Dental Tribune Indian Edition

6 Dental Tribune Indian Edition - January 2013 are activated by hydrolysis ([SiOR➝ [SiOH) before they react with the sur- face hydroxyl groups of the substrate.20 The first step of silane hydrolysis is the fast and reversible protonation of the alkoxy group at a low pH (3–5). Next, a bimolecular nucleophilic sub- stitution (SN2 ) reaction at the silicon atom takes place. A nucleophile, a water molecule, attacks backside to the silicon atom, an electrophile, to form a penta-coordinate transition state. A new bond is formed between the silicon and the nucleophile, and a bond is cleaved between the silicon and the leaving group, alcohol. This yields the product with an inversion of the configuration.21 A suggested mechanism for silane hydrolysis is shown in Figure 1. The silane hydrolysis rate depen- ds on the steric (size) and inductive (electronic) effects of alkoxy groups on the silane. The steric effect is the dominant factor that affects the silane hydrolysis rate.22 This effect is best il- lustrated using a ball-and-stick model (Fig. 2). As shown in Figure 2, the steric re- pulsion increases when the size of the alkoxy group is changed from a me- thoxy to butoxy group. The approach of a water molecule, a nucleophile, to the silicon atom is more difficult for the bulky butoxy groups. This may explain why ethoxysilanes are em- ployed in commercial dental products because of fast hydrolysis of small ethoxy groups. Methoxysilanes are not used, as the by-product methanol is very toxic. The organo-functional groups of silane coupling agents consist of, for example, >C=C<, a vinyl double bond that can react with the functio- nal groups of resin composite consi- sting of >C=C< bonds. The reaction is initiated by the initiators in the re- sin composite, which are decomposed by visible blue light to form free ra- dicals. These free radicals react with the >C=C< bond in the resin compo- site monomer or in the silane mole- cule to generate another free radical species. The reaction of these free radicals with resin composite mono- mers and silane molecules forms new C-C single bonds. Therefore, the sila- ne coupling agents connect the resin composite and the inorganic substrate surface.23 Application of silanes in dentistry Ceramic restorations and repairs Silane coupling agents are used in dental restoration, such as ceramic repairs of onlays, inlays, crowns and bridges. For most patients, repair is more economical and time-saving than the fabrication of new restora- tions, unless damage due to a fracture is beyond repair. The clinical proce- dure for repairing ceramic restoration usually involves the following steps: roughening the surface with diamond burs, sand-blasting the surface, acid etching, silanisation and finally bon- ding to resin composite.24 Glass fibre-reinforced composites A relatively new group of dental biomaterials, the glass fibre-reinforced composites, is used in fixed partial dentures, removable prosthodontics, periodontal splints and retention splints.25 The adhesion between the glass fibre and resin composite is im- proved by adding a silane coupling agent. The silane forms siloxane linka- ges with the surface hydroxyl groups of glass fibre. The organo-functional groups of silane react with the functio- nal group in the resin composite. Thus, the bonding strength is increased between resin composite and glass fibre.27 Resin composite filling materials Nowadays, dental resin composi- tes are composed of a resin matrix that contains monomers and cross- linking monomers, as well as a free- radical initiator, an inhibitor, colou- ring pigments, filler materials such as barium glass, silica, apatite and a silane coupling agent.27, 28 The latter enhances the bonding between the filler particles and the resin matrix.29 The filler particles added to the resin matrix also improve the physical and mechanical properties of the resin composite. Moreover, the addition of fillers reduces volume shrinkage after polymerisation, and improves the aes- thetic appearance and radiopacity.30 Titanium, noble metal and base metal alloys Titanium, noble metals and cobalt- chromium (base metal) alloys are commonly used for removable partial and complete dentures with a metal frame incorporated and metal-resin cement restorations.31,32 For these metal and metal alloys, surface con- ditioning by sand-blasting using sili- ca-coated alumina particles produces a silica-coated layer on the surface. Application of a silane coupling agent to the silica-coated surfaces forms a durable siloxane linkage. This is fol- lowed by cementation. Limitations of silanes as adhesion promoters Silanes are good at promoting adhesion between resin composites and dental restorative materials but there are some limitations to silane coupling agents. The adhesion of silane coupling agents and non-silica-based restorati- ve materials such as alumina, zirconia or metals is weaker than the silica co- ating of these materials.33, 34 Therefo- re, a surface pretreatment with silica coating is required so that durable bonds (siloxane bonds) are formed between silane and silica-coated re- storative materials. For noble metals