Unravelling intrinsic descriptors based on a two-stage activity regulation of bimetallic 2D c-MOFs for CO2RR.

The bimetallic 2D conductive MOFs of M1Pc-M2-O, possessing dual metal sites to realize flexible molecular-level structural modification, are brilliant catalysts for electrochemical CO2 reduction. However, the bimetallic centers bring about the complex regulatory mechanism of catalytic activity and obscure principles for catalyst design. Herein, systematical theoretical investigation unravels intrinsic descriptors to design favorable M1Pc-M2-O catalysts based on the discovered coarse-fine two-stage activity regulation mechanism. The reaction site controls the M-COOH distance of the key intermediate and therefore affects the reaction kinetics for the first stage of coarse regulation. The other metal site influents the d-band center of the reaction site and thus constitutes the second stage of fine regulation. The coarse and fine regulation are related to the valence electrons (V), electronegativity (E), and bond length (LM-N/O) between the metal and coordination atoms. The intrinsic descriptor ϕ = (4 × VM1 × (EM1 + EN/O)/EN/O + VM2 × (EM2 + EN/O)/EN/O) × LM1-N/O (with a coefficient ratio of 4 : 1) was eventually established and correlated well with the reported experiments. On this basis, the favorable catalysts CoPc-Zn-O and CoPc-Co-O were located. The research results could contribute to the diversity of bimetallic 2D c-MOFs in CO2RR.

Outcome of Different Endometrial Preparation Protocols Prior to Frozen-Thawed Embryo Transfer on Pregnancy Outcomes in Women with Repeated Implantation Failure.

Aim: To compare the pregnancy outcomes of frozen-thawed embryo transfer (FET) cycles among women with repeated implantation failure (RIF) treated with various endometrial preparation protocols. Methods: A total of 605 women with RIF were retrospectively recruited between January 2017 and December 2020 from Northern Theater General Hospital. Patients were divided into natural cycles, hormone replacement therapy (HRT) cycles, depot gonadotropin-releasing hormone (GnRH) agonist-HRT, and endometrial scratching (ES) plus depot GnRH agonist-HRT. The primary endpoint was clinical pregnancy rate, while secondary endpoints included live birth rate and pain assessment. Results: Of the 605 recruited patients, 63 were undergoing natural cycles, 281 were treated with HRT cycles, 141 treated with depot GnRH agonist-HRT, and 120 treated with ES combined with depot GnRH agonist-HRT. There were significant differences among protocols on clinical pregnancy rate (P=0.029), while no significant difference was observed among protocols on live birth rates (P=0.108). Multivariate analyses suggested that HRT (odds ratio [OR]: 0.50; 95% confidence interval [CI]: 0.28-0.89; P=0.019) and depot GnRH agonist-HRT (OR: 0.49; 95% CI: 0.27-0.91; P=0.021) cycles were associated with a lower clinical pregnancy rate as compared with natural cycles, while no significant difference between ES combined with depot GnRH agonist-HRT and natural cycles for clinical pregnancy rates (OR: 0.72; 95% CI: 0.38-1.36; P=0.313). Moreover, the HRT (OR: 0.70; 95% CI: 0.39-1.28; P=0.239), depot GnRH agonist-HRT (OR: 0.67; 95% CI: 0.35-1.29; P=0.229), and ES combined with depot GnRH agonist-HRT (OR: 1.11; 95% CI: 0.58-2.14; P=0.754) cycles had no significant effects on live birth rate as compared with natural cycles. A total of 87.50% patients treated with ES combined with depot GnRH agonist-HRT reported pain during the procedure. Conclusion: ES and depot GnRH agonists could be considered for RIF women with high-quality blastocysts, 14 days after verified transplantation failure.

[Determination of heavy metal by AAS in railway rock-cut slope soil].

Heavy metal contents in railway rock-cut slope soil have directly influenced ecosystem on rock-cut slope and eco-envi- ronment safety of farmland nearby. In the study heavy metal Pb, Cd, Zn, Cu and Mn was determined by AAS in railway rock-cut slope and control soil samples on Cheng-Da Railway crossing purple soil in Sichuan province. The results showed that Pb and Mn were significantly higher in rock-cut soil than in control soil, that is 29.7%-35.4%, while Cd, Zn and Cu were similar in both soils.

Application of digital positioning guide plates for the surgical extraction of multiple impacted supernumerary teeth: A case report and review of literature.

