In this section, we will have a look how dentists and optometrists apply chemistry in their works. This section will have a look about fillings and adhesive that are used by dentists, and contact lenses from optometrists. In general, this section will discuss how polymer plays important roles for dentists and optomerists.
Teeth
To begin with, we will start from teeth's clinic and the history is an appropriate start for this part. In early times natural minerals or animal bones were used for implant teeth of pre-historic civilisation. Later on, semi-precious and precious gem stones or ivory were used for teeth implant. Then, in 17th century the replacement teeth are from cadervas or human donors, but it had some problems such as there were often rejected by the body. In that time, many people died from rotten teeth as penicilin had not been invented yet. However, surprisingly there is archaeological evidence for oral function over extended periods of these crude implants. In 19th century, there was the beginning of experiments to determine materials for teeth replacement and in 20th century new materials technology applied to teeth such as adhesive, bulk materials and fillers.
Before we enter the discussion about chemistry, it might be better to know some dentistry terminologies and the oral environment or the saliva. There are 3 main terminologies in dentistry, firstly is endosteal which mean the devices that enter the bone tissue. Secondly is subperiosteal which is the devices that contact the exterior of bone surface. Lastly, dental caries which is an infectious disease, which damages teeth. The dental caries or toot decay is mainly caused by bacteria lactobacillus sp. which converts sugars into lactic acid. This cause the oral environment becomes more acidic where the acid attacks tooth enamel which leads to caries. Meanwhile, the oral environment or saliva is basically an aqueous environment which comprises of 98% of water. Besides water, it also contains salts (e.g. Mg2+, Na+, K+, Ca2+, Cl−, HCO3−, and PO43-), mucus which is mucopolysaccharides, antibacterials such as H2O2 and IgA (immunoglobulin), and enzymes such as α amylase, which starts digestion of starch, and lysozyme for lysing bacteria.
Firstly, the chemistry of dentistry is related to the fillings. The early filling was amalgam which is quite old method (around 150 years old) which is composed of 43-54% liquid Hg and the rest is an alloy of Ag, Sn, or Cu. This fillings is held in by mechanical forces with no adhesive layers, so the dentist need to drill the decayed tooth deeper than the actual to ensure the fillings is adhered to the enamel. Besides that, there is another filling such as composite resin or known as white fillings. This fillings is a glass powder mixed with resin such as bisphenol A acrylate. This polymer resin is made from the polymer of bisphenol-A epoxide reacts with acrylic acid, then it is formulated to polymerise by UV and the glass filler is added. The addition of acrylic acid to bisphenol-A epoxide to create a polymer network.
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| Bisphenol A Acrylate fillings (white resin) |
Another filling is glass ionomers which is a composites formed from glass powder and an acid functional polymer (ionomer). The glass is often a fluoroalumino silicate (basic component) and the ionomer such as polyacrylic acid. The fillings is neutralised and leaching of Al3+ and F- to form crosslinks and gels.
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| Glass ionomer |
Besides fillings, another component of dentistry is adhesives. This component is used to ensure the attachement of fillings and crowns because of the difficult oral environment. The difficulties inside oral environments such as:
- Thermal stress is caused by extreme changing temperature of intake materials into oral cavity such as hot coffee and then followed by ice cream.
- Mechanical stress is caused by active role of teeth for chewing.
- Wet environment due to saliva and this leads to hydrolysis of fillings.
- Oral bacteria which can enter the tooth cavity and cause infection.
From those factors, it is important to ensure the fillings is adhered to enamel by using adhesive. There are 2 examples of adhesive, zinc phosphate, Zn3(PO4)2, and zinc polyacrylate. Zinc phosphate is synthesised from the reaction of ZnO and H3PO4, and it is an amorphous binder which works by penetrating mechanically produced irregularities.
The second one is zinc polyacrylate where polyacrylate and zinc provides crosslinks, and polyacrylate bound to calcium in the tooth.
