___.Expanding composites. While general practitioners have more to deal with monomer shrinking during their cure (filling decay), orthodontists have problems with undesi–rable attachment late debondings and insufficient accuracy in molding. To reduce polymer contraction, it is necessary to increase the volume ratio between the inert part (moiety) of the monomer vs. the one which actively participates in the polymerization process (the aliphatic double bond). Another way may even lead to polymer’s expansion: this actually happens if the monomer is cyclic (i.e. contracted, with a smaller volume) and becomes linear in the setting process¹. While the methacrylate esters of large molecules such as cyclodextrin, a relative of starch, may theoretically fulfill these conditions, the actual replacement of the ones already on the market may take long time.
___.Genetically modified cavity fighters. Approved by the FDA, Oragenics Co. may help put in everybody’s mouth a genetically modified strain of S. mutans that does not produce lactic acid^2,3. Lactic acid erodes the tooth’s enamel and causes the great majority of tooth decay. The release of this acid may be responsible also for the adhesive’s attack, known to lead to composite decay and premature attachment debonding.⁴
___.High strength nano composites. Nano-silica-whisker-reinforced dental resin composites that possess twice the strength and toughness that of glass-filled composites⁵, as well as improved wear resistance⁶, are now being developed.
___.“Cold”-cure composites. Polymerizations are exothermic and can generate enough heat to damage tissues. LED curing lights do avoid this side effect, but cannot penetrate as deep as the tungsten halogen lights and have to be used in layers. LED (light emitting diodes do dozens of different jobs and are found in all kinds of devices. Among other things, they form the numbers on digital clocks⁷.)
References
1. Hussain LA, Dickens SH, R.L. Bowen RL, Shear Bond Strength
of Experimental Methacrylated Beta-Cyclodextrin-Based Formulations, J Dent Res 2004; 83 (Special Issue A); IADR Abstract No. 3941
2. Pollack A, Genetically modified cavity fighters, The New York Times, December 1, 2004;
3. http://www.oragenics.com product_replacement.php
4. Matasa CG, Microbial attack of orthodontic adhesives, Am. J. Orthod. Dentofac Orthop 1995; 108: 132-41
5. Deng Y, Xu HHK, Schumacher GE, Lawn BR, Nano-silica-whisker composites: cyclic contact fatigue, J Dent Res 2004, 83 (Special Issue A); IADR Abstract No. 3971,
6. Quinn JB. Xu HHK, Anotonucci JM, Giuseppetti AA, Nano-silica-whisker composites: wear and mechanical Properties, Idem, Abstract No. 3968,
7. Aravamudhan K, Dickens SH, Rakowski D, Flaim GM, Eichmiller FC, Fan PL, Depth of Cure and Temperature Rise with LED Curing Lights, Idem, Abstract No. 0415.

Not all latex gloves behave the same

___.“Politehnica” University, Bucharest, Romania, hosted in November 5-7, 2004, an international conference on Biomaterials & Medical Devices, “BiomMedD’2004”. As this conference was a precursor of the 19^th European Conference on Biomaterials to be held in Sorrento, Italy (at the same time as the “World of Orthodontics” congress to be held in Paris), it is useful to show that topics dealing in depth with dental and orthodontic materials may be thus missed. Indeed, while the last congress will reunite the World Federation of Orthodontists, the 6^th International Orthodontic Congress, the French Orthodontic Societies and other major European Societies, the advances occurring in the biomaterials’ field are not less important.
___.The participants of the BiomMedD’2004 congress were luminaries in the field such as David Williams, Editor-in-Chief of Biomaterials (the world’s leading journal in this field), Head of the Department of Clinical Engineering at the University of Liverpool and the Director of the UK Centre for Tissue Engineering; James Kirkpatrick, President of the European Society for Biomaterials and a Professor at the Institute of Pathology, University of Mainz, Germany; Jacques Lemaître, President of the Swiss Society for Biomaterials, Lausanne; Luigi Ambrosio, Head of the Institute of Composite and Biomedical Materials - C.N.R and of the Interdisciplinary Research Centre on Biomaterials and a Professor at the University Federico II of Naples, Italy (see adjacent photo taken in the second largest building in the world, the Romanian Parliament).
