1

Introduction

What I cannot create I do not understand.

Richard P. Feynman

Nanotechnologie ist in aller Munde

Deutsche Apotheker Zeitung

When the first author of this paper was a faculty member at Cornell University in the mid 1990s, the spirt of Richard P. Feynman, who was a Cornell faculty member until 1951, was still lingering on campus and in the labs. After Feynman’s famous and visionary lecture “ There’s Plenty of Room at the Bottom ” [ ] at Caltech in December 1959, many scientists saw him as the founding father of nanotechnology. In this lecture, he discussed information on a small scale, better electron microscopes, biological systems, small computers, miniaturization by evaporation, problems of lubrication, fabrication of materials and devices at the atomic/molecular scales and more. Although he did not refer to dentistry in his lecture, in a TV interview with Gell-Mann, Feynman used the example of brushing teeth to look at (common) things from a new point of view to develop new ideas [ ].

Since Feynman’s days, nanotechnology has moved from the state of a “vision”, or “science fiction”, to being a “real thing”, i.e., it has become reality. Many concepts Feynman dreamt up became true. Today, nanotechnology has left a major footprint in virtually all fields of science, engineering, technology, and medicine and dentistry are no exception to this.

Why is that so? It is so because nanomaterials and nanodevices have properties that were previously unaccomplishable and they make possible solutions to problems previously unsolvable [ ].

Applications of nanotechnology in dentistry are vast. They include but are not limited to dental diagnostics, preventive dentistry, dental materials, prosthodontics, endodontics, conservative and aesthetic dentistry, periodontics, implantology, and regenerative dentistry and nano-products [ ]. Nanotechnology plays also a major role in equipment used by dental practitioners such as LED light curing units [ ], protection of dental hard tissues against acid containing foods [ , ] and characterization of dental materials and dental hard tissues [ ] and many more.

Obviously, a detailed review of all these different and diverse areas in one paper does not make sense. In addition, to the best of our knowledge, no current review exists focused on nanomaterials for dentistry. Therefore, and since the journal this current review is intended for is called Dental Materials , we will focus in this review on nanostructures and nanomaterials relevant for dental materials.

In the past five years as of end of April 2020, the top ten journals ranked by impact factor (2018) [ ] in the categories Dentistry, Oral Surgery & Medicine published a total 6936 articles [ ]. Of these publications 146 have the word “nano” (including the truncated word “nano*) in the title and 415 have “nano” (including the truncation) as a topic [ ]. The number of citations of articles in these journals in the same time period with “nano” as a topic increased strongly from 55 in 2016 to 1461 in 2019. The journal Dental Materials published 859 articles in the past five years as of end of April 2020, of which 78 have the word “nano” in the title and 188 as a topic. The survey showed furthermore that among the top ten journals in the category Dentistry, Oral Surgery & Medicine, Dental Materials is by far the journal with the most nanotitles and nanosubjects in its articles.

Within these articles in Dental Materials , the most frequently covered topics (dental nanomaterials) are in this order: nanocomposites, nanoparticles/nanotubes/nanofibers including silver nanoparticles, antimicrobial materials, (bio-) mineralization and coatings. Therefore, we will focus on these topics in this review.

2

What is nano and nanotechnology?

The Greek word nάνoς means dwarf. In science nano refers to one billionth (American scale), denoting a factor of 10 ⁻⁹ . Since such a small number is hard to imagine for the human brain, analogies have been found to grasp the nanoscale. For example, the quotient of a meter (m) to a nanometer (nm) is approximately equal to the quotient of the diameter of the planet earth and the diameter of a hazelnut. One nanometer is approximately the length that a fingernail grows in one second [ ].