BACKGROUND: An extra tooth in the normal tooth sequence in any region of the dental arch is regarded as a supernumerary tooth (SNT). Due to the large variation in location and morphology, the extraction of impacted SNTs is an extensive and complex procedure with high risks of several complications. This report presents a rare case of seven impacted SNTs in the bilateral upper and lower arch that were successfully extracted with the use of digital positioning guide plates. CASE SUMMARY: In January 2022, a 21-year-old male was referred to our department with a chief complaint of pain in relation to tooth #36. Clinical examination showed a deep carious lesion with pulpal involvement in tooth #36 and lingual swelling of the bilateral mandibular posterior area. Radiographic examination revealed seven deeply impacted SNTs in the bilateral posterior area and bilateral impacted mandibular third molars. Based on these findings, the patient was diagnosed with bilateral, multiple impacted SNTs and tooth #36 chronic pulpitis. A root canal treatment and an all-ceramic crown restoration for tooth #36 were performed. An individualized digital positioning guide plate was designed by computer-aided design/computer-aided manufacturing technology and cone-beam computed tomography for extraction of the impacted SNTs. During the operation, the digital positioning guide plate allowed rapid positioning and exposure of the SNTs while avoiding adjacent important anatomical structures. At 3-month follow-up, regeneration of bone and soft tissues was visible. CONCLUSION: The application of digital positioning guide plates is useful for the individualized and minimalized extraction of impacted supernumerary teeth.

Upregulation of mitochondrial dynamics is responsible for osteogenic differentiation of mesenchymal stem cells cultured on self-mineralized collagen membranes.

Collagen membranes crosslinked with high molecular weight polyacrylic acid (HPAA) are capable of self-mineralization via in situ intrafibrillar mineralization. These HPAA-crosslinked collagen membranes (HCM) have been shown to promote osteogenic differentiation of mesenchymal stem cells (MSCs) and enhance bone regeneration in vivo. Nevertheless, the biological triggers involved in those processes and the associated mechanisms are not known. Here, we identified the contribution of mitochondrial dynamics in HCM-mediated osteogenic differentiation of MSCs. Mitochondriogenesis markers were significantly upregulated when MSCs were cultured on HCM, committing the MSCs to osteogenic differentiation. The mitochondria fused to form an interconnected mitochondrial network in response to the high energy requirements. Mitochondrial fission in MSCs was also triggered by HCM; fission slightly declined at 14 days to restore the equilibrium in mitochondrial dynamics. Mitophagy, another event that regulates mitochondrial dynamics, occurred actively to remove dysfunctioned mitochondria and isolate damaged mitochondria from the rest of network. The mitophagy level of MSCs was significantly elevated in the presence of HCM. Taken together, the present findings indicate that upregulation of mitochondrial dynamics via mitochondriogenesis, fusion, fission and mitophagy is responsible for HCM-mediated osteogenic differentiation of MSCs. STATEMENT OF SIGNIFICANCE: High molecular weight polyacrylic acid (HPAA)-crosslinked collagen membrane (HCM) was found to promote in-situ bone regeneration because of it can stimulate osteogenic differentiation of mesenchymal stem cells (MSCs). Nevertheless, the biological triggers involved in those processes and associated mechanisms are not known. This study identifies that activation of mitochondrial dynamics is centrally involved in HCM-mediated osteogenic differentiation of MSCs. The HCM accelerates mitochondriogenesis and regulates homeostasis of the mitochondrial network in response to the increased energy demand for osteogenic differentiation. Concomitantly, mitophagy actively occurs to remove dysfunctioned mitochondria from the rest of the mitochondrial network. Identification of the involvement of mitophagy in HCM-mediated osteogenic differentiation of MSCs opens new vistas in the application of biomimetic mineralization in bone tissue regeneration.

Corrigendum to "N-Acetyl Cysteine as a Novel Polymethyl Methacrylate Resin Component: Protection against Cell Apoptosis and Genotoxicity".

[This corrects the article DOI: 10.1155/2019/1301736.].

N-Acetyl Cysteine as a Novel Polymethyl Methacrylate Resin Component: Protection against Cell Apoptosis and Genotoxicity.

