Current Practicality of Nanotechnology

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Clinical, Cosm tic and In stigational D ntist y 2009:1 4761

Clinical, Cosmetic and Investigational Dentistry

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Cu nt p acticality of nanot chnologyin d ntist y. Pa t 1: Focus on nanocompositsto ati s and biomim tics

Scott A Saund sMi o Monito C ati ity,roy sfo d, PA USA

Co spond nc : Scott A Saund sP incipal, Mi o Monito C ati ity,1014 And way, roy sfo d,PA 19468-3134, USATl +1 610 948 2001Fax +1 610 948 2001email scott@mmc ati ity.com

Abstract: First described in 1959 by physicist Richard P Feynman, who saw it as an unavoidabledevelopment in the progress o science, nanotechnology has been part o mainstream scienti ctheory with potential medical and dental applications since the early 1990s. Nanoparticles,nanospheres, nanorods, nanotubes, nano bers, dendrimers and other nanostructures have been

studied or various applications to biologic tissues and systems. While many layers o nano-technologic capability have been envisioned or oral health in the last decade (eg, oral hygienemaintenance, local anesthesia, even whole-tooth replacement), ew o these applications have

been developed. Part 1 o a three-part series reviews the current clinical utili ty o nanotech-nologys most tangible contribution to dentistry to date: the restoration o tooth structure withnanocomposites. Characterized by ller-particle sizes o 100 nm, these materials can o er esthetic and strength advantages over conventional micro lled and hybrid resin-based composite(RBC) systems, primarily in terms o smoothness, polishability and precision o shade char-acterization, plus fexural strength and microhardness similar to those o the better-per orming

posterior RBCs. Available comparative data or nanocomposites and organically-modi ed ceramic (Ormocer ) restoratives are also reviewed. Finally, plausible next-phase trends incurrent nanorestorative research are judiciously examined, including 1) calcium-, phosphate-,and fuoride-ion-releasing nanocomposites or anticaries applications and 2) restorative systems

based on biomimetic emulation o the nanomolecular assembly processes inherent in dentalenamel ormation using nanorods, nanospheres, and recombinant amelogenins.Keywords: nanostructure, dental restorative, resin-based composite, biomimetic, amelogenin

Introduction Nanotechnology in medicine has been recently reviewed (2002-present) rom various perspectives relative to the human molecule-tissue inter ace. 14 A common trend inthis ongoing discussion is the capability to operate on a scale small enough to interactwith intracellular components including DNA. 58

Operating on a stage this minute provides a perspective to envision restoring toothstructure at a level that o ers progressively closer approximation o its individual ana-tomic structures. The ever-shrinking size o the nanoparticles in resin-based composite(RBC) ceramic restorative systems continues in a progression that might be envisioned as mimicking actual tooth structure.

Following this progression at the nanoscale, current laboratory-bench dentalresearch is exploring designs or restorative systems that biomimetically approximatethe very processes by which dental enamel is ormed. 912 Admittedly, this progress is

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slower than might be considered desirable or clinicianswishing to put cutting-edge technology to clinical use.

History of nanotechnologyin dentistryIn the last 10 years numerous theoretical predictions have

been made based on the potential applications or nano-technology in dentistry, with varying levels o optimism. 13,14 In 2000, Freitas echoed the 1959 prediction in the popular lecture by the late physicist Richard P Feynman (who wonthe Nobel prize or physics in 1965). This prediction accom-

panied the birth o nanotechnologys de nition and vision:that the atomic-level precision a orded by molecular devicesoperating at the nanoscale was an inevitable technologiceventuality. 13 Ironically, Feynman himsel re erred to theyear 2000 in this same lecture, predicting a retrospectiveincredulity that serious ocus on nanotechnology did nothappen until 1960. 15 The theme o anticipated versus real-ized transition o emerging technology to actual practice isnot new. 16 The pace o its applications to dentistry has beenless than revolutionary. Even so, nanotechnologys impact ondental education has captured the interest o academicians,who are assessing its impact on dental curricula. 17

This article will address the current major representa-tion o practical nanotechnology in dentistry: restoration o tooth structure with RBCs that make use o nanoparticles.In addition, it will summarize dental biomimetic researchcontexts. Speci cally, it will examine nanoscale processes

currently being studied in dental research laboratories that bear striking resemblances to natural processes such as dentalenamel ormation.

