Jing Mao

Limitations in Bonding to Dentin and Experimental Strategies to Prevent Bond Degradation

The limited durability of resin-dentin bonds severely compromises the lifetime of tooth-colored restorations. Bond degradation occurs via hydrolysis of suboptimally polymerized hydrophilic resin components and degradation of water-rich, resin-sparse collagen matrices by matrix metalloproteinases (MMPs) and cysteine cathepsins. This review examined data generated over the past three years on five experimental strategies developed by different research groups for extending the longevity of resin-dentin bonds. They include: (1) increasing the degree of conversion and esterase resistance of hydrophilic adhesives; (2) the use of broad-spectrum inhibitors of collagenolytic enzymes, including novel inhibitor functional groups grafted to methacrylate resins monomers to produce anti-MMP adhesives; (3) the use of cross-linking agents for silencing the activities of MMP and cathepsins that irreversibly alter the 3-D structures of their catalytic/allosteric domains; (4) ethanol wet-bonding with hydrophobic resins to completely replace water from the extrafibrillar and intrafibrillar collagen compartments and immobilize the collagenolytic enzymes; and (5) biomimetic remineralization of the water-filled collagen matrix using analogs of matrix proteins to progressively replace water with intrafibrillar and extrafibrillar apatites to exclude exogenous collagenolytic enzymes and fossilize endogenous collagenolytic enzymes. A combination of several of these strategies should result in overcoming the critical barriers to progress currently encountered in dentin bonding.

Intrafibrillar Collagen Mineralization Produced by Biomimetic Hierarchical Nanoapatite Assembly

Biomineralization templated by non-collagenous proteins is an example how nature uses nanotechnology to strengthen the load-bearing skeletons of vertebrates.[1–3] Mineralized collagen fibrils are responsible for the second level of hierarchy of bone structure[4] and account for its strength and toughness.[5] Calcium-deficient apatites are deposited in nanocrystalline form at the intrafibrillar and extrafibrillar spaces associated with the collagen fibrillar assembly.[6] Regulation of calcium phosphate phases during biomineralization is believed to exhibit two major characteristics.[7–9] The first is the sequestration of the calcium and phosphate ions into nanoscopic entities known as amorphous calcium phosphate (ACP). The second is the templating of mineral nucleation and growth through noncollagenous extracellular matrix proteins. These protein molecules contain polyanionic domains that are rich in polycarboxylic acid and phosphate function groups that bind to the collagen substrates at specific sites such as the gap zones of the collagen molecular assembly.[1,10] The immobilized proteins contain anionic charge sites wherein calcium binding and apatite nucleation occur at the aforementioned intrafibrillar locations. As a highly ordered manner of intrafibrillar mineralization at the nanoscale is responsible for the biomechanical properties of mineralized collagen,[11] this intricate biological process has inspired scientists to mimic its mechanism for creating mineralized type I collagen scaffolds to repair existing bone defects and/or generate new bone. Here, we demonstrate that the use of two phosphate-based templating analogs of matrix proteins in the presence of a polycarboxylic acid sequestration analog of ACPs results in a highly ordered intrafibrillar nanoapatite assembly that recapitulates the gap and overlap arrangement of collagen molecules within a collagen fibril. Adoption of a dual analog biomimetic strategy provides a means to produce advanced mineralized collagen scaffolds for tissue engineering applications.

Biomimetic remineralization of dentin

OBJECTIVES Remineralization of demineralized dentin is important for improving dentin bonding stability and controlling primary and secondary caries. Nevertheless, conventional dentin remineralization strategy is not suitable for remineralizing completely demineralized dentin within hybrid layers created by etch-and-rinse and moderately aggressive self-etch adhesive systems, or the superficial part of a caries-affected dentin lesion left behind after minimally invasive caries removal. Biomimetic remineralization represents a different approach to this problem by attempting to backfill the demineralized dentin collagen with liquid-like amorphous calcium phosphate nanoprecursor particles that are stabilized by biomimetic analogs of noncollagenous proteins. METHODS This paper reviewed the changing concepts in calcium phosphate mineralization of fibrillar collagen, including the recently discovered, non-classical particle-based crystallization concept, formation of polymer-induced liquid-precursors (PILP), experimental collagen models for mineralization, and the need for using phosphate-containing biomimetic analogs for biomimetic mineralization of collagen. Published work on the remineralization of resin-dentin bonds and artificial caries-like lesions by various research groups was then reviewed. Finally, the problems and progress associated with the translation of a scientifically sound concept into a clinically applicable approach are discussed. RESULTS AND SIGNIFICANCE The particle-based biomimetic remineralization strategy based on the PILP process demonstrates great potential in remineralizing faulty hybrid layers or caries-like dentin. Based on this concept, research in the development of more clinically feasible dentin remineralization strategy, such as incorporating poly(anionic) acid-stabilized amorphous calcium phosphate nanoprecursor-containing mesoporous silica nanofillers in dentin adhesives, may provide a promising strategy for increasing of the durability of resin-dentin bonding and remineralizing caries-affected dentin.