___.While the program has comprised advances in implants, dental alloys, nano-structured materials, ceramics, glasses, polymers in adhesives and composites and tissue scaffolds, in what follows will be presented a part of a conference dealing suggesting substitution of bis GMA, the present back bone of adhesives, restoratives, sealants and veneers with a new monomer... Coauthor of the study is the Texas A & M University tenured professor in organic chemistry Alexandru T. Balaban, who has to date authored nine books, edited seven and got 26 patents. He has written over 50 chapters in books edited by other authors, and published over 600 papers published in peer-reviewed journals.

Challenges in medical in-situ biomaterial polymerization
Claude G. Matasa, Alexandru T. Balaban

I. How well your basic resin, bis GMA, performs?
___.While there are over sixty monomers currently used in dentistry, only two “resins” are currently used in sealants, adhesives and restoratives. Some formulations also include a polyurethane dimethacrylate [1,6-bis (urethane-ethyleneglycol-methacrylate)2,4,4-trimethylhexane], short UDMA; in contrast, bis phenol A di (glicydyl methacrylate), widely known as bis GMA or “Bowen’s resin”¹ reigns supreme after almost half a century.
___.Interestingly, an analysis of properties of this monomer shows that there is plenty of room for improvement.
___.1. Shrinkage. While shrinking (phenomenon that leads to poor marginal adaptation in fillings and in premature attachment debonding), bis GMA can still be rated acceptable. Unfortunately, to be used it has to be diluted with TEGDMA ( triethylene glycol dimethacrylate) : this comonomer shrinks considerably, see Table I.
___.Bis GMA’s ratio between the volume of the molecule that remains unchanged and that which participates in the polymerization (the double bonds!) is relatively high, rendering the monomer acceptable.
___.2. Color. While the monomer is almost colorless, its polymer discolors when subjected to light or bleaching agents. Composites become colored due to the remaining (unpolymerized) double bonds, as well as to the tertiary aromatic amines used as polymerization kickers. Hydroquinone, otherwise a powerful polymerization inhibitor, can not be used for similar reasons. UV stabilizers often have to be added and opaque containers are a must. Despite these problems, bis GMA behaves acceptably.
___.3. Setting vs. shelf life. After initiation (chemical or photochemical), a dental resin should react fast to become solid within 3 min and strong enough to be worked out within two hours. In addition, the system should be stable enough at room temperature to allow proper use after years. This is realized with the help of polymerization inhibitors, compounds that often impede with the resin’s desired setting. As there are no other systems that behave remarkably better, bis GMA may be considered as acceptable.
___.4. Fluidity. The ideal monomer should allow a good bonding to substrates by penetrating in their fissures (mechanical interlocking), accept a high load of filler in composites and have a low freezing point. While it doesn’t freeze at the normal room temperatures (its T_g is -6^oC), it is too viscous to be used as such and has to be amply diluted with aliphatic monomers such as triethylene-glycol dimethacrylate (TEGDMA), Table II. This undesirable property seems to be due to hydrogen bonding, a property shared with water, a liquid which in their absence would be a gas.
___.When compared with other monomers, aromatic methacrylates exhibit, for the same number of atoms of carbon in their molecule, a higher freezing temperature, a phenomenon related to viscosity, Table 3. Based upon the above considerations, it is obvious that bis GMA has too high a viscosity for the purposes intended.
___.5. Water affinity. Since the resulting composites have to withstand exposures of decades in a (wet) oral environment, the monomer used as matrix has to be water-hating, even if this results in a lack of affinity toward the enamel (water loving). The polymer has been found to absorb almost one molecule of water for each bis GMA/TEGDMA molecule².
___.As the composites based upon bis GMA are undoubtedly a net advance from this point of view versus both silicate cements and polymethyl methacrylate matrices (see Table 4), bis GMA’s resistance to water is deemed better than the others, but still too low...
