, we demonstrate selective laser sintering of an iron doped fluorapatite on the surface of eroded enamel under low average power and mid-IR wavelength and the formation of a new layer to substitute the removed material. The new layer is evaluated through simulated brushing trials and nano-indentation. From the results we can conclude that is more acid resistant than natural enamel while, its mechanical properties are superior to that of current restorative materials. To the best of our knowledge this is the first time that someone demonstrated, laser sintering and bonding of calcium phosphate biomaterials on hard tissues. Although we here we discuss the case of dental enamel, similar approach can be adopted for other hard tissues, leading to new strategies for the fixation of bone/tooth defects.

ß-pyrophosphate: A potential biomaterial for dental applications.

Tooth hypersensitivity is a growing problem affecting both the young and ageing population worldwide. Since an effective and permanent solution is not yet available, we propose a new methodology for the restoration of dental enamel using femtosecond lasers and novel calcium phosphate biomaterials. During this procedure the irradiated mineral transforms into a densified layer of acid resistant iron doped ß-pyrophosphate, bonded with the surface of eroded enamel. Our aim therefore is to evaluate this densified mineral as a potential replacement material for dental hard tissue. To this end, we have tested the hardness of ß-pyrophosphate pellets (sintered at 1000°C) and its mineral precursor (brushite), the wear rate during simulated tooth-brushing trials and the cytocompatibility of these minerals in powder form. It was found that the hardness of the ß-pyrophosphate pellets is comparable with that of dental enamel and significantly higher than dentine while, the brushing trials prove that the wear rate of ß-pyrophosphate is much slower than that of natural enamel. Finally, cytotoxicity and genotoxicity tests suggest that iron doped ß-pyrophosphate is cytocompatible and therefore could be used in dental applications. Taken together and with the previously reported results on laser irradiation of these materials we conclude that iron doped ß-pyrophosphate may be a promising material for restoring acid eroded and worn enamel.

Observation of a cascaded process in intracavity terahertz optical parametric oscillators based on lithium niobate.

Cascaded difference frequency generation has been observed in intracavity optical parametric oscillators based on bulk lithium niobate and producing nanosecond pulses of terahertz radiation. Two idler waves are generated, namely: the primary idler wave associated with the parametric down conversion process itself; and a secondary idler wave, due to difference frequency generation. Experimental investigations of the frequency, temporal evolution, propagation direction, intensity, phase matching and oscillation threshold of the generated down-converted waves are reported. The overall generation efficiency for the terahertz radiation is enhanced, thereby overcoming the Manley-Rowe limit. Advantages of the present approach over schemes based on periodically poled lithium niobate are identified.

The effect of Coumadin upon thrombus forming on foreign surfaces.

Experience with the Stagnation Point Flow Experiment has revealed the earliest microscopic events surrounding thrombus inception and growth on foreign surfaces in a perfectly defined flow field. Using 40 and 80 X magnification, the effects of Coumadin upon these events was studied. These events include leukocyte adherence, fibrin deposition and platelet aggregation, deviation of flow streamlines around these aggregates, and subsequent downstream thrombus growth. Surface density of leukocytes was measured as a function of time and shear, and served as an index of the quantity of thrombus on the surface, which in turn, allowed direct comparisons of thrombus growth rate between experiments. Coumadin decreases the adhesive force between leukocytes or thrombi and the foreign surface. Emboli are more frequent with Coumadin, but are smaller. Thrombus inception is not altered. These results suggest that clinical benefit obtained from use of Coumadin derives from reduction in size of emboli.

A clinical evaluation of impedance rheography in peripheral arterial disease.

The impedance rheograph has been shown to give values of blood flow consistent with a history of vascular insufficiency; normal limbs having a mean flow of 6.9 +/- 1.6 milliliters per minute per 100 milliliters of limb volume and diseased limbs having a proportionately reduced flow. The most reduced flow was found in those with rest pain or severe claudication, 1.7 +/- 0.8 milliliters per minute per 100 milliliters of limb volume. It is suggested that this technique may be useful clinically in assessing patients before treatment and, subsequently, as an objective evaluation of that treatment.

An experimental analysis of some procedures to teach priming and reinforcement skills to preschool teachers.

This Monograph reports the results of teaching preschool teachers to be successful at increasing desired behaviors of their children, thus becoming successful teachers. Five teacher-training techniques were examined experimentally under single-subject designs: written assignments, feedback from viewing graphs, on-the-spot feedback from a wireless radio (Bug-in-the-Ear), feedback from an observer, and self-counting. Those teaching procedures that included prompt and frequent information to the teacher about the behavior under study were the most effective techniques. Self-counting, in which the teacher tallied the number of times she emitted the behavior of either priming or reinforcing social or verbal behavior of a child (or children), and observer feedback, in which the observer reported to the teacher periodically during the hour the frequency of her behavior, were the most reliable teaching techniques. The other procedures, while less reliable than self-counting and observer feedback, were effective with some teachers. Maintenance of teacher behavior across settings was examined with a group of Head Start teachers, and maintenance of teacher behaviors across different child behaviors and different children was examined with three student teachers. The results indicated that teaching was more likely to maintain if it occurred in the teacher's home setting rather than at another site. In all cases, when generalization occurred across settings, time, or children, the frequency of the teacher's behavior was not as high as when the relevant behavior had been trained directly. Results supported the proposal that it is possible to define effective teacher behavior, not just characterize it, as it occurs in the classroom, and that effectiveness can be measured by defining and observing the child behaviors to which teacher behaviors are directed.

The experimental modification of teacher attending behavior.

A method of observing and modifying teacher attention to appropriate child responses in preschool classrooms was developed. Two teachers with no formal training in reinforcement principles were observed for a baseline of eight days. Teacher A, who displayed a lower baseline rate of attending to appropriate child responses, was trained first. Teacher B was simply observed during the first part of the training condition for Teacher A. During training, A received feedback which included definitions of appropriate child responses, her frequency of attending to appropriate child responses, her total percentage of attending to appropriate child responses, and her frequency of failing to attend to appropriate child responses. Teacher B was then trained in a similar way. Both teachers showed an increase in attending to appropriate child responses subsequent to the onset of experimental feedback.