Differences between top-down and bottom-up approaches in mineralizing thick, partially demineralized collagen scaffolds

Biominerals exhibit complex hierarchical structures derived from bottom-up self-assembly mechanisms. Type I collagen serves as the building block for mineralized tissues such as bone and dentin. In the present study, 250-300 μm thick, partially demineralized collagen scaffolds exhibiting a gradient of demineralization from the base to surface were mineralized using a classical top-down approach and a non-classical bottom-up approach. The top-down approach involved epitaxial growth over seed crystallites. The bottom-up approach utilized biomimetic analogs of matrix proteins to stabilize amorphous calcium phosphate nanoprecursors and template apatite nucleation and growth within the collagen matrix. Micro-computed tomography and transmission electron microscopy were employed to examine mineral uptake and apatite arrangement within the mineralized collagen matrix. The top-down approach could mineralize only the base of the partially demineralized scaffold, where remnant seed crystallites were abundant. Minimal mineralization was observed along the surface of the scaffold; extrafibrillar mineralization was predominantly observed. Conversely, the entire partially demineralized scaffold, including apatite-depleted collagen fibrils, was mineralized by the bottom-up approach, with evidence of both intrafibrillar and extrafibrillar mineralization. Understanding the different mechanisms involved in these two mineralization approaches is pivotal in adopting the optimum strategy for fabricating novel nanostructured materials in bioengineering research.

Quaternary ammonium silane-functionalized, methacrylate resin composition with antimicrobial activities and self-repair potential.

The design of antimicrobial polymers to address healthcare issues and minimize environmental problems is an important endeavor with both fundamental and practical implications. Quaternary ammonium silane-functionalized methacrylate (QAMS) represents an example of antimicrobial macromonomers synthesized by a sol-gel chemical route; these compounds possess flexible Si-O-Si bonds. In present work, a partially hydrolyzed QAMS co-polymerized with 2,2-[4(2-hydroxy 3-methacryloxypropoxy)-phenyl]propane is introduced. This methacrylate resin was shown to possess desirable mechanical properties with both a high degree of conversion and minimal polymerization shrinkage. The kill-on-contact microbiocidal activities of this resin were demonstrated using single-species biofilms of Streptococcus mutans (ATCC 36558), Actinomyces naeslundii (ATCC 12104) and Candida albicans (ATCC 90028). Improved mechanical properties after hydration provided the proof-of-concept that QAMS-incorporated resin exhibits self-repair potential via water-induced condensation of organic modified silicate (ormosil) phases within the polymerized resin matrix.

Effects of an Experimental Calcium Aluminosilicate Cement on the Viability of Murine Odontoblast-like Cells

INTRODUCTION Quick-setting calcium aluminosilicate cement with improved washout resistance is a potential substitute for calcium silicate cements in endodontics. This study examined the effect of an experimental calcium aluminosilicate cement (Quick-Set; Primus Consulting, Bradenton, FL) on the viability of odontoblast-like cells. METHODS The biocompatibility of Quick-Set and white ProRoot MTA (WMTA; Dentsply Tulsa Dental Specialties, Tulsa, OK) cements and their eluents was evaluated using a murine dental papilla-derived odontoblast-like cell line (MDPC-23); 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay was used to examine the effects of the 2 hydraulic cements on mitochondrial metabolic activity. Flow cytometry and confocal laser scanning microscopy were used to identify the effects of the 2 cements on cell death-induced plasma membrane permeability to fluorescent dyes and DNA stains. RESULTS After the first week of immersion in culture medium, Quick-Set and WMTA were more cytotoxic than the Teflon-negative control (P < .05), and the cells exhibited more apoptosis/necrosis than Teflon (P < .05). After the second week of immersion, the 2 cements were as biocompatible as Teflon (P > .05), with cells exhibiting minimal apoptosis/necrosis. Eluents from the set cements at 1:1 dilution were significantly more cytotoxic that eluents at 1:10 or 1:100 dilution (P < .05). CONCLUSIONS Quick-Set and WMTA exhibited similar cytotoxicity profiles. They possess negligible in vitro toxicologic risks after time-dependent elution of toxic components.

An ORMOSIL-containing orthodontic acrylic resin with concomitant improvements in antimicrobial and fracture toughness properties

Global increase in patients seeking orthodontic treatment creates a demand for the use of acrylic resins in removable appliances and retainers. Orthodontic removable appliance wearers have a higher risk of oral infections that are caused by the formation of bacterial and fungal biofilms on the appliance surface. Here, we present the synthetic route for an antibacterial and antifungal organically-modified silicate (ORMOSIL) that has multiple methacryloloxy functionalities attached to a siloxane backbone (quaternary ammonium methacryloxy silicate, or QAMS). By dissolving the water-insoluble, rubbery ORMOSIL in methyl methacrylate, QAMS may be copolymerized with polymethyl methacrylate, and covalently incorporated in the pressure-processed acrylic resin. The latter demonstrated a predominantly contact-killing effect on Streptococcus mutans ATCC 36558 and Actinomyces naselundii ATCC 12104 biofilms, while inhibiting adhesion of Candida albicans ATCC 90028 on the acrylic surface. Apart from its favorable antimicrobial activities, QAMS-containing acrylic resins exhibited decreased water wettability and improved toughness, without adversely affecting the flexural strength and modulus, water sorption and solubility, when compared with QAMS-free acrylic resin. The covalently bound, antimicrobial orthodontic acrylic resin with improved toughness represents advancement over other experimental antimicrobial acrylic resin formulations, in its potential to simultaneously prevent oral infections during appliance wear, and improve the fracture resistance of those appliances.