The present study investigated the antiapoptotic and antigenotoxic capabilities of N-acetyl cysteine- (NAC-) containing polymethyl methacrylate (PMMA) resin. An in vitro Transwell insert model was used to mimic the clinical provisional restorations placed on vital teeth. Various parameters associated with cell apoptosis and genotoxicity were investigated to obtain a deeper insight into the underlying mechanisms. The exposure of human dental pulp cell (hDPC) cultures to the PMMA resin (Unifast Trad™) resulted in a rapid increase in reactive oxygen species (ROS) level beginning at 1 h, which was followed by time-dependent cell detachment and overt death. The formation of γ-H2AX and cell cycle G1 phase arrest indicated that oxidative DNA damage occurred as a result of the interactions between DNA bases and ROS, beyond the capacities of cellular redox regulation. Such oxidative DNA damage triggers the activation of p53 via the ataxia telangiectasia mutated (ATM) signaling pathway and the induction of intrinsic mitochondrial apoptosis. Oxidative stress, cell apoptosis, and DNA damage induced by the PMMA resin were recovered to almost the level of untreated controls by the incorporation of NAC. The results indicate that the PMMA resin induced the intrinsic mitochondrial apoptosis as a consequence of p53 activation via the ATM pathway in response to oxidative DNA damage. More importantly, the incorporation of NAC as a novel component into the Unifast Trad™ PMMA resin offers protective effects against cell apoptosis and genotoxicity. This procedure represents a beneficial strategy for developing more biocompatible PMMA-based resin materials.

Antibacterial effects of three experimental quaternary ammonium salt (QAS) monomers on bacteria associated with oral infections.

The aim of this study was to test the antibacterial effects of three experimental quaternary ammonium salt monomers in order to evaluate their potential applications as dental materials. In vitro susceptibility testing of the monomers was performed by the broth dilution method on bacteria associated with oral infections: Streptococcus mutans ATCC 25175, Actinomyces viscosus ATCC 15987, Staphylococcus aureus ATCC 29213 and Lactobacillus casei ATCC 393. The time-kill kinetics of the monomer with relatively higher antibacterial activity against S. mutans were also investigated. It was found that all the tested bacteria strains were susceptible to the three monomers, among which methacryloxylethyl cetyl ammonium chloride (DMAE-CB) exhibited the lowest minimal inhibitory concentrations, ranging from 1.2 to 4.8 microg/ml. The time-kill curve showed that DMAE-CB achieved 99.44% killing at 19.2 microg/ml (4 times the minimal bactericidal concentration) against S. mutans after 1 min and 100% killing within 10 min of contact. This result indicates that the quaternary ammonium salt monomer DMAE-CB may be a candidate antibacterial agent for incorporation into dental restorative materials.

Methyl 3-(4-{6-methyl-4-[3-(trifluoro-meth-yl)phen-yl]pyridazin-3-yl-oxy}phen-yl)propanoate.

In the title compound, C(22)H(19)F(3)N(2)O(3), the benzene rings of the trifluoro-methyl-phenyl and benzoyl-phenyl groups form dihedral angles of 41.89 (10) and 67.44 (10)°, respectively, with the pyridazine ring. The methyl-propanoate group is nearly coplanar with the attached benzene ring [dihedral angle = 3.9 (2)°]. The trifluoro-methyl group is disordered over two positions; the site-occupancy factors are ca 0.64 and 0.36. In the crystal structure, inversion-related mol-ecules are linked through C-H⋯O hydrogen bonds and C-H⋯π inter-actions.

3-[(Hydr-oxy)(4-isoprop-oxy-2-methoxy-phen-yl)methyl-ene]-1-isopropyl-pyrrol-idine-2,4-dione.

The title compound, C(18)H(23)NO(5), a potential herbicide, has an enol group that is intra-molecularly hydrogen bonded to a keto O atom. The dihedral angle between the six-membered ring formed by the enol group and the aromatic benzene ring is 53.35 (6)°.

[Latest Research Advance of Myeloproliferative Diseases Related Genes-Review].

JAK2, MPL and CALR gene mutations play an important role in the onset of myeloproliferative disease(MPD). The latest researches show that the difference of ATP binding ability between the wild type JAK2 protein and mutated JAK2 protein can help us understand the pathogenesis of the MPD further, and the clinical manifestation is related to the mutation burden of the JAK2. In some ET and PMF patients, research find the expression of MPL mutation, which can affects the progress of the disease by collaborating with the JAK2 mutation. CALR mutation is a gene related with the MPD that has been found recently. The pathogenesis of the CALR is similar to that of the JAK2, while there are some features in clinical manifestation comparing with the other mutations.

The antifungal effects and mechanical properties of silver bromide/cationic polymer nano-composite-modified Poly-methyl methacrylate-based dental resin.

