The purpose of this study was to evaluate the effect of the size of gold nanoparticles (AuNPs) on Streptococcus mutans (S. mutans) cell attachment and initial biofilm formation. Three different sizes of polyvinyl-pyrrolidone coated AuNPs (5, 10, or 20 nm) were used in this study. The ability of AuNPs to adhere to bacterial cells and the inhibition of biofilm formation were studied through a transmission electron microscope (TEM) based morphological examination and a biofilm formation assay, respectively. Only the 5 nm AuNPs caused a slight inhibition of S. mutans biofilm growth. The 5 nm size AuNPs easily adhered to the bacterial cell surface as observed through TEM; however, the adhesion of 10 nm and 20 nm particles to bacterial cells was low. It is hypothesized that the attachment of AuNPs to S. mutans disrupts the initial biofilm formation.

To accelerate bone formation, we prepared an injectable bone-inducing sol-gel material. Bone morphogenetic protein (BMP) was incorporated into a cross-linkable hyaluronic acid (cHLA) sol with –SH functional groups with and without nano-hydroxyapatite (nHAP). The test cHLA/BMP/nHAP and control cHLA/BMP sols were injected into the back subcutaneous and thigh intra-muscular tissues of 10-week-old male mice (n=10 each), and allowed to gel for 20 min. After feeding for 8 weeks, calcification at the injection sites was evaluated by micro-computed tomography (CT) and histological examination. CT revealed calcification at the sites of test cHLA/BMP/nHAP sol-gel injection in 80–90% of the mice, while control cHLA/BMP sol-gel could not induce any calcification. Histological observations clarified that new bone formed in the calcification zones at both sites by endochondral ossification. It was thought that nHAP functioned as a carrier that retains and slowly release osteo-inductive BMP. The cHLA/BMP/nHAP sol-gel has potential as an injection-type osteo-inductive biomaterial.

Silver nanoclusters (AgNCs) are ultrasmall in size (< 2 nm) and are expected to be an effective antibacterial substance to combat oral infective diseases. In the present study, we synthesized AgNCs (Ag_∼75) as the main component for esthetic and antibacterial application against the caries pathogen Streptococcus mutans. The results showed that AgNCs significantly reduced the turbidity and viability of S. mutans. In addition, the bactericidal effects of AgNCs were confirmed by LIVE/DEAD staining. After AgNC application to human dentin, no discoloration of dentin was observed, as compared to silver diamine fluoride application, and AgNC-treated dentin showed an inhibitory effect on colony formation by S. mutans. Therefore, AgNCs appear to be beneficial for dental therapy as an antibacterial and/or esthetic substrate.

Dental metals have toxic effects on human development, including the high toxicity of the constituent element vanadium of the standard product ASTM F136, i.e., the medical titanium material of the Ti-6Al-4V alloy used in dental implants for its mechanical strength due to the use of a large amount of pure Ti. In addition, the corrosion of pure Ti and Ti alloy implants due to fluorides in the oral cavity is a problem. Elution of the constituent element vanadium due to corrosion may have toxic effects on development. Therefore, each element composing Ti-6Al-4V was examined in an in vitro embryotoxicity test by the Embryonic Stem Cell Test (EST) protocol. As a result, only vanadium had a risk of embryotoxicity. In addition, the Ti-6Al-4V alloy was corroded with hydrofluoric acid to examine its effects on the differentiation of mouse-derived ES and iPS cells, and was found to affect the differentiation of both cell types.

An in vitro test to assess the toxicity levels of chemicals based on the recovery of cell growth in a normal culture environment after transient exposure is used as a cell recovery test. This test is generally conducted in a two-dimensional culture. As human tissues and organs have a complicated three-dimensional structure, cell recovery tests developed in three-dimensional tissue models may be superior to those in two-dimensional cultures.

Fluorides are commonly employed in the dental field to prevent dental caries. However, their effects on human health are attracting social attention. In the present study, KF and NaF were evaluated by comparing the results of cell recovery tests between two- and three-dimensional cultures using mouse-derived ES-D3 and 3T3 cells. As a result, ES-D3 cells exhibited more significant recovery than 3T3 cells after exposure to both chemicals in the three-dimensional culture compared with in the two-dimensional culture.