Third-molar development in relation to chronologic age in young adults of central China

SummaryThe estimation of chronologic age based on the stages of third-molar development was evaluated by using the eight stages (A-H) method of Demirjian and the third-molar development was compared, in terms of sex and age, with results of previous studies. The samples consisted of 291 orthopantomograms from young Chinese subjects of known chronologic age and sex (including 139 males with a mean age of 14.67±3.62 y and 152 females with a mean age of 14.85±3.70 y). Statistical analysis was performed by employing the Mann-Whitney U-test and the t-test. Regression analysis was conducted to obtain regression formulas for calculating dental age from the chronologic age. Our results showed statistically significant differences (P<0.05) in third-molar development between males and females, at the calcification stages D, E and H. And a strong correlation was found between age and third-molar development in both males (r2=0.65) and females (r2=0.61). New equations (Age=8.76+1.32 Development stage) for estimating chronologic age were derived. It is concluded that third-molar genesis took place earlier in males than in females. The use of third molars as a developmental marker is appropriate in young adults of Central China. The formula obtained in the present study can be used as a guide for estimation of dental maturity and a standard for age estimation for young adults of Central China.The estimation of chronologic age based on the stages of third-molar development was evaluated by using the eight stages (A-H) method of Demirjian and the third-molar development was compared, in terms of sex and age, with results of previous studies. The samples consisted of 291 orthopantomograms from young Chinese subjects of known chronologic age and sex (including 139 males with a mean age of 14.67±3.62 y and 152 females with a mean age of 14.85±3.70 y). Statistical analysis was performed by employing the Mann-Whitney U-test and the t-test. Regression analysis was conducted to obtain regression formulas for calculating dental age from the chronologic age. Our results showed statistically significant differences (P<0.05) in third-molar development between males and females, at the calcification stages D, E and H. And a strong correlation was found between age and third-molar development in both males (r 2=0.65) and females (r 2=0.61). New equations (Age=8.76+1.32 Development stage) for estimating chronologic age were derived. It is concluded that third-molar genesis took place earlier in males than in females. The use of third molars as a developmental marker is appropriate in young adults of Central China. The formula obtained in the present study can be used as a guide for estimation of dental maturity and a standard for age estimation for young adults of Central China.

Effect of water-aging on the antimicrobial activities of an ORMOSIL-containing orthodontic acrylic resin.

Quaternary ammonium methacryloxy silicate (QAMS), an organically modified silicate (ORMOSIL) functionalized with polymerizable methacrylate groups and an antimicrobial agent with a long lipophilic alkyl chain quaternary ammonium group, was synthesized through a silane-based sol-gel route. By dissolving QAMS in methyl methacrylate monomer, this ORMOSIL molecule was incorporated into an auto-polymerizing, powder/liquid orthodontic acrylic resin system, yielding QAMS-containing poly(methyl methacrylate). The QAMS-containing acrylic resin showed a predominant contact-killing effect on Streptococcus mutans (ATCC 35668) and Actinomyces naeslundii (ATCC 12104) biofilms, while inhibiting adhesion of Candida albicans (ATCC 90028) on the acrylic surface. The antimicrobial activities of QAMS-containing acrylic resin were maintained after a 3month water-aging period. Bromophenol blue assay showed minimal leaching of quaternary ammonium species when an appropriate amount of QAMS (<4wt.%) was incorporated into the acrylic resin. The results suggest that QAMS is predominantly co-polymerized with the poly(methyl methacrylate) network, and only a minuscule amount of free QAMS molecules is present within the polymer network after water-aging. Acrylic resin with persistent antimicrobial activities represents a promising method for preventing bacteria- and fungus-induced stomatitis, an infectious disease commonly associated with the wearing of removable orthodontic appliances.

Peptide aptamers against titanium-based implants identified through phage display

Commercially pure titanium (cp-Ti) is widely used in the field of long-term clinical oral implantology owing to its ability to allow close bone-implant apposition. The optimization of its function based on artificial proteins has become a key issue in the development of improved cp-Ti implants. Here, we set out to identify peptide aptamers with preferential adsorption towards titanium-based implants through the phage display methodology. Fifteen sequences were selected in the third round of biopanning. One sequence, ATWVSPY (named TBP1), had a 40% repetition rate and exhibited the strongest binding affinity to cp-Ti disks. Ten sequences were selected in the fourth round, among which the repetition rate is 80% for TBP1 and 20% for TBP2 (GVGLPHT). The peptide aptamers against cp-Ti disks can provide an alternative method of functional coating for biomaterial surfaces.