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| Zinc polyacrylate adhesive |
Eyes
In this part we will have a look about the contact lenses and to begin with we will have a brief look about the eyes and tears.
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| Cross section of an eye |
The eyes are such complicated and unique organs. Although it is similar in some way with mouth (open cavity), it has immunoprivilage due to no decent blood supply. This uniqueness was found at the World War when the pilots of war aircraft had a bit perspex in their eyes after their plane was shot, and it did not cause infection. Therefore, the idea of contact lenses was came as the material would not cause complication. The appliance of lenses should consider 3 main aims. Firstly is the lenses would be removable such as contact lenses or it will implanted. The implanted lenses is used for intraocular lenses (IOL), intracorneal implant, and implant for glauocoma. Secondly is due to drug delivery and thirdly is for vitreous replacement.
Besides that, the eyes also quite wet environment that is caused by tears. The normal tears (basal tears) has functions to wet and to nourish the cornea, to lubricate the eye, and to clear the eye of dust. In normal healthy tears the composition of tears are comprised of water, mucin, lipids, proteins (lysozyme, lactoferrin, lipocalin, lacritin, immunoglobulins), urea, sodium and potassium salts. Therefore, the adsorption of these biomolecules can effect performance of a device, such as a contact lens. The contact lenses should have fulfil some properties for the comfortable of the users. The required properties are:
- Clarity
- Optical power/correction
- Chemical stability
- Cost effective manufacture
- High oxygen transmission-needed by the retina. Lack of oxygen may cause the formation of fibrin in the retina which may lead to blindness.
- Tear film wettability for the comfortableness
- resistance to adsorption of mucus, proteins, and lipids.
- Easy to clean and disinfect. The infected contact lenses is the main causes of infectious diseases in the eyes.
The soft contact lens is prepared by in-situ radical copolymerisation, which means the polymerisation is done inside the mould. This reaction is generated by UV radiation (most of the contact lenses is manufactured by this method) or heat. The general manufacturing scheme of contact lens is shown below.
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| The schematic diagram of soft contact lens manufacture |
The mould is filled not only by the monomers, but also with the cross-linker and photo initiator if the process is generated by UV radiation. The high intensity of UV light is used as well for sterilisation, besides for the reaction. Moreover, heat is also used to remove the residual polymer.
The monomers of contact lens is a hydrophilic monomer to ensure the water content. One of the example is methyl methacrylate which provide the toughness and to control water content and clarity. Another monomers are shown below.
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| Soft contact lens monomers |
The difunctional monomers polymerise both ends with the other monomers producing crosslinked polymer called a network. Then, water interacts with polymer but it cannot dissolve it because of the crosslinks so it swells. The polymer will understress as it is swollen and this stress limited the expansion.Hence, the water swollen polymer is called a hydrogel, and soft contact lenses are hydrogels. The role of water in contact lenses are to make the material soft and comfortable to wear-swelling and plastisization, to make it permeable to oxygen, and to provide very low surface energy and polymer is mobile so proteins etc do not stick or foul the lens.
The last factor that a contact lens should have is high permeability of oxygen. Oxygen permeability is vital for a healthy retina and increased water content provides increasing in oxygen permeability. However, the price of increased water is increased susceptibility to drying on eye. One of the polymer that provides high oxygen permeability is silicone polymers. Silicone polymers contain silicon with the proper name is poly(siloxane), but they are very hydrophobic (lipid stick to them) and fluid cannot diffuse through the material. Therefore, the solution to make a hybrid of a silicone hydrogels, which are conetworks. The networks composed of more than one block and a typicla strategy is to make a silicone macromonomer. The example of silicone hydrogel is shown below.
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| An example of silicone hybrid hydrogel |
Besides fillings, another component of dentistry is adhesives. This component is used to ensure the attachement of fillings and crowns because of the difficult oral environment. The difficulties inside oral environments such as:
The second one is zinc polyacrylate where polyacrylate and zinc provides crosslinks, and polyacrylate bound to calcium in the tooth.
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