___.6. Leaching. The matrix of an ideal restorative/ adhesive should both be deprived of monomers and oligomers that may be released and capable of entangling leachable components. While the bis GMA polymer per se is too bulky to leach, bis GMA/TEGDMA-based composites release twice as much TEGDMA than these made of UDMA/ TEGDMA³. While large, bis GMA’s molecule has insufficient functional sites and not enough cross-linking to retain leachable compounds. ___.As shown in Fig. 1, the testing for oxidable releases of a number of commercial composites based upon the classic system bis GMA-TEGDMA has shown that in all cases leaching can be a problem⁴.
___.7. Adequate molecular weight & structure. An adequate polymer matrix should exhibit a low dispersion and be deprived of monomers and oligomers. For illustration, Fig. 2 shows a poor distribution of the polymer length (pink): an abundance of short chain polymers translates into poor strength and solubility. In contrast, marked with blue is a desirable distribution where most chains have almost the same, longer dimension.
___.As the present bis GMA/TEGDMA system starts from relatively small molecule monomers, it leads to copolymers that comprise undesirable short, water-soluble chains.
___.8. Risk. A chapter entitled “Orthodontic polymers: a worrisome present?”⁵ and a fully dedicated web site⁶ show that bis GMA and its hydrolysis product, bisphenol A, are far from being safe. Bis GMA has been found to be allergenic, toxic, cytotoxic, mutagenic and oestrogenic, while its parent, bisphenol A, has been described as toxic, teratogenic, oestrogenic, antiandrogen, aneuploidic and resistant to biodegradation ^7-21.
___.These problems are due to the aromatic character which renders phenols acid, and to the biomimetism of the structures involved. The body confuses these compounds for estrogens, leading to undesirable consequences and possibly to death.
___.9. Resistance to wear & erosion. The main reason why bis GMA composites are still preferred resides in the exceptional strength of their matrix. As it can be seen from Table 5, such composites have mechanical properties similar to the enamel.
___.Sealants that better withstand erosion, restoratives that last and adhesives that bond for years render bis GMA a winner despite of the many attempts to replace it.. Out of the nine properties previously examined, only one is outstanding, remaining unequaled after almost half a century despite the many attempts to replace bis GMA. In what follows we will examine just few of these, along with a hypothesis that may lead to success.
II. What has been done to replace bis GMA?
___.Bis GMA is currently used in restoratives, adhesives, sealants, veneers, onlays, inlays. For over forty years (as shown in Table 6), it has been the basic ingredient for most composites (marked with asterisk). The attempts to replace it are numerous: perhaps its inventor, Rafael Bowen, has tried the most, alone or with his team at the National Institute for Standards and Technology, Gaithersburg, MD. After decades of trials and errors, the scientific community seems to have reached the consensus that the ideal resin should be a polyfunctional methacrylate. Polyfunctional, to provide cross linking, i.e. generate three-dimensional, strong copolymers: methacrylate, to provide a moiety that polymerizes fast and better withstands water. As the double bond needed for polymerization has to be provided by an acid which cannot be used as such (methacrylic acid is both toxic and corrosive), the search was, and still is, for its most suitable counterpart (moiety) which would check these properties while possibly adding desirable ones. From the biocompatibility standpoint, suitable such counterparts are the alcohols (distinctive function -OH), the amines (-NH-) and the thiols (-SH).
___.Actually, the first choice was the methacrylate of methylic alcohol used even today. To minimize the shrinking generated during polymerization (see Table I for some methacrylates), the volume of the -OH bearing moiety has to be as large as feasible, or even to expand during the polymerization process. This has led to the study of cyclic ethers, lactones, lactames, spiro-carbonates, etc., compounds that incur in such conditions the opening of a ring, i.e. an expansion in volume.^22-35 If these attempts would prove to be successful, the resulting monomers would be less toxic that the diglycidyl ether of bisphenol A (DGEBA) used today. Efforts in this direction, spearheaded by RL Bowen at NIST/Gaithersburg and DJ Eick at the U. of Missoury/Columbia are continuing.
