**3. Bonding materials in dentistry (cements and fillings)**

In human oral cavities, it is possible to distinguish two types of dentitions: primary and permanent one. First of the mentioned occurs in children aged 2–6 and involves incisors, canines and molars. Primary dentition is with the time replaced by permanent one. This type includes 32 teeth, and in comparison to the previous one, it comprises additionally premolars [11]. The

In the construction of every tooth, three basic elements can be mentioned, i.e. crown, neck and root. Among tissues that form teeth, enamel, dentin, cementum, and pulp have to be mentioned. Enamel—the toughest tissue in the human body—in the vast majority (95%) is made of inorganic compounds. The most innervated and vascularized tissue from the above-

Inadequate oral hygiene which consequently results in the accumulation of food debris and in the development of bacterial flora leads to the development of various types of teeth and periodontal diseases. In some cases, the process of healing of this type of illnesses is complex and lengthy. Therefore, it is necessary to develop this field of medicine whose role is to deal with functioning but also with treatment of teeth, periodontal, all tissues and any other elements contained in the oral cavity. The origins of this science should be sought several thousand years ago. Literature reports indicate that the traces of actions aimed at preventing tooth decay were discovered in the skull of young man whose remains were found more than 14,000 years ago. Areas of dentistry such as orthodontics and prosthetics also are characterized by a long tradition. The hygiene and the aesthetics of oral care were taken in ancient times. Therefore, high attention has been paid to the development of this field of medicine. With the passing of years, new elements that had to meet the growing demand on the market

Nowadays, it is necessary to use dental materials associated with artificial restorations or with technologies that in perfect way will enable a reconstruction of the missing piece of the dentition. Such materials are undoubtedly dental bridges, crowns, dentures and dental braces [11, 12].

different types of teeth in dentition were shown in **Figure 5**.

mentioned is a tooth pulp [11, 12].

30 Acrylic Polymers in Healthcare

of dental materials have been introduced [11, 12].

**Figure 5.** Kinds of teeth in permanent dentition.

The concept of 'cement' is associated unambiguously with the construction industry. However, it should be noted that this term also refers to the materials constituting the binder in dentistry. There is a wide diversity of dental cements including zinc-phosphate, glass-ionomer, compomer or composites (on the basis of resin). Last of the mentioned due to their characteristics are becoming more and more popular on the market of medical products [13].

Composites based on acrylic resins are materials widely used for dental restorations. These are becoming increasingly popular displacing from the market of medical products amalgam fillings. The use of those fillings caused the problem in terms of safety due to the fact that their main component is toxic mercury. However, it should be noticed that in some ways, they outweigh the fillings on the basis of acrylate resins. In fact, they were characterized by greater durability and better resistance to fracture. Therefore, nowadays, researchers are working on the improvement of the physico-chemical properties of resin based on acrylates [13].

Acrylic materials are included in glass-ionomer cements. This filling is composed of two elements, i.e. powder and liquid. The liquid is an aqueous solution of copolymers of itaconic and polyacrylic acids, while powder contains fluoro-calcium-aluminium-silicon glass. Interesting feature that distinguishes this type of cement from the others on the market of dental products is a very good adhesion to enamel and dentin. Furthermore, glass-ionomer cements are characterized by an ability of forming a chemical bond with the tissue of the tooth which contributes to the cariostatic action of such filling (protection against tooth decay). Polyacrylates contained in the liquid react with the compounds forming the powder and consequently release of various types of ions with a predominance of fluoride anions occurs. As a result of their reaction with polyacids, formation of a gel matrix occurs which combines chemically with the dentin. This is a middle layer of a tooth which is composed of fluid-filled tubules, connective tissues and nerves. The elements included in the dentin are mainly calcium (27%) and phosphorus (13%) [14].

Combining of glass-ionomer cement (**Figure 6**) with the tooth is a result of forming a chemical bond between ions contained in cement and ions of phosphorus and calcium included in the structure of enamel or dentin. Tooth surface must be previously cleaned that has an impact on the effectiveness of the formed combination [14].

Glass-ionomer cement is characterized by good resistance to bending. However, these materials have also some disadvantages including low tensile strength. Undoubtedly, one of the most favourable features of these fillings is their antibacterial activity which derives from the presence of fluorine in their structure [15].

Hybrid cements such as glass-ionomer fillings or those one modified with resin are characterized by slightly better strength properties. It is worth noting that these materials are chemo or light cured. Besides liquid and powder, such cements contain in their composition catalysing system of a micrometric size disposed in the form of capsules. The mentioned system consists of ascorbic acid and potassium persulfate, whereas liquid comprises an aqueous solution of polyacrylic acid containing additionally 2-hydroxyethyl methacrylate (HEMA). The connection

**Figure 6.** Glass-ionomer cement.

between this kind of filling and a tooth is stronger that a bond that is formed by glass-ionomer cement [16, 17].

Acrylates play an important role also in the compositions of zinc oxide-polyacrylic cements (known also as zinc oxide-polycarboxylate cements). The aqueous solution of poly(acrylic acid) acts as a liquid, while powder is composed of properly processed oxides of magnesium and zinc. While application of the mentioned filling, poly(zinc acrylate) is synthesized. That polymer is characterized by gel consistency. Combining this type of cement with the tooth is accompanied by process of chelating of calcium occurring in dentin by carboxyl groups present in the polymer. However, it is worth mentioning that these cements are characterized by a slightly weak strength properties (such as small bending strength in comparison to other commercially available cements as well as the relatively large thickness that hinders adjusting and deposition of filling in the place of application) [16, 17].

Acrylates represent an important group of components of selected cements based on resin. Particularly key role is played by methyl methacrylate. This compound is almost from the half of the twentieth century used for the deposition of various types of prostheses. Widely used are also composite cements that consist of a mixture of methacrylates (or acrylates) with a glass or with a suitably modified silica. Mentioned restorations belong to the group of materials chemo or light cured. On the market of dental products, a significant role is also played by composites modified with polyacids, i.e. compomers that also can contain acrylates [16, 17].
