Nanodentistry

Growing interest in the future medical applications of nanotechnology is leading to the emergence of a new field called nanomedicine [11] – the science and technology of diagnosing, treating, and preventing disease and traumatic injury, of relieving pain, and of preserving and improving human health, using nanoscale-structured materials, biotechnology and genetic engineering, and eventually complex molecular machine systems and nanorobots. Similarly, development of “nanodentistry” will make possible the maintenance of near-perfect oral health through the use of nanomaterials [26, 27], biotechnology [27-31] including tissue engineering [32, 33], and nanorobotics. The first two approaches have been discussed extensively elsewhere. This paper offers a first look at the third and most exciting (though somewhat more technologically remote) approach to nanodentistry – dental nanorobotics.

When the first micron-size dental nanorobots can be constructed, perhaps 10-20 years from today, how might they be applied to dentistry? Freitas [11] has described how medical nanorobots might utilize specific motility mechanisms to crawl or swim through human body tissues with navigational precision, acquire energy, sense and manipulate their surroundings, achieve safe cytopenetration (e.g., pass through plasma membranes such as the odontoblastic process without disrupting the cell), and employ any of a multitude of techniques to monitor, interrupt, or alter nerve impulse traffic in individual nerve cells, in real time. These nanorobot functions may be controlled by an onboard nanocomputer executing preprogrammed instructions in response to local sensor stimuli. Alternatively, the dentist may issue strategic instructions by transmitting his orders directly to in vivo nanorobots via acoustic signals (e.g., ultrasound) or by other means – like an admiral commanding a fleet. (Biocompatibility of medical nanorobots and other important clinical issues will be discussed in Volume II of the Nanomedicine [11] series.)

One of the most common procedures in dentistry is the injection of local anesthetic, involving long waits and varying degrees of efficacy, patient discomfort, and complications [34]. Well-known alternatives such as Transcutaneous Electronic Nerve Stimulation (TENS) [35, 36], Cell Demodulated Electronic Targeted Anesthesia (CEDETA) [37], and other transmucosal [36], intraosseous [36], or topical [38] techniques are of limited clinical effectiveness.

To induce oral anesthesia in the era of nanodentistry, a colloidal suspension containing millions of active analgesic micron-size dental nanorobots will be instilled on the patient's gingivae. After contacting the surface of the crown or mucosa, the ambulating nanorobots reach the dentin by migrating into the gingival sulcus and passing painlessly through the lamina propria [39] or the 1-3 micron thick layer of loose tissue at the cemento-dentinal junction [40]. Upon reaching the dentin, the nanorobots enter 1-4 micron diameter dentinal tubule holes [41-43] and proceed towards the pulp, guided by a combination of chemical gradients, temperature differentials, and even positional navigation [11], all under onboard nanocomputer control. There are many pathways to choose from. Dentinal tubule number density is typically 22,000 mm^-2 near the dentino-enamel junction, 37,000 mm^-2 midway between junction and pulpal wall, and 48,000 mm^-2 close to the pulp [41] in coronal dentin [41], with number density slightly lower in the root (e.g., 13,000 mm^-2 near the cementum [44]). Tubule diameter increases nearer the pulp [43, 45] and may thus facilitate nanorobot movement, though circumpulpal tubule openings vary in number and size [46]. Tubule branching patterns may present a significant challenge to navigation, because they show an intricate and profuse canalicular anastomosing system crisscrossing the intertubular dentin, with dentinal branching density most abundant in locations where tubule density is low [47]. Dentinal tubules are continuous between primary dentin and regular secondary dentin in young and old teeth, but not between primary and irregular secondary dentin [48]. Regular secondary dentin becomes highly sclerosed in older teeth [48], and many tubule openings on the outer dentin surface can become completely occluded in some circumstances [49], probably necessitating significant detouring by the dental nanorobots. (On the other hand, a small number of “microcanals” [50, 51], “large tubules” [52], or “giant tubules” [52, 54] of diameter 10-50 microns or even larger [54] may exist in some cases, possibly affording easier transit.)

