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Trends Biomater. Artif. Organs, Vol 21 (2), pp 94-97 (2008) http://www.sbaoi.org

Bio-Smart Dentistry: Stepping into the Future!

Pawan Gautam and Ashima Valiathan*

Dept. of Orthodontics and Dentofacial Orthopedics,Manipal College of Dental Sciences, ManipalKarnataka 576 104 India*corresponding author e-mail: [email protected]

Received 29 August 2007; published online 18 December 2007

Today the most promising technologies for lifetime efficiency and improved reliability include the use of smart materials

and structures. Biomedical applications of smart materials involve their use delivery of therapeutics, tissue engineering,cell culture, bioseparations, biomimetic actuators, immobilized biocatalysts, drug delivery and thermoresponsivesurfaces. The use of biocompatible smart materials has revolutionized many areas of dentistry. Some of the smartmaterials used in dentistry include Shape memory alloys for orthodontic wires, coils and springs, Cercon smartceramics, a dental restoration, offering extremely natural aesthetics paired with excellent durability, biocompatibility,“Smart composites with amorphous calcium phosphate stimulating repair of defective teeth, and Smart fibers which arehollow-core photonic-crystal fibres (PCFs) for the delivery of high-fluence laser radiation for ablating tooth enamel. Animportant aspect of smart materials used in various areas of dentistry is their excellent biocompatibility. Theseinnovations in the material science have marked the beginning of an era of Bio-Smart Dentistry, a step into the future!

Introduction

The term ‘smart materials’ refers to a class ofmaterials that are highly responsive andhave the inherent capability to sense andreact according to changes in the

environment. For that reason they are oftenalso called ‘responsive materials’. Earlysmart material applications started withmagnetostrictive technologies. This involvedthe use of nickel as a sonar source duringWorld War I to find German U-boats by Alliedforces. Depending on changes in someexternal conditions, "smart" materials changetheir properties (mechanical, electrical,appearance), their structure or composition,or their functions.

Classification of Smart Materials

Passive smart materials that respond toexternal change without external control;

Active smart materials that utilize a feedbackloop to enable them to function like a

cognitive response through an actuatorcircuit;

Very smart materials that sense a change in

the environment and respond (e.g., byaltering one or more of their propertycoefficients, tuning their sensing, or actuationcapabilities); and

Intelligent materials that integrate thesensing and actuation functions with thecontrol system.

Biomedical Applications (1,2)

Recent advances in the design of stimuli-responsive polymers have createdopportunities for novel biomedical

applications. Stimuli-responsive changes inshape, surface characteristics, solubility,formation of an intricate molecular self-assembly and a sol-gel transition enabledseveral novel applications in the delivery oftherapeutics, tissue engineering, cell culture,



Bio-Smart Dentistry: Stepping into the Future! 95

bioseparations, biomimetic actuators,immobilized biocatalysts, drug delivery andthermoresponsive surfaces.

Smart pressure bandages: Polyethyleneglycols bonded to various fibrous materialssuch as cotton and polyester possess theintelligent properties of thermal adaptabilityand reversible shrinkage. Reversibleshrinkage involves imparting a "dimensionalmemory" to the material such that when thematerial is exposed to a liquid (e.g., water) itshrinks in area. Such materials could beused for pressure bandages that contractwhen exposed to blood, thereby puttingpressure on a wound.

A smart suture: A smart suture that t ies itselfinto the perfect knot is a potential medicalapplication for new biodegradable plasticswith "shape memory". The materials are alsobiocompatible and safe for use in a livinganimal.

Hydrogel: Hydrogels exhibit plasticcontraction with changes in temperature, pH,magnetic or electrical field, and have a vastnumber of applications, for example softactuators in the biomedical field or forcontrolled drug release.

Smart shirt: Developed by Georgia Techalong with Sensa Tex, Inc., "Smart Shirt," isa T-shirt that functions like a computer, withoptical and conductive fibers integrated intothe garment. The shirt monitors the wearer’sheart rate, EKG, respiration, temperature,and a host of vital functions, alerting thewearer or physician if there is a problem. TheSmart Shirt also can be used to monitor thevital signs of law enforcement officers, firemen, astronauts, military personnel,chronically ill patients, elderly persons livingalone, athletes, and infants.

In countering radioactive rays: Compositecontainment structures can be used tocounter radioactive or chemical wastematerials. Fibres with chemically sensitivecoatings or radiation sensitive coatings maybe provided which are adapted to release

scavenger compounds when radiation orchemical waste is detected.

