| Since early 1960s composite resins have been widely used in clinical dentistry due to the advantages of good esthetics, improved mechanical properties, lower dissolubility, etc. Yet, there are still some concerns with their polymerization shrinkage which will lead to micro-leakage, secondary caries and the failure of prosthetic restorations. It has also been reported that compared with other restorative materials, more plaque accumulation or bacterial adherence are possibly prone to occur on composite resin, and consequently result in caries recurrence and gingivitis. To deal with these problems, several attempts has been subjected to develop antibacterial composite resins by addition of antibiotics such as chorhexidine, fluoride-containing filler and silver-loading materials. However, incorporation of these agents frequently resulted in compromised mechanical properties, high toxicity, short life expectancy, discoloration and high producing cost, which were not suitable for clinical use. Developing antibacterial composite resin that could be used in clinics requires an optimum antibacterial agent with the advantages of consistent strong antibacterial activity, high safety, low-cost, etc. and should not jeopardize the basic properties of the parent materials. Thus the antibacterial resin could both inactivate the invading bacteria and resist occlusal load, which will surely prolong the life expectancy of restorations.Tetrapod-like zinc oxide whiskers (T-ZnOw) is a new type of antibacterial agents which possesses good comprehensive properties such as super high strength, antibacterial activity, semi-conductivity, wear resistance, microwave absorption, vibration insulation, etc. Compared with other antibacterial agents, the T-ZnOw are particularly suitable for endowing dental materials with antibacterial activity because of the advantages of better antibacterial effects, low decomposability, long life expectancy, non-discoloration, low producing cost, etc[1]. Moreover, they could improve the mechanical properties of the materials at the same time. However, to the best of our knowledge, there has been no report on the use of T-ZnOw as an antibacterial agent to be incorporated with a composite resin.The aim of the present study was to investigate the effect of incorporating tetrapod-like zinc oxide whiskers on the antibacterial activity and mechanical properties of a self-cured composite resin. It was hypothesized that whiskers substantially provided composite resin with antibacterial effects. It was further hypothesized that the whiskers significantly improved the mechanical properties of the resin at the same time.Major results and conclusions from the studies are as following:1 The pod length of the T-ZnOw was about 30μm. The diameter of the pod in basal part was about 0.25～2μm (average value= 0.8μm )and the diameter of the pod ends was in nano-grade. Zn, O, C and Si were the main elements of the agent. The optical micrographs showed that the whiskers dispersed uniformly in the resin and formed networks in the resin containing 5% of the T-ZnOw. In the resin containing 3% of the T-ZnOw the network of the whiskers didn't exist and in the resin containing 10% of the T-ZnOw the agglomeration of the whiskers could be detected.2 The broth dilution tests indicated that both the powder and liquid components of the resin and the cured resin didn't show antibacterial effect against Streptococcus mutans. And the MIC (MBC) of the T-ZnOw compound antibacterial agent and silver-based inorganic antibacterial agent were 156.25 (312.5)μg/ml and 156.25 (156.25)μg/ml respectively.3 The direct contact tests showed that when the concentrations of T-ZnOw added to composite resin were 3%, 5% and 10%(w/w), the antibacterial rates of the resin specimens were 84.85%±5.16%, 94.22%±3.73% and 99.43%±0.48% respectively. When the concentrations of the silver-based antibacterial agent added to composite resin were1%, 3% and 5%, the antibacterial ratios of the resin specimens were 71.01%±7.67%, 90.76%±5.91% and 97.19%±4.10% respectively. With the same concentrations of the two antibacterial agents added to composite resin, the antibacterial rate difference of the two composite resins was not significant.4 After aging treatment for 3 months, the antibacterial rate decreased significantly when the concentration of T-ZnOw was 10%, whereas it did not changed significantly when the concentration of T-ZnOw was 3% or 5%.In addition, after aging treatment, the antibacterial activity of the resin specimens did not show any difference between the 5% subgroup and the 10% subgroup. Thus it seems there are no needs to increase the concentration of the agent to 10% in the view of long-term effectiveness. Besides, the antibacterial rates of the resin specimen containing 5% of the T-ZnOw antibacterial agent showed better long-term effectiveness than its silver-based antibacterial agent counterpart.5 The effect of incorporating T-ZnOw into the experimental composite resin was significant for all mechanical parameters with the peak value got at the concentration of 5% and then decreased in 10% group. Compared with the blank control group, the FS, CS and DTS of the 5% group were significantly improved by 38.36%,14.42% and 41.63% respectively. Whereas, compared with the 5% group, the strengths of the 10% group occurred somewhat decrease, especially the FS and DTS. And there was no significant difference between the 10% group and the control group in FS and CS. The SEM observations showed that the fracture surface of the 5% group was tougher than that of the blank control and 10% group. Massive whiskers and whisker pull-out could be found in 3% and 5% groups and there were more bubbles in 10% group.6 Before aging treatment, color changes of the T-ZnOw antibacterial agent group and the silver-based antibacterial agent group both increased with the increase of concentrations of the antibacterial agents. After aging treatment, there was not significant difference between the color of the T-ZnOw group and the blank control, while the silver-based antibacterial agent group showed significant color changes towards dark–green-blue.In conclusion, the incorporation of T-ZnOw antibacterial agent could both improve the antibacterial activity and mechanical properties of the experimental composite resin. The composites containing 5% T-ZnOw showed strong antibacterial activity, better antibacterial long-term effectiveness and enhanced mechanical properties. It was suggested that T-ZnOw fillers may be an appropriate candidate for the antibacterial modification of the composite resin.
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