or noble metal alloys, thione or thiol- based coupling agents are used to promote adhesion.35 These coupling agents have different bonding mecha- nisms with various dental restorative materials. Current trends and future development of coupling agents in dentistry Nowadays, other coupling agents (such as phosphate ester) are added to self-adhesive resin cements and adhesive primers, metal and alloy primers, and carboxylic acid primers used in dental restoration.36,37 Pho- sphate esters can bond directly to non-silica-based ceramics such as zir- conia.38 It has been reported that using this phosphate ester can enhance the hydrolytic stability of bonding more than using silane coupling agents can.39 The main problem of resin compo- sites bonded to silica-coated restora- tive materials with the application of commercial silane coupling agents is the bond degradation over time under artificial ageing.40, 41 In order to in- crease the hydrolytic stability of the bonding at the interfacial layer, novel surface treatments of restorative ma- terials and the design of novel silane monomers can solve this problem. Si- lane coupling agents with long hydro- carbon chains are more hydrophobic than those with short hydrocarbon chains. The bonding at the interfa- cial layer is more resistance to water ageing. These two approaches could resolve the problem. It could be said that silane coupling agents can fulfil the clinical require- ments for dental restorations. Nowa- days, a standard laboratory protocol for dental restorations entails surfa- ce conditioning of dental materials, silanisation and cementation. The problem of hydrolytic stability of the siloxane linkage formed from silane coupling agents with resin composites and dental restorative materials is currently being addressed. It is not an exaggeration to claim that silane cou- pling agents have wide application in industry, dentistry and medicine and will play an important role in bioma- terials science. This review is based on the article “Aspects of silane coupling agents and surface conditioning in dentistry: An overview”, Dental Materials, 28 (2012): 467–77. A complete list of references is available from the publisher.DT Dr Christie Ying Kei Lung is a postdoctoral fellow in Dental Ma- terials Science at the Faculty of Dentistry at the University of Hong Kong. He can be contacted at cyklung@hku.hk. Contact Info Trends & Applications The sensor consists of a graphene layer printed on to a bioresorbable silk substrate. (DTI/Photo Princeton University, USA) Daniel Zimmermann DTI PRINCETON, NJ, USA: Princeton University researchers have successful- ly tested a special kind of biosensor that could help to prevent disease by detec- ting even small amounts of harmful bacteria more quickly than conventio- nal methods. Using a “tattoo” made from silk and gold and attached to a cow’s tooth, they were able to transmit a signal wirelessly to a nearby receiver. With the method, developed in collaboration with the US Air Force and the American Asthma Founda- tion, the researchers hope one day to be able to detect not only bacteria but also DNA or particular viruses. In lab tests conducted at Princeton’s School of Engineering and Applied Scien- ce this year, they were able to detect pathogens responsible for surgical infections and stomach ulcers, among others. The signals are received from a gold antenna on a tattoo that is atta- ched to an array of graphene—very small particles of carbon—that trig- gers a signal when in contact with bacteria through attached proteins called peptides. Therefore, the device does not require any power supply, the researchers said. The sensor is held in place by a water-soluble silk base derived from insect cocoons. In this way, the rese- archers said, the sensor can be used on different kinds of biomaterials, like teeth or skin, and washed away or dissolved by body enzymes after use. According to the researchers, there is still a long way to go before such a biosensor could be in regular use, since the sensor is still too large to fit on human teeth and its lifetime and transmission distance are short. They admitted, however, that a few modi- fications to the design of the sensor could increase its transmission distan- ce in the future. Most traditional biosensors are ba- sed on substrates like silicon, which makes them heavy and uncomfortable to wear.DT Novel biosensor for use on teeth India council under scrutiny A new reported issued by the Comptroller and Auditor General of India has painted a poor picture of how the country’s Dental Council is managing dental education. Among other misconducts, it found that a significant amount of dental institu- tions have not been inspected by the governmental body for years and that fees worth more than US$1, 3 million to be paid by these colle- ges for the recognition and renewal of certain dental courses are outstan- ding. In addition, more students were admitted in some of the colleges than actually allowed by the 1948 Den- tists Act. Dental colleges have mushroomed in India over the last few years, now adding 30,000 new dentists annually to an already massive dental work- force of 1.3 million. DT