Smart Materials used in Dentistry

Shape Memory Alloys. SMAs have comeinto wide use because of their exceptionalsuperelasticity, their shape memory, theirgood resistance to fatigue and wear, andtheir relatively good biocompatibility.Theshape-memory effect was first observed incopper–zinc and copper–tin alloys byGreninger and Mooradian in 1938, but it wasonly in the early 1960s that Buehler and hiscolleagues discovered and patented Nitinol(Nickel Titanium Naval OrdnanceLaboratory), a nickel–titanium alloy created

in the Naval Ordnance Laboratory. Theycurrently seem to hold the most promise inradiology, cardiovascular applications,urology, and other medical applications foruse as prostheses, tissue connectors, andendovascular stents. For biomedicalapplications, the most attractive SMA isNitinol, an almost equiatomic nickel-titaniumalloy. This SMA, however, is 55% nickel byweight and may thus have allergic, toxic, orcarcinogenic effects. For short-term use, in-vitro and clinical data strongly support Nitinolas a safe biomaterial, at least as good asstainless steel or titanium alloys, such as

Ti6Al4V (3,7). Another commerciallyimportant application is the use ofsuperelastic and thermal shape recoveryalloys for orthodontic applications (4). Archwires made of stainless steel have beenemployed as a corrective measure formalaligned teeth for many years. Owing tothe limited flexibility and tensile properties ofthese wires, considerable forces are appliedto teeth, which can cause a great deal ofdiscomfort. Visits may be needed to theorthodontist for re-tensioning every three tofour weeks in the initial stages of treatment.Superelastic wires are now used for these

corrective measures. Owing to their elasticproperties and extendibility, the level ofdiscomfort can be reduced significantly asthe SMA applies continuous, gentle forceswhich are in physiological range, over alonger period. Visits to the orthodontist are



P. Gautam and A. Valiathan96

reduced significantly. Apart from thetensioned archwires, other superelasticorthodontic devices exist which can move

teeth linearly where there is uneven toothspacing. Movements of 6mm in 6 months arepossible with minimum discomfort. Devicesalso exist that can apply torsional forces inthe case of a “twisted” tooth. Other wire-forms can then be fitted to the brackets topush, pull, twist or force other movementsthat facilitate corrective measures forcosmetic or clinical reasons. Other similarSMA devices are also being used for healingbroken bones - staples of the shape memorymaterials are attached to each part of thebone, and these staples then apply aconstant, well-defined force to pull the two

pieces together as the SMA is warmed bythe body and tries to return to its originalconfiguration. This force helps knit the twopieces of bone back together (5). Such smart‘healing’ powers are the reason why SMAsare being borne in mind for manyapplications in the medical, dentistry andother fields in the future. SMA are alsoemployed in various DistractionOsteogenesis appliances which do not needto be adjusted over a long period of time,hence the dependence on patientcompliance is greatly eliminated (6). Nitinolendodontic files, for root canal procedures,

offer superior flexibility, durability, andtorqueability as compared to stainless steelfiles. This is a fundamental advantageproviding ease of use and increased patientcomfort: when the dental procedure iscarried out the file can effectively fit intominiscule and tight positions without beingforced. Super-elastic files provide benefit bymaintaining close contour to the canal shapewithout concern of file breakage (8).

Smart Composites containing ACP(amorphous calcium phosphate) (9) ACP isone of the most soluble of the biologically

important calcium phosphates, exhibiting themost rapid conversion to crystallinehydroxyapatite (HAP). ACP, whenincorporated into specially designed andformulated resins to make a compositematerial, will have an extended time release

nature to act as a source for calcium andphosphate, useful for preventing caries.Hydroxyapatite is the basic building block of

tooth enamel and the inorganic componentof dentin. During a carious attack, the HAP isremoved from the tooth structure resulting incavities or white spots. The carious attack isusually the result of exposure to low pHconditions (acid attack) either from bacteria,other biological organisms releasing acid,food (carbohydrate decomposition products),or acidic beverages. ACP at neutral or highpH remains ACP. When low pH values (at orbelow 5.8) occur during a carious attack, ACP converts into HAP and precipitates,thus replacing the HAP lost to the acid. Sowhen the pH level in the mouth drops below

5.8, these ions merge within seconds to forma gel. In less than 2 minutes, the gelbecomes amorphous crystals, resulting incalcium and phosphate ions.

