## 3 Photonic Technologies
### Dental Materials  
Shrinkage of resin-based dental materials has been measured by several methods. Common clinical problems caused by the imperfect interfacial bonding are marginal staining, bacterial micro leakage, and recurrent cavities. Optical fiber sensors have been used to monitor the setting chemical reaction of different dental materials, like dental cements, dental composite resins, dental gypsum, and fiber-reinforced composites [133, 134, 135, 136, 137]. In particular, the amount of shrinkage and the rise in temperature was studied during the setting chemical reaction of dental gypsum for different water/powder (_w/p_) ratios (Figure 10) [133]. [PERSON] _et al._[135] presented, in 2000, a new method to characterize the amount of shrinkage of the cementing materials and the consecutive stresses appearing in the tooth cement-facing structure during bonding using highly birefringent single-mode optical fiber sensors and, in 2012, introduced a fully-automated setup to measure the Bragg wavelength shift of the FBG strain sensors and to accurately monitor the linear strain and shrinkage of dental resins during curing [136]. In a similar study, FBGs were used to monitor the linear strain due to the polymerization shrinkage and hydroscopic expansion, due to water sorption, of four fiber-reinforced biocomposites [137]. The polymerization shrinkage was monitored during the light curing process and the hydroscopic expansion of the samples was measured for 132 days, during which they were immersed in water. The authors have used a silicon mold to pour the samples and let them cure with blue light with embedded sensors. Three types of materials were monitored with only one sensor and the other with two sensors in orthogonal directions (Figure 11). Duplicate samples without embedded fiber sensors were made of each material in order to monitor the mass increase, due to water  
Figure 10: **Left:** Setting expansion variations; **Right:** Temperature variations; during the setting chemical reaction of dental gypsum type IV, for different _w/p_ ratios. Updated from [133], Copyright The Japanese Society for Dental Materials and Devices, 2011.  
sorption. The strain and temperature sensitivities of the FBG sensors used within this study were 1.20 pm/\(\upmu\)e and 11 pm/\({}^{\circ}\)C, respectively. The strain resolution was 0.4 \(\upmu\)e and the temperature resolution was 0.05 \({}^{\circ}\)C.  
FBG were also used to characterize the expansion of dental gypsum, for different ratios of gypsum powder and water [133]. Thermal variations were also evaluated, during the setting chemical reaction, and the linear thermal expansion coefficient was obtained. For that purpose, the authors carried out an experiment where they flowed the mixture of gypsum with water into a cylinder and used two FBG sensors. One of the sensors was in direct contact with dental gypsum, and hence sensitive to strain and temperature variations, and the other sensor was placed inside a double needle to overcome the cross sensitivity and to be only sensitive to temperature. In 2000 [PERSON] _et al._[138] developed a diagnostic device to permit simultaneous quantitation of multiple proteases within a single periodontal pocket, using a chemical fiber optic sensor, for the diagnosis of periodontal disease. Recent efforts were made to push forward further investigation to permit the use of FBG in _in vivo_ studies, to perform strain measurements in bones, and that the osteoblastic biocompatibility be evaluated [139]. There is a lack of information in this topic of experimental biomechanics which is important to give further ideas on the bone remodeling process around implants, which has been so well studied by other methods. Recently, some review reports appeared in the literature, presenting the application of FOS in biomechanics, in particular dental biomechanics and in biomedical applications [140, 141, 142, 143, 144, 145, 146, 147, 148], clearly showing the great interest in this type of sensor in research.