Poly-methyl methacrylate (PMMA)-based dental resins with strong and long-lasting antifungal properties are critical for the prevention of denture stomatitis. This study evaluated the antifungal effects on Candida albicans ATCC90028, the cytotoxicity toward human dental pulp cells (HDPCs), and the mechanical properties of a silver bromide/cationic polymer nano-composite (AgBr/NPVP)-modified PMMA-based dental resin. AgBr/NPVP was added to the PMMA resin at 0.1, 0.2, and 0.3 wt%, and PMMA resin without AgBr/NPVP served as the control. Fungal growth was inhibited on the AgBr/NPVP-modified PMMA resin compared to the control (P < 0.05), and the antifungal activity increased as the incorporation of the AgBr/NPVP antimicrobial composite increased. Confocal laser scanning microscopy (CLSM) showed that the number of fungal cells attached to the modified PMMA resin was considerably lower than in the control. The relative growth rate of HDPCs of modified groups were higher than 75%. The flexural strength (FS) and flexural modulus (FM) were not significantly different (P > 0.05) between the experimental and control groups. These data indicate that the incorporation of AgBr/NPVP conferred strong and long-lasting antifungal effects against Candida albicans to the PMMA resin, and it has low toxicity toward HDPCs, and its mechanical properties were not significantly affected.

Antibacterial activity of a modified unfilled resin containing a novel polymerizable quaternary ammonium salt MAE-HB.

Resins with strong and long-lasting antibacterial properties are critical for the prevention of secondary dental caries. In this study, we evaluated the antibacterial effect and the underlying mechanism of action of an unfilled resin incorporating 2-methacryloxylethyl hexadecyl methyl ammonium bromide (MAE-HB) against Streptococcus mutans UA159 (S. mutans UA159). MAE-HB was added into unfilled resin at 10 mass%, and unfilled resin without MAE-HB served as the control. Bacterial growth was inhibited on 10%-MAE-HB unfilled resin compared with the control at 1 d, 7 d, 30 d, or 180 d (P < 0.05). The growth inhibitory effect was independent of the incubation time (P > 0.05). No significant differences in the antibacterial activities of eluents from control versus 10%-MAE-HB unfilled resins were observed at any time point (P > 0.05). The number of bacteria attached to 10%-MAE-HB unfilled resin was considerably lower than that to control. Fe-SEM and CLSM showed that 10%-MAE-HB unfilled resin disturbed the integrity of bacterial cells. Expression of the bacterial glucosyltransferases, gtfB and gtfC, was lower on 10%-MAE-HB unfilled resin compared to that on control (P < 0.05). These data indicate that incorporation of MAE-HB confers unfilled resin with strong and long-lasting antibacterial effects against S. mutans.

Effects of type I collagen degradation on the durability of three adhesive systems in the early phase of dentin bonding.

OBJECTIVE: This study was designed to evaluate the effects of type I collagen degradation on the durability of three adhesive systems in the early phase of dentin bonding. METHODS: Bonded dentin specimens were prepared using three different types of adhesive systems. Micro-tensile bond strength and degradation of collagen were tested before, and after 1 month or 4 months of aging in artificial saliva. The relationship between micro-tensile bond strength and collagen degradation was analyzed by calculating their Pearson's correlation coefficient. RESULTS: Aging induced time-dependent reduction in micro-tensile bond strengths for all the tested adhesive systems, although such reduction for the single-step self-etching adhesive G-Bond (GB) was not statistically significant. The bond strength of the two-step self-etching primer adhesive system Clearfil SE Bond (SEB) was similar to that of the two-step etch-and-rinse self-priming adhesive system Single Bond 2 (SB), and they were both significantly reduced after one or four months of aging. A negative correlation was found between the degree of collagen degradation and magnitude of micro-tensile bond strength (r = -0.65, p = 0.003). The Pearson's correlation coefficient was 0.426, indicating that 42.6% of the aging-induced reduction in bond strength can be explained by the degradation of collagen. CONCLUSIONS: In the early phase of dentin bonding, there was a negative correlation between the degree of collagen degradation and the magnitude of micro-tensile bond strength. The reduction of bond strength was accompanied by the degradation of collagen. These results provide evidence for the causative relationship between the degradation of collagen and the deterioration of dentin-adhesive interface.

The role of grape seed extract in the remineralization of demineralized dentine: micromorphological and physical analyses.