Overview of nanostructuresfor dental applicationsNanopa ticl s

Nanoparticles (molecular units typically de ned as havingdiameters o between 0.1 and 100 nm) o various composi-tion represent the most widespread use o nanoscale units indentistry. They are currently being used in RBC restorations;two-year clinical results have been published or several o them and will be discussed. 1821

Together with the evolution o nanoparticles or dentalcomposites, sharper ocus is being applied to re ormulationso inter acial silanes. These have been used or many years tocoat and bond inorganic llers into RBC matrices in dentalrestoratives. Considerable research related to nanocompositesis ocusing on tailoring newer types o silane bonding agents

or optimal use with nanoparticles in RBCs. Organosilanessuch as allyltriethoxysilane have demonstrated good compat-ibility with nanoparticle llers such as TiO 2.

22 In addition,3-methacryloxypropyltrimethoxysilane has also beendemonstrated to enhance dispersion o silica nanoparticles(525 nm) within the restorative resin matrix. 23

Silanization has been reported by Chan and colleagues 24 as one o several theoretical avenues or increasing rac-ture toughness o nanocomposites. Xu and colleagues alsoreported that silanization increased the strength o a novelion-releasing calcium phosphate (CaPO 4) composite, butdecreased the level o release. 25 Another study by Karabelaand Sideridou also ound that di erent silane compositionsused to bind composites containing silica nanoparticles had di erent e ects on sorption o organic solvents and water bythe RBC, as well as solubility o the RBC. 26 Nanoparticlesand associated modi cations o existing RBC systems havea considerable record o demonstrated clinical utility and widespread use. Nanohybrid RBCs are currently the mostubiquitous example o such technology.

Nano ods Nanorods are o particular interest in a restorative context.Chen and colleagues have synthesized enamel-prism-likehydroxyapatite (HA) nanorods that have exhibited sel -assembly properties. 10 Since they are similar to the enamelrods that make up the basic crystalline structure o dentalenamel, nanorods could contribute to a practical arti cialapproximation o such a naturally-occurring structure.

Nanosph sIn a similar direction, such a potential transition to restorativesystems that also mimic nanoscale processes already inherentin natural tooth development will also be explored in thisarticle. Speci cally, nanosphere assembly in conjunction withcalcium phosphate deposition and amelogenin nanochainassembly will be discussed in a restorative context. 12

Nanotub s Nanotubes o various types have been investigated or dentalapplications in a number o interesting directions. Titaniumoxide nanotubes have been shown in vitro to accelerate thekinetics o HA ormation, mainly in a context o bone-growthapplications or dental implant coatings. 27 More recently,modi ed single-walled carbon nanotubes (SWCNTs) have

been shown to improve fexural strength o RBCs. TheseSWCNTs had silicon dioxide applied to them in conjunctionwith specialized organosilane bonding agents. 28


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micro lls. 37 Nanocluster particles as small as 220 nm arecurrently being used as nano lled restoratives. 38 A practicalaim o this paring down o particle size to the nanoscale has

been described by Mitra and colleagues in terms o criticallength scale, a limit that applies to every physical property.By creating particles smaller than this limit, a materialsdesirable per ormance (eg, optical properties, with a potential

bearing on esthetics) can be maximized. 38

Top-do n sus bottom-upmanufactu ing app oach

Nanotechnology has rede ned the ocus on how newer and smaller materials are created. Traditional manu acture o ller

particles or dental composites has required the comminu-tion o larger particles o quartz, glass, or ceramics throughgrinding or milling. Since this process cannot e ectively

produce particles less than approximately 100 nm in diameter,direct molecular assembly, or bottom-up processes thatinvolve synthetic chemical processes must be used. 38

Physical p op ti s of nanocomposit sand ational fo th i ad antag s

Nanohybrid and nano lled RBCs are generally the twotypes o composite restorative materials re erred to under the term nanocomposite, usually in a context o particlesize. 16,39 These are usually distinguished rom their predeces-sors, microhybrids and the older conventional micro lled RBCs, mainly in terms o particle size combinations and distributions, which can vary greatly. Mean individual

particle size o the older composites has exceeded 1 m,and experimental ller analyses in relation to physical

properties o dental RBCs have involved ller particles aslarge as 15.5 m.40 Comparative in vitro studies evaluating

physical properties o the various types o RBCs are sum-marized in Table 1.