A general definition of a material being a nanomaterial is, when it is smaller than 100 nm in one dimension [ ]. This definition is based on the material’s structure . A more rigorous definition considers in addition to the structure the properties of the nanomaterial, i.e. that the material possess properties that are specific for the smallness of the material [ ]. This suits materials scientists, since they aim to develop structure-property relationships of materials. An important example relevant for nanomaterials in dentistry are the nanoparticles in nanofilled dental composites. Although synthetic nanomaterials are state of the art cutting edge materials, one should realize that living nature has developed and used natural nanomaterials for billions of years. For example, the proteins in our blood stream are functional nanomaterials.

Nanotechnology can be defined as a technology that deals with small structures or small sized materials [ ]. Nanostructures or nanosized materials can be created by two different strategies. The top down strategy starts normally with a bulk material much larger than “nano”, and then uses externally applied forces (mechanical or other), to break the material down for example into nanoparticles. The bottom up strategy, essentially proposed by Feynman [ ], assembles materials or structures atom by atom or molecule by molecule to obtain the desired result, although – given the magnitude of Avogadro’s Number – such a process must be massively parallel. A relevant example for nanostructures that may be useful in dentistry are nanostructured titanium surfaces for implants in which the nanostructures have an antimicrobial effect without utilizing antibiotics [ ].

Although nanoscience and technology are among the fastest growing areas in science and technology and interesting per se , “nano” is not always “better”. Sometimes, the term nano is used to market products in dentistry without sufficient clinical evidence that the nanoversion of a material is significantly better than the conventional non-nanoversion of the product. But it seems that “nano” sells.

3

Basic physics and chemistry of nanomaterials

Materials on the nanoscale, i.e., nanomaterials have unique properties. These properties are physical and chemical in nature. Nanomaterials, often also called nanostructures, may be categorized by their dimension [ ]. Zero-dimensional nanostructures are nanoparticles, and one-dimensional nanostructures are nanowires and nanorods. Thin films are two-dimensional nanostructures. All these structures fulfill the definition of a nanomaterial or nanostructure mentioned above: they are smaller than 100 nm in one dimension. As discussed below, nanoparticles in different varieties are important nanomaterials in dentistry, hence we focus on them in this work.

One of the most striking features of nanomaterials such as nanoparticles is the large ratio between their surface and their volume. The radii of atoms range approximately between 0.30 nm and 3.00 nm [ ]. If one considers a nanoparticle of a few nm in diameter, this means that all atoms are either at the surface of the nanoparticle, or within a few atomic distances from the surface inside the particle, depending on the size of the atoms and the size of the nanoparticle. For example, for a cube of iron with an edge length of 1 nm, every atom constituting this cube is a surface atom [ ].

The number of atoms that constitute a nanoparticle of a given size can be calculated to good accuracy using simple arithmetic, if the type of unit cell of the material and its lattice constant are known and assuming a spherical shape of the nanoparticle. The latter assumption is realistic, considering the forces resulting from the surface energy.

In a bulk material all atoms bond to their neighbors. Surface atoms, however, possess fewer nearest neighbors and thus have dangling or unsatisfied bonds [ ]. The lack of binding partners for surface atoms leads to an inwardly directed force towards the center of the particle and a change of lattice constants (relaxation) or even the lattice type (reconstruction) [ ]. If these surface atoms are part of a nanoparticle this affects the physical properties of the whole particle significantly.

As a consequence of the unsatisfied bonds at the surface of the nanoparticle, the surface atoms possess extra energy called surface energy or surface free energy or surface tension [ ]. The surface energy is

where, G is the Gibbs free energy and A is the surface area. This is the energy necessary to create a unit area of “new “surface [ ]. For a new surface to be created, bonds need to be broken, which requires energy.

These considerations show that not only the geometrical surface is important for understanding the unique properties of nanoparticles, but also the volume of the particle that is influenced by the surface. This is sometimes called physical surface [ ].