OBJECTIVES: Grape seed extract (GSE) is known to have a positive effect on the demineralization and/or remineralization of artificial root caries lesions. The present study aimed to investigate whether biomodification of caries-like acid-etched demineralized dentine, using proanthocyanidins-rich GSE, would promote its remineralization potential. DESIGNS: Dentine specimens were acid-etched for 30s, then biomodified using proanthocyanidin-based preconditioners (at different concentrations and pH values) for 2min, followed by a 15-day artificial remineralization regimen. They were subsequently subjected to microhardness measurements, micromorphological evaluation and X-ray diffraction analyses. Stability of the preconditioners was also analyzed, spectrophotometrically. RESULTS: A concentration-dependent increase was observed in the microhardness of the specimens that were biomodified using GSE preconditioners, without pH adjustment. Field emission scanning electron microscopy revealed greater mineral deposition on their surfaces, which was further identified mainly as hydroxylapatite. The absorbances of preconditioner dilutions at pH 7.4 and pH 10.0 decreased at the two typical polyphenol bands. CONCLUSIONS: Transient GSE biomodification promoted remineralization on the surface of demineralized dentine, and this process was influenced by the concentration and pH value of the preconditioner. GSE preconditioner at a concentration of 15%, without pH adjustment, presented with the best results, and this may be attributed to its high polyphenolic content.

Antibacterial effect of a resin incorporating a novel polymerizable quaternary ammonium salt MAE-DB against Streptococcus mutans.

The antibacterial properties of resins incorporating MAE-DB and the underlying mechanisms of action were evaluated. Antibacterial effects against Streptococcus mutans were tested using the film contact method, with accumulation and membrane integrity observed by scanning electron microscopy and confocal laser scanning microscopy, respectively. Quantitative PCR was used to determine expression of the S. mutans glucosyltransferase B (gtfB) gene on the surface of resins containing 10% MAE-DB. Bacterial growth was inhibited on resin containing 10% MAE-DB as compared with the control at 1 day, 7 days, 30 days, or 180 days (p < 0.05). For the 10%-MAE-DB resin, no significant differences in bacterial viability were found regardless of the time of incubation (p > 0.05). The number of bacteria attached to resin containing 10% MAE-DB was considerably lower than the control. The proportion of bacteria with damaged cell membranes was increased in the experimental resin over controls. Expression of gtfB was reduced by 10% MAE-DB compared with the control (p < 0.05). These findings demonstrate that MAE-DB can be incorporated into resin materials at sufficient concentrations for long-term antibacterial effects against S. mutans after polymerization by attenuating gtfB expression and impairing membrane integrity.

[Influence of nano-silica content on flexural properties of the aluminum borate whisker and silica filler composite resins].

OBJECTIVE: To discuss the influence of nano-silica content which was hydrolyzed by tetraethyl orthosioate (TEOS) on the aluminum borate whisker (AlBw) and silica filler composite resins on flexural properties. METHODS: The nanometer-size silicon dioxide (SiO2) particles were prepared by sol-gel method based on tetraethyl orthosioate. Different proportion of AlBw and SiO2 were fused and attached onto the surface of AlBw through high temperature, then polymerized with resin matrix after surface siliconization and their flexural strength and flexural modulus were determined. The effects of heat treatment to the surface morphology of AlBw and the shapes of the mixture at various proportions were characterized by TEM. RESULTS: The flexural properties of dental composite resins with AlBw-SiO2 compound as inorganic fillers were significantly improved. The flexural property of a new type of dental composite resins was(130.29 +/- 8.38) MPa, when the mass ratio of AlBw and nano-SiO2 particle was 3:1. CONCLUSION: Nano-silica content which was hydrolyzed by tetraethyl orthosioate improved flexural properties of the aluminum borate whisker and silica filler composite resins.

The cytotoxicity of methacryloxylethyl cetyl ammonium chloride, a cationic antibacterial monomer, is related to oxidative stress and the intrinsic mitochondrial apoptotic pathway.