Nanohybrid composites possess a wider range o particlesizes, and multiple ller compositions. For example, inorganic

particles averaging 7 nm together with larger glass particlesaveraging 2000 nm are used in NANOSIT nanohybrid composite (Nordiska Dental, Angelholm, Sweden). 41 Nano-lls have a more uni orm particle size range (eg, 75 nmand 520 nm or translucent and nontranslucent shades o Filtek Supreme Plus [3M ESPE], respectively). 42

The use o combinations o nanomeric particles and nano-clusters was introduced with Filtek Supreme (3M ESPE).The intent was to promote better wear patterns by using nano-sized primary particles (between 2 and 75 nm, depending onthe shade) breaking o rom the bottom-up designed larger

clusters (0.61.4 m). This would result in a smoother sur acethan would breakage o the much larger, nonsubdividable

particles (produced via top-down manu acturing) contained in many hybrids. 38

Materials with this cluster structuring have been reported to have distinct mechanical and physical properties compared with conventional RBCs, including better maintenance o

biaxial fexural strength during six months o water immer-sion, in contrast to a microhybrid tested. 43

Another study by Turssi and colleagues 44 ound consider-able variation among di erent nanocomposites in a three-

body wear test conducted in an oral wear simulator, as well asfexural atigue limit (FFL) analysis by the staircase method.O these materials, a micro ll control (Heliomolar ; Ivoclar Vivadent), a nanohybrid (Grandio ; Voco GmbH, Cuxhaven,Germany) and an agglomerated cluster/ ne-particle nano ll(Filtek Supreme) showed signi cantly higher FFL thaneither an ormocer-based RBC (CeramX mono; DentsplyInternational, York, PA, USA), or another nanohybrid (Premise; Kerr Corp., Orange, CA, USA). Ormocers arediscussed below in greater detail.

The rationale or broader particle-size range o hybridsincludes manu acturer statements o superior esthetics, sur aceand polishability. 41,45 While comparisons with older materialsmay prove the hypothesis, comparisons with nano lls tend to show that their esthetics are at least nonin erior to thoseo nanohybrids. A comparative study by Yap and colleaguesdemonstrated that materials based on ormocer and nanomer technology were signi cantly smoother than those based onmicro llers and nanoclusters. These composite materials,in turn, were signi cantly smoother than glass ionomers and compomers. For the material group that was nished and

polished (as opposed to the group that received polymeriza-tion against a matrix strip only), roughness values observed or the ormocer and nano ll were signi cantly lower thanthose observed or the micro ll and nanocluster composites. 46

The ongoing hypothesis or the use o progressivelydecreasing ller particle sizes is a model o better disper-sion and increased inter acial area between matrix and ller. This should translate into increased fexural strength,sur ace microhardness, and thus polishability o the nished restoration.

Tanimoto and colleagues published a computationalapproach that quanti ed an observed progressive decrease infexural strength as the mean ller-particle diameter increased.This investigation was limited to silica llers ranging rom3.3 to 15.5 m,40 which is considerably above the maximum

particle size range o nanohybrids or nano lls.


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However, a study by Beun and colleagues 47 compared the physical properties o nano lled, universal hybrid and micro-lled composites, and observed a higher elastic modulus withthe nano lled RBC than most o the hybrids tested. While allmaterials tested exhibited similar fexural strengths, micro llsshowed the poorest physical properties overall.

Glass-ionomer cements have been reported to have sig-ni cantly poorer sur ace- nish and esthetic properties than thenewer composites. 46 However, manu acturers have begun toincorporate nanoscale structuring to produce nanoionomers,in an e ort to make their sur ace nish more closely approxi-mate that o a hybrid composite. One recent in vitro study byOxman and colleagues compared Ketac Nano (3M ESPE),a paste/paste nanoionomeric hybrid resin-modi ied glassionomer (NHRMGI) with two fuoroalumiosilicate RMGIs(Fuji II LC and Fuji Filling LC [GC America, Inc., Alsip,IL USA]) and a nanohybrid composite (Tetric EvoCeram )

by ANOVA with Tukeys comparison at P 0.05. Ketac Nano showed signi cantly higher gloss than the other RMGIs(initial polish numbers by gloss meter were 36.5 1.3 versus64.2 11.1 or Tetric EvoCeram), with greater similarity to thato a hybrid composite (three-body wear depth at 80,000 cycleso 21.8 1.3 m versus 6.8 0.4 m or Tetric EvoCeram ).Wear rates or the other RMGIs were signi cantly higher thanor Ketac Nano .48

O moc sOrmocer is an acronym or organically modi ed ceramics,a registered trademark o Fraunho er-Gesellscha t (Munich,Germany). Ormocers represent a new technology based on sol-gel synthesis using particles comprising silicones,organic polymers, and ceramic glasses that is applicableto dental composites. 49 Ormocer composite technology isused in conjunction with nanoparticle llers such as ZrO 2 that are widely used in nanocomposite restorative systems.Some ormocers (such as CeramX [Dentsply International])contain particles as small as 23 nm in diameter. 50