Cercon - Smart Ceramics (10). In 1995 thefirst "all ceramic teeth bridge" was inventedat ETH Zurich based on a process thatenabled the direct machining of ceramicteeth and bridges. Since then the processand the materials were tested and introducedin the market as CERCON - SmartCeramics. The strength and technology ofCercon allows bridges to be produced

without stainless steel or metal. TheZirconia-based all ceramic material is notbaked in layers on the metal, but is createdfrom one unit with no metal. The overallproduct is metal-free biocompatible life likerestoration with strength that helps resistcrack formation. With Cercon unsightly darkmargins and artificial grey shadows from theunderlying metal are no longer a problem.Whether for "front" or "back" teeth, single unitor multi-unit bridges, Cercon Smart Ceramicsdeliver outstanding aesthetics withoutreservations or compromise. Zirconium oxide(ZrO2) is a highly stable ceramic oxide,

typically used in industrial applicationsrequiring high strength and stability, and hasa history as a biomaterial dating back tothe1970s. It is used in implants and othernon-dental applications extensively, and iscurrently the material of choice for use in



Bio-Smart Dentistry: Stepping into the Future! 97

total hip replacements. Hip replacementsparallel dental applications in that they areboth load bearing situations with close

proximity to vascular and osseous tissue.They are also both articulated joints thatexperience wear. The fracture toughnessand flexural strength of zirconia aresignificantly higher than that of alumina orany other currently available all ceramic. TheCercon system offers a comprehensivesolution to these needs by taking advantageof the strength, toughness, reliability, andbiocompatibility of zirconium oxide.

So the

cercon ceramics are said to be smartmaterial as they are bioresponsive.

Smart Fibres for Laser Dentistry (11).

Hollow-core photonic-crystal fibres (PCFs)for the delivery of high-fluence laser radiationcapable of ablating tooth enamel have beendeveloped. Sequences of picosecond pulsesof Nd:YAG-laser radiation are transmittedthrough a hollow-core photonic-crystal fibre

with a core diameter of approximately 14 µmand are focused on a tooth surface to ablatedental tissue. The hollow-core PCF is shown

to support the single-fundamental-moderegime for 1.06 µm laser radiation, servingas a spatial filter and allowing the laser beamquality to be substantially improved. Thesame fibre is used to transmit emission fromplasmas produced by laser pulses on thetooth surface in the backward direction fordetection and optical diagnostics.

Conclusion

Applications of stimuli-responsive, or ‘smart’,polymers in areas like dentistry, biomedicalengineering, delivery of therapeutics, tissue

engineering, bioseparations, sensors oractuators are an indicator of the potentialand rapid progress in this area. Thenumerous applications they have been putto, no doubt that "Smart Materials" hold areal good promise for the future.

References

1. Tony Anson. Shape memory alloys: Medical applications.Materials World, 1999 Vol. 7, No. 12, 745-747

2. El Feninat, F.; Laroche, G.; Fiset, M.; Mantovani, D. Shape memory materials for biomedical applications . Adv.Eng. Mater. 2002 , 4, 91–104.

3. M. Haïdopoulos, F. Elfeninat, D. Mantovani, Memory Metals , in "Encyclopedia of Biomaterials and BiomedicalEngineering", Wnek G., Bowlin G., Eds. Marcel Dekker, New York, 2004, 1340-1347.

4. Fukuizumi, M.; Kakigawa, H.; Kozono, Y. Utility of Ni–Ti shape memory orthodontic wire. Dent. Mater . 1999 , 18,413–424.

5. Drugacz, J.; Lekston, Z.; Morawiec, H.; Januszewski, K. Use of TiNiCo shape-memory clamps in the surgical

treatment of mandibular fractures. J. Oral Maxillofac. 1995 , 53, 665–671.

6. Mehrdad M., Nozomu, Anthony P, Craig A., Paul N. Spring-Mediated Mandibular Distraction Osteogenesis. Journal of Craniofacial Surgery. 2003;14(5):756-762,

7. Shabalovskaya, S.A. Surface, corrosion and biocompatility aspects of Nitinol as an implant material . Biomed.Mater. Eng. 2002 , 12, 69–109.

8. Revathi M, Rao CVN, Lakshminarayanan L Revolutions in Endodontic instruments-A review Endodontology 2001Vol.13,43

9. D. Skrtic J. M. Antonucci, E. D.Eanes Amorphous Calcium Phosphate- Based Bioactive Polymeric Composites for Mineralized Tissue Regeneration [J. Res. Natl. Inst. Stand. Technol. 2003; 108, 167-182

10. www.cercon-smart-ceramics.com

11. Stanislav O Konorov et a.l Hollow-core photonic-crystal fibres for laser dentistry2004 Phys. Med. Biol. 49 1359-1368

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