Assuming a ~10 mm total path length from tooth surface to pulp, a very modest nanorobot travel speed [11] of 100 microns/sec completes the journey into the pulp chamber in ~100 sec. The presence of natural cells that are constantly in motion around and inside the teeth – including human gingival and pulpal fibroblasts [55], cementoblasts at the cemento-dentinal junction [56], bacteria inside dentinal tubules [57], odontoblasts near the pulp/dentin border [43, 46, 58, 59], and lymphocytes within the pulp or lamina propria [60] – suggests that such journeys should be feasible.

Once installed in the pulp and having established control over nerve impulse traffic [11], the analgesic dental nanorobots may be commanded by the dentist to shut down all sensitivity in any particular tooth that may require treatment. When the dentist presses the icon for the desired tooth on the handheld controller display, the selected tooth immediately numbs (or conversely later, upon command, awakens). After the oral procedures are completed, the dentist orders the nanorobots (via the same acoustic data links) to restore all sensation, to relinquish control of nerve traffic, and to egress from the tooth by similar pathways used for ingress, followed by aspiration. Nanorobotic analgesics offer greater patient comfort and reduced anxiety, no needles [61], greater selectivity and controllability of analgesic effect, fast and completely reversible switchable action, and avoidance of most side effects and complications [34].

New treatment opportunities in nanodentistry may include:

(1) Tooth Repair. Nanodental techniques for major tooth repair may evolve through several stages of technological development, first using genetic engineering, tissue engineering [32, 33, 62-67] and tissue regeneration [33, 63-68], and later growing whole new teeth in vitro [69, 70] and installing them. Ultimately, the nanorobotic manufacture and installation of a biologically autologous whole replacement tooth including both mineral and cellular [71-73] components – e.g., complete dentition replacement therapy – should become feasible to undertake within the time and economic constraints of an ordinary office visit, using an affordable desktop manufacturing facility in the dentist's office.

(2) Tooth Renaturalization. Dentition renaturalization procedures may become a popular addition to the typical dental practice, providing perfect methods for esthetic dentistry. This trend may begin with patients who desire to have their old dental amalgams [74] excavated and their teeth remanufactured with native biological materials. But demand will grow for full coronal renaturalizations in which all fillings, crowns, and other necessary 20th century modifications to the visible dentition are removed, with the affected teeth remanufactured so as to be indistinguishable from the natural originals.

(3) Hypersensitivity Cure. Dentin hypersensitivity is another pathologic phenomenon that may be amenable to a nanodental cure. Dentin hypersensitivity may be caused by changes in pressure transmitted hydrodynamically to the pulp. This etiology is suggested by the finding that hypersensitive teeth have 8 times higher surface density of dentinal tubules – and tubules with diameters twice as large – than nonsensitive teeth [75]. There are many therapeutic agents for this common painful condition that provide temporary relief [76], but reconstructive dental nanorobots could selectively and precisely occlude selected tubules in minutes, using native biological materials, offering patients a quick and permanent cure.

(4) Orthodontic nanorobots. Orthodontic nanorobots could directly manipulate the periodontal tissues including gingiva, periodontal ligament, cementum and alveolar bone, allowing rapid painless tooth straightening, rotating, and vertical repositioning in minutes to hours, in contrast to current molar uprighting techniques which require weeks or months to proceed to completion [77].

(5) Dental Durability and Cosmetics. Tooth durability and appearance may be improved by replacing upper enamel layers with covalently-bonded artificial materials such as sapphire [78] or diamond [16] which have 20-100 times the hardness and failure strength of natural enamel [11] or contemporary ceramic veneers [79], and good biocompatibility [80]. Like enamel, sapphire is somewhat susceptible to acid corrosion [81], but sapphire can be manufactured in virtually any color of the rainbow [11], offering interesting cosmetic possibilities (e.g., iridescence) as alternatives to standard whitening [82] and sealant [83] procedures. Pure sapphire and diamond are brittle and prone to fracture if sufficient shear forces are imposed [84] but can be made more fracture resistant as nanostructured composites [85], possibly including embedded carbon nanotubes [86].