Modi ying ormocers with organic moieties such asmethacrylate-substituted ZrO 2 or SiO 2 organosol nanopar-ticles was ound by Moszner and colleagues to improve themechanical properties o RBCs. 51 This study also describesormocers as being more biocompatible, 51 a claim echoed

by one manu acturer (Voco GmBh). 52 Ormocers alsoclaim decreased sur ace roughness, which is supported byin vitro evidence involving a variety o polishing techniques(see Table 1). 46,53

An in vitro study by Montanaro and colleagues examined the adhesion o Streptococcus mutans strain ATCC 25175

to the ormocer Admira (Voco GmBh), glass ionomersincluding Fuji IXFAST (GC Amercia), a compomer, fow-able composites, and microhybrids. Admira and Fuji IXFAST were more susceptible to S. mutans adhesion than a

polystyrene control. The other materials showed adhesionsimilar to the control. 54 Clinical evaluations o ormocer-based composites compared with nano lls are discussed below.

esth tics of nanocomposit sAn early goal o nanocomposite development was theintroduction o materials that possessed the strength tounction under the stresses o Class I and Class II occlusalapplications, while at least replicating the esthetic standardso hybrids and micro lls. With this objective, Mitra and colleagues used bottom-up manu acturing design to producea nanocomposite that contained a combination o nanomeric-

particle and nanocluster nano llers that possessed physical properties comparable to hybrids and with esthetic propertiescomparable to micro lls. 38 This ormulation is currently used in Filtek SupremePlus nanocomposite (3M ESPE).

Nanocomposites have been reported or the past severalyears to o er desirable overall esthetics, unction and biocom-

patibility or anterior restorations. A two-part series published in 2004 by Terry 55,56 provided initial clinical recognition o nanocomposites role in the esthetic dentists armamentarium.The series recaps the history o nanocomposite developmentand provides clinical guidance speci cally or their use inanterior restorations. 55 A case report by Milnar also illustrates

predictable replication o esthetics via combined use o adirect nano ll and calorimetric analysis or shade selection. 57 A study by Beun and colleagues that primarily highlights theelastic strength o nano lls comments on their esthetic utilityin anterior restorations as well. 47 Favorable compatibility o nano lls with esthetic dentistry has also been reported byWard. 58 However, larger-scale clinical colorimetric estheticstudies comparing nanocomposites with older-generationRBCs are lacking to date.

Signi cant improvement in sur ace smoothness/polishretention have been reported or nano lls compared withconventional micro lls. 59,60 Yap and colleagues reported that a nanomer-based RBC (Filtek Supreme Translu-cent) as signi icantly smoother than nanocluster-based RBCs (Filtek Supreme [dentin]). 46 This is an interestingobservation in view o a more recent study by Senawongseand Pongprueksa, in which the same nanocluster RBC system

produced the smoothest overall nish a ter polishing or brushing, measured by both scanning electron microscope(SEM) and sur ace roughness tester. 61


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Additional comparative studies show that the physical properties o nanocomposites are comparable to or better than those o the later-generation microhybrids, 43,62 includinga two-year clinical assessment by Ernst and colleagues thatcompared a nano lled RBC with a ne-particle hybrid and showed similar per ormance. 19 Studies o this type are

perhaps the best initial clinical predictor o nanocompositesevolving track record.

Clinical studies: the rst two yearsof nanocomposit p fo manc dataTerrys early review o the evolution o the nanocompositein 2004 made assessments regarding increased strength,durability and longevity o nanocomposites. 56 To date, the

predominant trends in studies comparing two-year dataor the various gradations o nanocomposites to those o conventional composites have shown clinical acceptabilityand nonin eriority, but probably not a clearly revolutionaryadvantage in terms o actual clinical per ormance, at leastnot at the current time point.

Several clinical studies published in the last three yearshave begun to benchmark the clinical record o nanocompos-ites. Results across these studies are similar and consistent,which is a positive sign or the clinician making greater use o nanocomposites in restorative practice. Similarly,a comprehensive summary o such similar results is importantrom a benchmarking perspective. Retrospective evaluationmay identi y nuances in study design that have a bearingon the continued evolution and speci ic utility o suchrestoratives.

As stated above, Ernst and colleagues 19 compared thenano ll Filtek Supreme with a conventional ne hybrid (Tetric Ceram, Ivoclar Vivadent) in Class-II applications.Each o 50 patients received one restoration with each mate-rial in a split-mouth design. A ter two years there was nostatistically signi cant di erence in durability between thetwo materials as measured by Ryge criteria. A summary o studies involving the longest-term clinical evaluations to dateor nanocomposite per ormance is presented in Table 2.