The size of nanoparticles or more precisely their curvature affects their chemical potential μ, which is essentially the driving force of a substance to react with another, for phase changes or for diffusion. The change of surface chemical potential from atoms in a flat surface to a nanoparticle is inversely proportional to the radius of the particle:

where, Ω is the atomic volume and R is the particle radius [ ]. Thus, nanoparticles have a large chemical potential which results in a high chemical reactivity of these particles and a high atomic diffusion from these particles. As a result, for two particles in a solvent with R ₁ ≫ R ₂ the larger particle will grow at the expense of the smaller particle through diffusion [ ]. This is called Ostwald ripening.

The physical and chemical phenomena mentioned above have several important consequences for the properties of nanoscaled materials and their synthesis. Nanoparticles tend to agglomerate in order to reduce their surface energy, resulting in a loss/change of their unique properties. This effect is a challenge when producing or working with nanoparticles, for example during the production of nanoparticle filled dental composites.

During the sintering of ceramics based on nanoparticles, Ostwald ripening may lead to an undesired grain growth and inhomogeneous microstructure leading to inferior mechanical properties of the ceramic [ , ]. This is relevant to produce high strength dental ceramics. Based on their high surface energy content, nanoparticle based ceramics may have lower sintering temperatures than the macroparticle based counterparts. This is useful when dimensional stability is paramount.

The surface energy is also responsible for a drastic change of other physical properties of nanoparticles compared to bulk materials. For example, bulk gold has a melting point of 1064 °C. However, melting of gold nanoparticles is below room temperature for particle sizes less than 1.4 nm [ ]. For gold nanoparticles larger than 15 nm the melting point approximates the melting point of the bulk material. Since the nanoparticles are much smaller than the wavelength of light, they do not scatter light when dispersed in transparent media [ ].

4

Synthesis, stabilization and processing of nanomaterials

A major challenge in nanotechnology is the controlled and purposeful synthesis of nanomaterials and nanostructures. The methods to synthesize nanomaterials are vast and depend on several factors such as dimension of the materials created (0D, 1D, 2D, 3D) and the material class produced.

The most frequently used type of nanomaterial in dentistry and a common denominator in dental nanomaterials are 0D nanoparticles. Especially nanoscale particles and surface structures represent nanotechnology among consumers [ ]. Therefore, we focus on the synthesis of nanoparticles in this section.

The Latin word “pars” means part. Nanoparticles can be created by both the top down or the bottom up approach. In addition, synthesis methods for nanoparticles depend of the material class the particle belongs to: metal, ceramic or polymer. Nanoparticles can be synthesized via the solid, liquid or gas phase. Depending on the method of synthesis, nanoparticles have an irregular or regular shape and broad or narrow particle size distributions.

The classic top-down approach for creating ceramic nanoparticles is very fine grinding or colloid milling. Ball mills use steel or other balls rotating in a hollow cylinder to crush the material by impact and attrition. A ring and ball mill consists of two types of rings separated by a series of large balls, like a thrust bearing [ ], that crush the material in between two particles. The attrition mill mechanically reduces solid particle size by intense agitation of a slurry of material being milled and coarse milling media [ ]. Such mills are frequently used for the creation of ceramic nanoparticles with sizes down to a few tens of nanometres.

Physical and technological factors limit the smallest particle sizes that can be obtained by top-down milling of materials. First, the smaller the particles, the stronger they are. This is because smaller particles are single crystals and/or have normally fewer defects, such as grain boundaries, than larger ones. This makes it more difficult to crush particles to sizes smaller than a few tens of nanometres. A second limit of milling are the properties of the milling balls: their active (crushing) volume, Young’s modulus, and kinetic energy ( Fig. 1 ). As a rule of thumb, the ratio of the diameters of the milling balls and the particles are 1000:1, i.e., to obtain 100 nm particles the milling balls should have a diameter of 100 μm.

Principle of particle crushing in a ball mill. The large milling balls crush particles within their active volume (shaded area between the milling balls). Image courtesy J. Bossert und I. Firkowska-Boden, FSU Jena.