Antibacterial monomers incorporated in dentin bonding systems may have toxic effects on the pulp. Thus, the cytotoxicity of antibacterial monomers and its underlying mechanisms must be elucidated to improve the safety of antibacterial monomer application. The influence of an antibacterial monomer, methacryloxylethyl cetyl ammonium chloride (DMAE-CB), on the vitality of L929 mouse fibroblasts was tested using MTT assay. Cell cycle progression was studied using flow cytometry. Production of intracellular reactive oxygen species (ROS) after DMAE-CB treatment was measured using 2,7-dichlorodihydrofluorescein diacetate staining and flow cytometry analysis. Loss of mitochondrial membrane potential, disturbance of Bcl-2 and Bax expression, as well as release of cytochrome C were also measured using flow cytometry analysis or Western blot to explore the possible involvement of the mitochondrial-related apoptotic pathway. DMAE-CB elicited cell death in a dose-dependent manner and more than 50% of cells were killed after treatment with 30 µM of the monomer. Both necrosis and apoptosis were observed. DMAE-CB also induced G1- and G2-phase arrest. Increased levels of intracellular ROS were observed after 1 h and this overproduction was further enhanced by 6-h treatment with the monomer. DMAE-CB may cause apoptosis by disturbing the expression of Bcl-2 and Bax, reducing the mitochondrial potential and inducing release of cytochrome C. Taken together, these findings suggest that the toxicity of the antibacterial monomer DMAE-CB is associated with ROS production, mitochondrial dysfunction, cell cycle disturbance, and cell apoptosis/necrosis.

Antibacterial activity and cytotoxicity of two novel cross-linking antibacterial monomers on oral pathogens.

OBJECTIVES: The antibacterial activity and cytotoxicity of two novel cross-linking antibacterial monomers, 2-methacryloxylethyl dodecyl methyl ammonium bromide (MAE-DB) and 2-methacryloxylethyl hexadecyl methyl ammonium bromide (MAE-HB) were tested in this study. DESIGN: The minimum inhibitory concentration (MIC) and the minimum bactericidal concentration (MBC) of unpolymerized MAE-DB and MAE-HB against eight strains of oral bacteria were tested using a broth dilution test. Time-kill determinations were performed to examine the kinetics of unpolymerized MAE-DB and MAE-HB against Streptococcus mutans UA159 and Streptococcus sanguinis ATCC6715. Bacterial morphology was observed using a field emission scanning electron microscope (Fe-SEM). The cytotoxicity of unpolymerized two new monomers and Bis-GMA on the human gingival fibroblast cell line H2620 was assessed using a methyl thiazolyl tetrazolium assay. RESULTS: Unpolymerized MAE-DB and MAE-HB showed strong bactericidal activity against oral bacteria. The MBC value of MAE-DB ranged from 12.2 to 24.4µg/ml and the MBC value of MAE-HB ranged from 6.2 to 48.8µg/ml. Time-kill determinations indicated that unpolymerized MAE-DB and MAE-HB had rapid killing effects against S. mutans UA159 and S. sanguinis ATCC6715 at the concentration of 4× MBC. The Fe-SEM observation showed that MAE-DB and MAE-HB could disturb the integrity of bacteria and cause lysis of bacterial cells. The median lethal concentration values on human gingival fibroblast for both monomers were between 10 and 20µg/ml, and greater than that of Bis-GMA. CONCLUSIONS: Unpolymerized MAE-DB and MAE-HB monomers had strong bactericidal activity against eight strains of oral bacteria. Their cytotoxicities were less than that of Bis-GMA.

[Effects of acid etching time on the degradation of type I collagen in dentin].

OBJECTIVE: To evaluate the effects of acid etching time on the degradation of type Icollagen in dentin. METHODS: Dentin was conditioned with 37% phosphoric acid for 10, 15, 30 and 60 s. There was no treatment for the control group. Quantity of collagen degradation in each group was determined by enzyme linked immunosorbent assay. Observations were carried out by means of a field emission in-lens scanning electron microscope (FEISEM). RESULTS: Samples conditioned with 37% phosphoric acid for 60 s showed the most degradation of collagen, which was 4.86 (1.55) mg/g, followed by 30 s group and 15 s group, which were 2.76 (0.87) mg/g and 1.93 (0.88) mg/g, respectively. Group of 10 s was 0.95 (0.38) mg/g. The control group showed the least degradation of 0.06 (0.03) mg/g. Significant differences in collagen degradation were found among groups (P < 0.005). Smear layer were removed well but tubular orifices and collagen fibrils were covered by particles after dentin being etched with 37% phosphoric acid for 10 s, while open and clear tubular orifices were observed for 15 s group. Smoother surfaces of exposed collagen fibrils and fewer globular particles were found in 30 s group than in 15 s group. In the 60 s group, the number of major fibrils decreased while minor branching fibrils increased, which indicate that the intratubular structure collapsed and fibrils fractured. CONCLUSIONS: Dentin conditioned with 37% phosphoric acid for 15 s can result in mineral dissolution without collagen structure damage. However, longer applications of 37% phosphoric acid within 60 s may increase collagen degradation.