Eighteen-month results o a clinical study comparing thenano ll Filtek Supreme (3M ESPE) with the nanohybrid Grandio (Voco GmbH) to restore carious lesions in com-

bination with an antibacterial adhesive by Ergucu and Turkun 63 were also avorable. The only statistically signi cantdi erence was greater sur ace roughness with Grandio thanwith Filtek Supreme ( P 0.05, see Table 2).

Additional clinical studies reporting two-year dataor per ormance o nano lls, nanohybrids, and ormocers

compared with conventional microhybrid RBCs used inocclusal restorations have been published by Mahmoud and colleagues 64 and E es and colleagues. 18,65

Mahmoud and colleagues analyzed 140 restorations(Admira [Voco GmbH] an ormocer RBC; Tetric EvoCeram ,a nanohybrid; Filtek Supreme, a nano ll; Tetric Ceram , amicrohybrid RBC) in 35 patients, each o whom received 1restoration o each type. A ter two years, all three types o the newer composites per ormed similarly to the microhy-

brid RBC tested, with no statistically signi cant di erence( P 0.05) in United States Public Health Service (USPHS)modi ed Ryge scoring criteria. 64

E es and colleagues compared two types o newer RBCs:the ormocer Admira and the nano ll Filtek Supreme withRenew (a conventional hybrid). Over a two-year period, theyevaluated 90 Class-I maxillary restorations in 90 patients,with speci c endpoints o hypersensitivity and secondarycaries. Except or one ormocer restoration, which had ailed at two years, no secondary caries or hypersensitivity wasevident or any RBC tested ( P 0.05). All materials per or-mance was clinically acceptable (see Table 2). 65 An additionalstudy by E es and colleagues also showed excellent two-year

per ormance by the same nano ll and ormocer . Two previ-ously calibrated dentists evaluated the two restoratives usingthe same criteria with regard to secondary caries, marginaladaptation, sur ace texture and other variables. This studyalso evaluated the use o a fowable liner with each o thesematerials; it o ered no additional bene t or either one. 18

Finally, Schirrmeister and colleagues 21 compared theormocer RBC CeramX with the microhybrid TetricCeram,with a potentially con ounding variable o the use o K-0127

primer/adhesive or both RBCs; 31 o 43 patients (each o whom had received one Class-I or Class-II molar restorationwith each RBC) returned or the two-year recall visit. OneCeramX restoration had ailed (removal was necessary dueto pulpitis and need or root canal treatment). While somemarginal discoloration was noted or both RBCs, no statisti-cally signi cant changes were noted or sur ace texture, and no sensitivity or recurrent caries occurred ( P 0.05). 21

In summary, the comparative clinical per ormance todate among the various types o nanocomposites as well asin comparison with older hybrids does not yet show a clear advantage or nano lls. Continued systematic benchmarkingo longevity and clinical acceptability data rom this type o comparative study are essential or any such advantage to benoted as RBCs continue to evolve. Nevertheless, consistentclinical acceptability and low ailure rates evident rom thesestudies are encouraging.


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In vitro nanotechnologic dentalresearch approaching clinicalfeasibilityNano brillar reinforcementTwo recent in vitro studies by Tian and colleagues 31,32 have

provided interesting insights based on the theme o nano bers

and nanocrystals. Nano brillar silicate crystals can be used either alone 31 or in conjunction with nano bers, to rein orce

bis-GMA/TEGDMA-based nanocomposites. 32 Both studiesound that the addition o such nanostructures in smaller amounts up to an e ective threshold improved mechanical

properties o the experimental RBC.

Ca i s p ntionOptimal delivery o molecules that acilitate tooth structureremineralization and orestall caries is an active area o nanostructure-based research. Much o this work involvesnanoparticles in conjunction with RBC systems.

Xu and colleagues recently published in vitro data on astress-bearing, fuoride (F)-releasing nanocomposite whosefexural strength (110 11 MPa) matched that o a com-mercial non-F-releasing RBC (108 19 MPa). This materialcontains novel CaF 2 nanoparticles in a whisker-rein orced resin matrix, and had sustained F-release values exceedingthose o conventional and resin-modi ed glass ionomers. Thisstudys results are summarized in Table 3. Such an RBC could o er the previously unavailable combination o the strengtho a hybrid, and the F-releasing capacity o an RMGI that

by itsel is not suitable or high-stress occlusal restorations. 66 Whisker rein orcement o RBCs has been known or sometime to o er reduced brittleness and increased racturetoughness compared with conventional composites. 67 Itsuse with nanoparticles as in the study described above could reduce rates o restoration racture and secondary caries(see Table 3).