___.Monomers comprising aromatic (benezene) cycles continue to be proposed^36,37,43, as such or as large monomers³⁸. Such polymers limit the amount of smaller, unreacted molecules that may weaken the structure and leach, but may also hydrolyze and release phenols and other noxious compounds. Dentsply International researchers and especially J. Klee have issued a number of patents in this field. Being based upon monomers that contain benzenic (aryl) cycles, not common in nature and therefore likely to generate problems, such resins and the resulting composites may present risks. While polyurethane dimethacrylates (UDMA) are less toxic than those containing benzenic cycles^{39, 40}, these do not exhibit the superior mechanical properties shown by bis GMA.
___.Recent works have suggested acidic monomers that contain acid groups such as phosphoric, phosphonic, phosphinic, sulfuric, sulfonic or sulfinic: their functionality may enhance adhesion to teeth tissues, but their resistance to water remains doubtful^41,42.
___.Recent advances in metal-catalyzed and controlled polymerizations could lead to star-branched, comb-like or dendritic macromonomers which, restricted to a determined molecular weight, could solve the leaching problem. Unfortunately, the need for monomer exhibiting low viscosities may render this avenue improbable.
III. A safer and improved version of bis GMA?
After decades of attempting to find a substitute for bis GMA, the researchers of the most focused institution on the matter at NIST/Gaithersburg, have arrived to the conclusion that “except for improvements in water-resistance, monomers with better mechanical properties than Bis GMA and UDMA could not being synthesized”.⁴⁴
___.In what follows, based upon the data we gathered, we will try to explain why this monomer has not been replaced even after almost five decades ( and this in a world where half the products found ten years ago on the market were withdrawn...)
___.As previously shown, to improve on bis GMA, the new monomer has to have both its outstanding mechanical properties and be less toxic, leach less and have a lower viscosity. In addition, it should cure fast, leading to insoluble, three-dimensional polymers. These conditions may be fulfilled by some terpene derivatives, as will be shown below.
Toxicity. Bis GMA can hydrolyze to bisphenol A, a substance exhibiting a variety of health risks, as already discussed. In general, aromatic (In contrast, terpenes and analog alicyclic compounds are less toxic, see Table. 7). Substances having LD-50 greater than 500 mg/kg or 1000 ppm are considered only slightly toxic.
___.Terpenes are widespread in nature, mainly in plants as constituents of essential oils. Many terpenes are hydrocarbons, but oxygen-containing compounds such as alcohols, aldehydes or ketones (terpenoids) are also found. Their structure may comprise two cycles, as in pinenes, or even three, like in camphor.
Viscosity. The lower the freezing temperatures, the more fluid a liquid normally is. As shown in Table 3, the freezing point of a terpene that has ten carbon atoms, isbornyl methacrylate, is -50oC: in contrast, the smallest aromatic methacrylate, freezes at room temperature (20^oC) despite it lower number of atoms (the smaller the molecule, the lower the melting point). In Table 8 are shown the viscosities of three terpenes which may be approximate to that of a solution of 65% bis GMA in TEGMA, see Table 2.
Mechanical properties. What makes bis GMA polymers strong is the rigid structure of a monomer unit derived from bis phenol A. This structure, specific to polycarbonates, is amorphous (thereby displaying excellent mechanical properties and high dimensional stability) and leads to Lexan®-type polymers commonly used as “bullet-proof” glass in military aviation, as well as for riot shields, vandal proof glazing and safety helmets. Better known applications are the CDs and the Invisalign® retainers.
___.Polymers have been likened with a heap of boiled, randomly bent spaghetti, each stick being made out of a large number of interconnected units (monomers). If the connection is also extended between sticks, i.e. in three dimensions, the polymer becomes by far stronger and less soluble. To allow the last property to happen, the monomers have to be polyfunctional, i.e. have chemical groups that can generate many bonds.
___.What distinguishes these phenol A-derived units from other monomers is that these bend less due to a phenomenon known as steric hindrance. As can be seen in the bis GMA’s formula, Fig.3, the two methyl (-CH₃) groups keep this rather large molecule long (straight)⁴⁵ In an assembly of other, irregular-shaped molecules, such a fibrilar structure reinforces the material.