Bottom-up approaches for the creation of nanoparticles use changes in thermodynamic equilibrium to induce phase transformations of materials. A starting point for this can be supersaturated non-equilibrium solutions (step 1) in which nuclei form (step 2) and then grow to particles (step 3). Nanoparticles can be obtained through heterogenous or homogenous nucleation [ ]. Nanoparticles form if the number of nuclei is large and the growth is limited with the aim of obtaining many nanoparticles with a narrow size distribution. This can for example be accomplished by strong supersaturation, limiting diffusion, supercooling, low concentrations and increasing the viscosity.

Metallic nanoparticles in metal colloidal dispersions are typically synthesized via reduction of metal complexes in dilute solutions under controlled reduction reaction conditions [ ]. Dental materials frequently used silver nanoparticles due to their antimicrobial properties. Metal cations such as silver in sufficient dose have a damaging effect to living cells. The antimicrobial action of these ions is based on cell wall and cell membrane damages, oxidation of proteins and lipids and disrupting hydrogen bond between DNA strains. Since silver has a strong tendency to oxidize and nanoparticles have a large surface to bulk ratio, silver nanoparticles may have a high silver oxide content [ ]. Silver nanoparticles are typically synthesized via wet chemistry methods from AgNO ₃ complexes in the presence of a reducing agent [ ].

The manufacturers of dental composites use different approaches to produce nanofillers for their nanofilled composites and only publish their methods to a limited extent. Microfillers in dental composites are sometimes aggregates of nanoparticles. Some dental composite manufactures produce many of their filler particles by a sol gel process [ ]. The sol gel process is suitable to produce metal oxide nanoparticles such as silicon oxide, zirconium oxide or hybrid polymers from colloidal dispersions. Particles are made from small molecule liquid precursors, called sol (i.e. from solution). Nanoparticles form through hydrolysis reactions. The particles grow and aggregate in solution which leads to an increase of viscosity of the sol and then subsequently form a gel (viscoelastic solid). The gels are then dried and further treated; for example by tempering or sintering.

The sintering process may be modified to produce loosely agglomerated nanoparticles, i.e., nanoclusters [ ]. Such nanoclusters behave similarly to the densified particles found in other composites in terms of providing high filler loading [ ]. The resulting composite is claimed to have high strength and wear resistance with significantly improved polish retention and optical properties [ ].

Other manufactures use highly dispersed and non-aggregated nanofillers, aiming for homogeneous dispersion and complete resin wetting of nano-sized filler particles, to improve the aesthetic and mechanical properties of composites. For example, organically modified ceramic nanoparticles can be produced via controlled hydrolysis and condensation reactions [ ]. The nanoceramic particles have a size of 2.3 nm as investigated by X-ray diffraction [ ].

Polymer nanoparticles including nanospheres and nanocapsules are frequently used for drug delivery purposes [ ]. They often contain active pharmaceutical ingredients within each particle or have adsorbed macromolecular substances on their surface. The preparation of polymer nanoparticles can be divided into two approaches, i.e., two-step and one-step procedures.

Polymer nanoparticles can be prepared by polymerization-based methods such as emulsion polymerization, dispersion polymerization or interfacial complexation or by polymer participation methods such as single/double emulsion, solvent displacement or salting out [ ].

Drugs can be incorporated either during nanoparticle preparation or after. Typical polymers used for polymer nanoparticles are chitosan, polyacrylamide, polyacrylate and polyesters [ ].

In order to release drugs from the nanoparticle into the human body, often biodegradable polymer nanoparticles are used. Polymer nanoparticle based drug delivery has several advantages, compared to conventional drug application, such as targeting to specific tissues and cells via ligand specificity, absorption of polymer nanoparticles into cells, lower doses of drugs necessary, reduced toxic effects, sustained drug release at the target site and enhancement of therapeutic potential of drugs [ ]. Nevertheless, several major hurdles such as particle aggregation or control of release kinetics need to be overcome to accomplish all these advantages.