In a similar direction, several other recent studies byXu and colleagues 25,68,69 have evaluated the incorporationo nanosized CaPO 4 particles into RBCs, with a resultingimprovement in stress-bearing capacity as well as ion releasethat could inhibit caries. 68 Further investigation o this modelusing dicalcium phosphate anhydrous (DCPA) incorpo-rated with nanosilica- used whiskers ound that it increased the strength o the RBC by as much as three old whilereleasing CaPO 4. This release was greater with decreasingCaPO 4 particle size.

25 The authors hypothesize that sucha system could provide a desirable combination o caries

prevention and increased restoration strength. 25,69Lee and

colleagues ound a similar trend with reducing the particlesize o a zirconia-amorphous calcium phosphate RBCller, ie, good release properties were observed in addi-tion to an increase in biaxial fexural strength. 70 Finally,HA and fuorapatite bioceramic nanoscale bers have

been studied, with the interesting nding that nanostruc-ture solubility could be manipulated to produce e ectiverelease o fuorine ions rom fuorhydroxyapatite. 30 Whenenvisioned as part o a nanocomposite restorative or sealantsystem, such a process could have important applicationsin caries prevention.

Biomim tics: am log nins, hyd oxyapatit ,nam l plication and pai

Perhaps the most tempting venue or speculation on thenext phase o nanorestoration o tooth structure is that o nanotechnology mimicking processes that occur in nature(biomimetics), such as the ormation o dental enamel.Salient research over the last six years on the theme o studying and replicating enamel ormation is summarized in Table 3.

The central theme in the study o ways to mimic naturesalready-e icient use o nanotechnology surrounds thecooperative interaction between sel -assembled nanosphereso the proline-rich protein amelogenin (the most abundant

protein in dental enamel), and the ormation and directionalorientation o HA crystals that compose enamels hard-tissuemineral phase. Spanning a period between publicationo earlier work by Fincham and Moradian-Oldak 71,72 and Fincham and colleagues 73,74 rom the early 1990s to the

present, studies o various types o amelogenins continueto elucidate this proteins cooperational versatility among

processes that occur at the nanoscale during the ormationo dental enamel. 75,76

A recent in vitro study by Wang and colleagues hasurther elucidated mechanisms o interaction among amelo-genin nanospheres, nanoparticles and nanorods at critical

points during the HA crystal-growth process. The resultso er urther evidence or cooperativity in inter acial match-ing between organic and inorganic nanophases that mayresemble processes that occur in actual enamel ormation. 77 In another attempt to mimic enamel ormation, Uskokovicand colleagues also recently described such a synergy among

protein sel -assembly, proteolysis (through a pivotal role o matrix metalloprotease-20 [MMP-20], also known as enam-elysin) and crystallization. They used an in vitro model withull-length human amelogenin (rH174) in a saturated aqueouscalcium solution. 78 Tarasevich and colleagues have described


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a process o disassembly or shedding o a variety o smaller substructures rom nanospheres onto various organic and inor-ganic substrates, including those with methyl- and carboxyl-group unctionality as well as crystal fuoroapatite. The authorshypothesize that amelogenin may have di erent unctionaland sur ace-interactive speci cities. 79,80 The emergence o anamelogenin-interactive role in macromolecular sel -assemblyand enamel mineralization or a second protein, enamelin, alsohas been reported recently by Fan and colleagues. 81

A study o rat enamel organ by Brookes and colleaguessuggests that nanospheres ormed in vivo (through interac-tion with amelogenin) appear to be a basic assembly unit o enamel ormation. Secretion and assembly o monomericsubunits likely results in mature nanospheres that containull-length amelogenins. 9 Hence, nanosphere assembly prob-ably occurs intracellularly as a precursor to secretion o theenamel matrix. Sel -assembly o synthesized and modi ed nanorods into an enamel prism-like structure has also beensuccess ully done or human and rat enamel by Chen and colleagues. 10 Regularity o amelogenin nanosphere assemblyinto microribbons was also observed by Du and colleagues,who hypothesized amelogenins pivotal role in directing and ordering apatite crystal growth. 11

Thus, pivotal roles or various nanostructure types appear inherent in dental enamel ormation in nature. These lines o research suggest that such modes o assembly in nature arenot dissimilar to the arti cial assembly o nanostructures.The next logical step in this process would be success ulemulation o natures use o nanotechnology to developa restorative that could be delivered practically as a tooth-structure replacement.

