Spectroscopic Imaging of Biointerfaces
Work from Dr. Spencer’s laboratories provided the first molecular structural analysis of acid-etched smear layers (Spencer et al. 2001; Wang and Spencer 2002). This work represented the first study to quantify dentin demineralization under conditions that permit hydration of the specimen throughout the analysis. Hydration is critical to these efforts since it is widely accepted that the collagen within the demineralized dentin will collapse if it is allowed to dry; such collapse would lead to inaccurate characterization of the extent of degree of dentin demineralization. The micro-Raman spectral data indicate that 15 sec of acid-etching with 35% phosphoric acid gel demineralized dentin to a depth of ~ 10 micrometers. The spectral results presented in this study indicated that collagen within the smear layer is disorganized but not denatured. This disorganized collagen is denatured by the 15-second acid-treatment used in this study.
Development of Non-destructive Techniques for Analyzing Material/Tissue Interfaces
Because of the non-destructive nature of the analytical characterization techniques, the same specimen and the same small region of the specimen was analyzed using both scanning acoustic microscopy (SAM) and micro-Raman spectroscopy (µRS). Thus, the structure as determined by measurement of the molecular features can be related directly to the acoustic impedance (modulus of elasticity) within the same small region of the sample. These complementary techniques allow us to relate differences in the micro-mechanical properties -specifically the modulus of elasticity- to the molecular structure within the region analyzed.
Multi-scale Characterization and Modeling of Tissues, Materials and Tissue/Material Interfaces
In the exploration of new biomaterials, one area that has been largely overlooked is chemical and mechanical characterization of the material/tissue interface. This is a particularly challenging area of investigation since many of the current analytical techniques do not offer the required spatial resolution to study reactions occurring at the interface of the conditions, i.e. temperature, vacuum, etc. under which the sample must be analyzed destroy or significantly damage/alter the biologic tissue. To address these problems, we have developed nondestructive techniques to characterize and quantify reactions at the material/tissue interface.
Multi-Scale Computational Modeling of Hierarchical Biologic Tissues
While there may be as many as five or six distinct length scales in the hierarchical structural architectures of native tissues and material/tissue interfaces, the influence of small features in the hierarchy on the overall mechanical properties is not well understood. The generation of mechanistic models that relate global mechanical response to initial degradation requires the development of experimental techniques that allow the measurement of mechanical properties of small substructures, e.g. in the range of 10-1000nm. In addition, mathematical models that meaningfully use the measurements at small scales to predict behavior at larger scales are also not widely available. The combination of experimental measurements with the modeling is expected to provide insights beyond what could be accomplished if either of the approaches were applied independently. Moreover, the combination of experimental and modeling techniques will help alleviate the typical disadvantage of mathematical models that stems from the less-than-perfect empirical information available to make the models realistic. The multi-scale modeling approach offers the advantage that the parameters that cannot be easily modified in the laboratory may be easily varied in the models and the models may be exercised for a variety of conditions.
Synthesis of Monomers and Evaluation of Dentin Adhesives
The lifetime of methacrylate adhesives in the mouth has fallen far short of expectations. Though many factors may contribute to the premature breakdown of these adhesives, their chemical “Achilles heel” may prove to be the ester linkages in the methacrylate matrix since these are susceptible to attack by water and esterases. Our group has designed and synthesized new monomers, and characterized and evaluated the interface and the enzymatic degradation of dentin adhesives containing new monomer.
Photoacoustic Imaging of Biological Tissue
Photoacoustic imaging (PAI) is a novel, hybrid, and nonionizing imaging modality that combines the meritsof both optical and ultrasonic imaging methods. It is highly sensitive to the optical absorption of biological tissue. PAI provides greater spatial resolution than purely optical imaging in deep regions while simultaneously overcoming the disadvantages of ultrasonic imaging regarding both biochemical contrast and speckle artifact. PAI can provide high spatial resolution images with optical contrast in a region up to 5 cm deep in biological tissue, whereas purely optical imaging techniques cannot provide high spatial resolution in regions beyond the quasiballistic regime (1 mm deep) because of the strong scattering in biological tissues. With PAI, we can image the structure of biological tissue with optical contrast, detect functional changes (through measuring blood oxygenation, blood volume) in brains, and interrogate tissue molecular information (molecular imaging).