___.Fibers measuring 25 mm x 2 mm⁴⁶ are currently used to reinforce high-strength composites: at a smaller scale, reinforcing fibers are called whiskers and measure around 10 x 1 microns. At an even smaller scale, nano-whiskers such as those of cellulose are measured in billionth of a meter, or nanometer. Taking in account the fact that atoms radii are between 0.25 and 3 Å (1Å=0.1 nanometers), and the lengths of chemical bonds are typically between 1 and 2 Å, the phenomenon of composite reinforcement goes as far as the atomic scale. In the bis GMA’s case, the length of these “nano-whiskers” is of about four times their width, as one can see from Fig. 4 where the straight part of the monomer unit is shown extrapolating X-ray diffraction data for anthracene.⁴⁷
___.It is our presumption that this monomer, which exhibits aromatic (benzene) cycles that are not found in living beings, can be substituted for other ones paralleling its structure but starting from naturally occurring materials such as terpenes or their analogs, as shown in Fig 5.
___.Starting materials for such compounds have a structure analog with terpenes and can be found in commerce. Thus, the larger alcohol above, i.e. tricyclodecane dimethanol (CAS Number 26896-48-0), having the formula shown in Fig. 6, is sold by Celanese Chemicals as TCD Alcohol (Dallas, TX 75234).
___.To obtain an analog of bisphenol A in which the aromatic rings are replaced by the tricyclo [4.3.0.12,5] decane scaffold, shown above, it is necessary to link the two molecules by a 2,2-propylidene group, common procedure in organic synthesis. Instead of the two methyl groups thus introduced, it is feasible to insert larger substitutes that could enhance the rigidity of the new polyalcohol
___.The resulting molecule can then be acylated with methacrylol chloride, leading to the dimethacrylate shown in Fig. 7.
___.While the ratio length /width in the case of bis GMA is 4, the “fibrilar effect” in this case is 5.5, possibly providing additional strength to the resulting polymer.
___.Technology today is capable of providing such compounds on a large scale: the same way Sartomer Company (Exton, PA 19341) sells commercial quantities of the diacrylate of dimethanol tricyclodecane (its SR833S), it can also deliver its dimethacrylate version, Fig. 8.
Discussion
___.Bis GMA has been described in the scientific literature as toxic, cytotoxic, mutagenic, carcinogenic and oestrogenic. It is a bomb waiting to explode in a litigious society where “experts” having medical degrees are ready to help. Due to “Proposition 65”, now law in California and in its way to expand, the physician is liable for the materials he uses. In this particular case, we are not aware about composite kits indicating the ingredients used or about information available to the public.
___.While there is another resin which shares the market, polyurethane dimethacrylate or UDMA, see Table 6, the monomer leads to weaker composites, and therefore bis GMA may very well be in the devices you use. The current study does not take an attitude against these, but against the fact that not enough avenues have been explored to make available to the public safer products. Investigating US patents, it is striking to see that, with the exception of the Paffenbarger Research Center of NIST/ADA and Dentsply International, there are very few companies that pursue new avenues in this direction. The improvements deal mostly with initiation systems, fillers (nano) and other topics for which the support is not too costly: it is true; to launch a new resin today may take a decade or so, at the level of drugs.
___.In our view, while expanding (ring opening) monomers may be the best solution, the fact that in decades no sizeable progress has been made and no related commercial product is on the market leads to the conclusion that other, new approaches should be taken. One of these may be the idea advanced here, i.e. to purposely tailor long, planar and therefore rigid monomers starting from less toxic natural products or their structural analogs.
Conclusion
___.This study suggests a way to substitute a monomer that has open an era, but which has problems, with others in which a similar, rigid structure is proposed. The starting materials and the syntheses are known, similar products already being already commercialized. As their structure approaches that of some natural products, the health risks involved should be reduced. While the synthesis of these monomers appears to be reasonable, the testing required and the launching of the products require involvement of superior means.
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