Disadvantages of polymer nanoparticles for drug delivery include the relatively high costs of production, the challenge of controlling release kinetics and the tendency of polymer nanoparticles to agglomerate.

Agglomeration is the major challenge when processing and working with nanoparticles. It needs to be avoided or reduced to maintain the unique properties of nanoparticles. Two major strategies to stabilize nanoparticles in suspension are electrostatic stabilization and steric stabilization or combinations of both [ ].

In electrostatic stabilization, the attractive forces between nanoparticles are counterbalanced by repulsive Coulomb forces. This is, for example, accomplished by attaching negative surface charges on the nanoparticles through ion adsorption or the creation of a Stern layer [ ].

Steric stabilization is based on multiple short polymer chains adsorbed on the surface of the nanoparticles. If nanoparticles mutually approach, the surface polymer segments of the different particles also get closer and have less freedom to move. This causes a reduction of entropy (ΔS < 0) which increases the Gibbs free energy [ ]. As a result of this thermodynamic penalty the particles do not agglomerate. If the interactions of the polymer chains with the surrounding medium (e.g. water) is stronger than that between the polymer molecules, enthalpy contributes to the non-agglomeration of the nanoparticles.

As mentioned above, the most common application of nanoparticles in dentistry is their use in dental composites. Dispersing nanoparticles in a polymer matrix is intrinsically challenging because of unfavorable entropic interactions between the matrix and the nanoparticles [ ]. However, the simplest ways to create a resin-composite incorporating discrete (non-agglomerated) nanoparticles is to start with the unfilled monomer mixture. To the liquid monomer mixture may be added up to ca. 30% vol/vol of non-agglomerated nanofiller without an enormous increase in the viscosity of the system. This nano-incorporating monomer mixture can then be mixed with larger particles in the micron and sub-micron size range to form a nano-hybrid paste. Since highly-filled composites only contain ca. 20% vol/vol monomer, the final proportion of nano-filler in the composite may be as low as 6–8%. Nevertheless, the structural benefit of nanoparticles in a nano-hybrid is that they can fit between the spaces remaining when larger particles are in direct contact. Furthermore, they have an effect of reducing the absolute monomer content and thereby the magnitude of properties such as polymerization shrinkage that can have adverse consequences. Some manufactures add nanoparticles (aerosil SiO ₂ ) to keep the larger particles suspended in the resin and not settle out.

Further methods of dispersing nanoparticles in polymer matrix composites include chemical methods such as sol-gel routes (see above), surface treatment and functionalization such as grafting of polymer chains on the nanoparticle filler surface. Dense polymer brushes can lead to a good dispersion of nanoparticles in (uncured) polymer matrixes when the length of the grafted chains is comparable to that of the matrix molecules [ ].

Nanoparticles can also be dispersed in polymer matrixes using mechanical methods such as high performance milling (see above). A three-roll mill (calender) applies high shear forces to particle agglomerates in polymer matrixes to break them up by calendaring. Furthermore ultrasonication, stirring with high shear forces [ ] or attrition milling are useful for breaking up nanoparticle agglomerates in the preparation of polymer-based nanocomposites.

The resulting nanoparticles and nanoparticle clusters can be mixed in the mills with solvents and activated resins to form a fluid dispersion incorporating well dispersed particles [ ]. Subsequently, the suspension is transferred to a spray granulator where small droplets form from which the solvent evaporates during heat treatment, resulting in resin-particle spheres with a narrow size distribution [ ]. The spheres are then cured producing completed fillers for composites.

5

Typical nanomaterials of high relevance for dentistry

As mentioned above, our survey of articles in the journal Dental Materials showed that the most frequently covered topics in dental nanomaterials are in this sequence: nanocomposites, nanoparticles/nanotubes/nanofibers including silver nanoparticles, antimicrobial nanomaterials, (bio-)nanomineralization and nanocoatings.

Table 1 shows a ranking of the nanotopics published in the past five years up to the end of April 2020 in Dental Materials .

Table 1

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