Other recent studies by Fan and colleagues show perhapsthe greatest advancement in this direction. One describesthe development o an amelogenin-apatite composite. 12

produced by electrolytic deposition o calcium phosphatesimultaneously with the sel -assembly o amelogenin nano-chain structures, using a ull-length recombinant amelogenin(rP172). Induction o parallel bundles o calcium phosphatenanocrystals was also evident. The authors noted that use o the ull-length amelogenin (as compared with a truncated one) was critical to the optimal sel -assembly o the apatitecomposite. The strength o the resulting composite wasalso dependent on use o the ull-length amelogenin. Theyhypothesize that the organized-bundle morphology o suchan amelogenin-assembled composite has important potentialor its use as a restorative.

Fan and colleagues also ound that dose-dependentenamel remineralization (via HA crystal growth) occurred e

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N o n o f t h t h m a t i a l s x h i b i t d s c o n d a y

c a i s o p o s t o p a t i

s n s i t i i t y a t s i x - m o n t h ,

o n - y a o t o - y a c a l l ( P

0 . 0

5 ; F i d m a n

a n d / o r

W i l c o x o n s i g n e

d r a n k t e s t s ) . A

f t e r t w o y e a r s ,

x c p t f o t h o n f a i l d A d m i a s t o a t i o n , n o

o t h c i t i o n a s c l i n i c a l l y u n a c c p t a b l .

A b b r e v i a

t i o n s : U S P H S , U n i t d S t a t s P u b l i c H a l t h S i c ; P O S , p o s t o p a t i s n s i t i i t y .


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Table 3 In vitro studies with relevance to caries prevention and the development of enamel/dentin restoratives involving biomimeticp oc ss s and nanost uctu sAuthor Material/process tested Key fndings

Xu t al 66 Fluo id - l asing, st ss-b a ing nanocompositcontaining no l CaF 2 nanopa ticl s in ahisk - info c d d ntal sin

Fl xu al st ngth (m an SD; n = 6) as 110 11 MPafo th composit containing 30% CaF 2 and 35% hisk sby mass ( qui al nt to 108 19 MPa fo st ss-b a ing,non l asing comm cial composit ; Tuk ys at 0.05). Thcomposit containing 20% CaF 2 had a cumulati F l as of 2.34 0.26 mmol/L at 10 weeks. This formulation may reduceth occu nc of s conda y ca i s and sto ation f actu .

Zhou t al 89 Synth sis and antibact ial acti ity of ananohydroxyapatite (n-HA)/zinc oxide (ZnO)compl x

T o phas s of ZnO and n-HA combin d clos ly; this compl xposs ss d st ong antibact ial capability (99.45% to Staphylcoccusaureus and 95.65% to Escherichia coli und light, sp cti ly).

Zhou t al 83 G n tically ngin d p ptid s fo ino ganics(GePIs)

Sinc an id al d ntal sto ati ould mimic natu al toothst uctu , an analog of d ntal ha d tissu (i , hyb id of GePIsand hyd oxyapatit c ystals), might b ngin d using thcognition p op ti s b t n GePIs and HA c ystal. GePIs

can b us d in th ass mbly of functional nanost uctu s.

Fan t al 82 Am log nin ( combinant P172 at conc nt ationsabo o b lo 33 g/mL) used with a modi ed

biomim tic d position m thod in th p s nc of uoride to remineralize etched enamel

At a conc nt ation of 33 g/mL rP172 and 1 mg/L uoride,am log nin initiat d o i nt d bundl fo mation of fus d

needle-like uoridated hydroxyapatite crystals from enamelsurface in a dose dependent manner. Restorative/reparatived ntal biomat ial could b d lop d using an am log ninuoridated hydroxyapatite nanocomposite.

Fan t al 12 P pa ation of a mat ial mimicking d lopingnam l (comp ising calcium phosphat min al,at , and nam l mat ix p ot ins, mainlyam log nins); simultan ous ass mbly of am log ninand calcium phosphat p cipitat s by l ct olyticd position

Fo mation of o ganiz d bundl s in am log nin-apatitcomposit s sults f om am log nin nanochain ass mbly; suchmat ials ha pot ntial applications as d ntal sto atimat ials.

B ook s t al 9 Investigation of speci c intracellular/secretoryprocesses/locations of in vivoass mbly of am log nin nanosph s using at nam l

o gans using a bifunctional c oss-link , dithio bissuccinimidyl propionate and gel electrophoresis/st n-blot p obing

Int ac llula am log nin monom s a in clos n ighbocontact, fo ming compl x s comp ising up to six indi idualam log nin monom s; autho s sugg st that th s initial

compl x s a p fab icat d int ac llula ly b fo s c tion;post-s c tion th p fab icat d subunits ass mbl into full-siznanosph s containing num ous indi idual am log nins(as in nam l mat ix).