(6) Nanorobotic Dentifrice (dentifrobots). Effective prevention has reduced caries in children [87] and a caries vaccine may soon be available [88], but a subocclusal-dwelling nanorobotic dentifrice delivered by mouthwash or toothpaste could patrol all supragingival and subgingival surfaces at least once a day, metabolizing trapped organic matter into harmless and odorless vapors and performing continuous calculus debridement. These invisibly small (1-10 micron) dentifrobots, perhaps numbering 10³-10⁵ nanodevices per oral cavity and crawling at 1-10 microns/sec, might have the mobility of tooth amoebas [89] but would be inexpensive purely mechanical devices that would safely deactivate themselves if swallowed and would be programmed with strict occlusal avoidance protocols. (Even diamondoid nanomachines can be crushed by dental grinding unless their outer shells are at least 10% of device radius thick [11].) Properly configured dentifrobots could identify and destroy pathogenic bacteria residing in the plaque and elsewhere, while allowing the ~500 species of harmless oral microflora to flourish in a healthy ecosystem. Dentifrobots would also provide a continuous barrier to halitosis, since bacterial putrefaction is the central metabolic process involved in oral malodor [90]. With this kind of daily dental care available from an early age, conventional tooth decay and gum disease will disappear into the annals of medical history.

The visions described above may sound unlikely, implausible, or even heretic. Yet, the theoretical and applied research to turn them into reality is progressing rapidly. Nanotechnological developments are expected to accelerate significantly through new governmental [91] and private-sector [92] initiatives. It is unimportant which ones of these scenarios will actually come to pass. The important point is that advances in nanotechnological research and development have made such applications theoretically possible. Time, specific advances, resources, and needs will determine which ones will become reality.

Nanotechnological advances should be viewed in the context of other expected developments relevant to oral health in the coming decades. Biologic approaches such as tissue and genetic engineering [32, 33, 62-67] will yield new diagnostic and therapeutic approaches much sooner than nanotechnology. At the same time, continual refinement of traditional methods, development of advanced restorative materials, and new medications and pharmacologic approaches will continue to improve dental care.

Oral health and disease trends may also change the focus on specific diagnostic and treatment modalities. Increasingly preventive approaches will reduce the need for curative or restorative interventions, as has already happened with dental caries. Deeper understanding of the etiology and pathogenesis of other disease processes, such as periodontal disease, developmental craniofacial defects, and malignant neoplasms should make prevention for most of them a viable approach.

The role of the dentist will continue to evolve along the lines of currently visible trends. In the U.S., for example, cases evidencing simple neglect will become fewer, while cases involving cosmetic procedures, acute trauma, or rare disease conditions will become relatively more commonplace. Diagnosis and therapeutics will be customized to match the preferences and genetics of each individual patient. Treatment options will become more numerous and exacting. All this will demand, even more so than today, the best technical abilities, professional judgment, and strong doctor-patient interpersonal skills that are the hallmark of the contemporary dentist.

Nanodentistry faces many significant challenges [11] in bringing its promises to fruition. Basic engineering problems run the gamut from the precise positioning and assembly of molecular-scale parts, to economical nanorobot mass production techniques, to biocompatibility and the simultaneous coordination of the activities of large numbers of independent micron-scale robots. There are larger social issues of public acceptance, ethics, regulation and human safety that must be addressed before molecular nanotechnology can enter the modern medical armamentarium. But there are equally powerful motivations to surmount these various challenges, such as the vision that the 80% of the world's population that currently receives no significant dental care could enjoy a similar level of oral health to which citizens of the industrialized nations are already accustomed.

References and the complete text are available in the printed version of this paper, or online for ADA members and for paid subscribers, at the JADA Archives for the November 2000 issue of JADA, at http://www.ada.org/prof/pubs/jada/index.asp.

Last updated on 3 December 2002