Du t al 11 In-vitrofo mation of bi f ing nt mic o ibbonst uctu s that g n at d th oughth sup amol cula ass mbly of am log ninnanosph s

Autho s obs d g o th of apatit c ystals o i ntat d alongc-axis and pa all l to long ax s of th mic o ibbons in vitro.Chains of s lf-ass mbl d am log nin nanosph s obs d asint m diat stat s b fo mic o ibbon fo mation sugg st a k yol fo am log nin in cont olling th o i nt d apatit c ystalg o th du ing min alization of nam l.

B niash t al 90 Clari caction of the role of amelogenin (the mostabundant p ot in in d ntal nam l) in nam lmin alization

Am log nins la g hyd ophobic po tion is in ol d in inhibitionof c ystal g o th. Impo tantly, its 13-amino-acid hyd ophilicC-t minal domain is ss ntial fo th alignm nt of c ystals into

pa all l a ays, hich only occu s ith full-l ngth monom icam log nin. enaml is fo m d th ough coop ati int actionsb t n fo ming c ystals and ass mbling p ot ins.

Bou opoulos andMo adian-Oldak 91

In stigation of int actions of combinant mousam log nin nanosph s ith hyd oxyapatitc ystals in solution using a Langmui mod lof adsorption site speci city of amelogenin-hyd oxyapatit binding

Autho s d sc ib d a num ical lationship b t n numbof am log nin nanosph s and hyd oxyapatit c ystal su faca a co d by ach population of nanosph s at maximumadso ption. Th y hypoth siz d that am log nin bindingonto apatit su fac is s l cti and p obably occu s only atc tain sit s.

Abbreviations: GePI, G n tically ngin d p ptid s fo ino ganics; HA, hyd oxyapatit ; SD, standa d d iation.


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under the direction o rP172 amelogenin (at a concentrationo 33 g/mL) in the presence o fuoride (F) at a concentra-tion o 1 mg/L. These results urther emphasize the pivotalimplications o amelogenin F-HA nanocomposite develop-ment or the restorative setting. 82

Finally, genetically engineered peptides or inorganics(GEPIs) have recently been hypothesized by Zhou and colleagues to have practical implications or tooth repair,i they can be engineered to recognize inorganic HA and orm a hybrid with it (see Table 3). 83

I some commercially viable combination o these proc esses could be deve lope d in to an amelogen in -nanosphere-based, HA-replicating, F-releasing, enamel-replicating restorative, its clinical implications would besigni cant. A conservative estimate might still place com-mercial availability o such a restorative system one to twodecades away. However, a progression o improving tech-nology systems based on biomimetic use o nanostructured materials is not an unreasonable expectation.

ConclusionRestorative dentistry relies on time-tested methods.Currently, nanotechnology has had its greatest impact onrestorative dentistry by o ering re inements to alreadyclinically proven RBC systems.

Esthetic dentistry has rom its inception attempted torecreate the natural appearance o tooth structure. So, perhapsi the clinician can envision tooth development as a long-standing natural process to be emulated by human technology,tooth-structure restoration becomes a continuum encompass-ing nature itsel , rather than just the stepwise advancemento chemical and physical technology.

Esthetic intent (and incorporation o greater strength) hasdriven the use o RBCs or an expanding range o applica-tions, and has met with increasing clinical acceptance over the past 30 years. Such acceptance has embraced nanotech-nologic restorative techniques on a number o esthetic restor-ative ronts, including nano lled RBCs and nanoionomers.

In parallel with this, research over the last 15 years hascategorized many nanotechnologic processes inherent in thenatural ormation o tooth structure. The perspective o emu-lating natures appearance revolutionized esthetic dentistrythrough to the present mainstream use o nano-RBCs. Thissame perspective could supply the impetus or introductiono HA-F-amelogenin nanochain restorative technology intomainstream dentistry. Made practical and durable (as RBCsgradually evolved to be), such nanorestorative biomaterialscould very credibly be the next trans ormative clinical leap.

Using retrospect or perhaps hindsight this leap is at leastcomparable in magnitude to todays widespread use o nano-hybrid and nano lled RBCs, compared with composites inuse three decades ago.

AcknowledgmentsThe author reports no nancial disclosures or conficts o interest in this work. The author would like to thank Gordon JChristensen, DDS, MSD, PhD o The Scottsdale Center or Dentistry, Scottsdale, AZ, USA; Rudol J Michl, PhD o Chemichl AG Vaduz, Liechtenstein; and William K Deal,DMD o Bryn Mawr Dental Health Group, Inc., Bryn Mawr,PA USA, or their invaluable critical review o this article.

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