Abstract
Objectives
The present study was carried out to create composites and fibers using polyurethane (PU) with hydroxyapatite (HA) that could be used for dental applications.
Methods
Composites with varying HA concentration were prepared by solution casting technique. Similarly, PU–HA fibers with varying PU hard and soft segments and fixed HA concentration were also prepared. Various characterization techniques, such as, X-ray diffractometry, differential scanning calorimetry, scanning electron microscopy and Fourier transform infrared spectroscopy in conjunction with photo-acoustic sampling cell were employed to study the composites and fibers for changes in their physicochemical properties before and after immersion in artificial saliva at 37 °C for up to 5 days.
Results
The results indicated formation of amorphous apatite layers with maximum amorphicity in composites containing highest amount of HA with 5 days of immersion in artificial saliva. Similarly, fibers with more PU hard segment resulted in better transformation of crystalline HA to its amorphous state with increasing immersion time thus confirming the bioactive nature of the HA–PU fibers.
Significance
Concentrations of HA and PU hard segment along with the duration of immersion in artificial saliva are two major factors involved in the modification of solid-state properties of HA. The amorphous apatite layer on the surface is known to have tendency to bind with living tissues and hence the use of optimum amount of HA and PU hard segment in composites and fibers, respectively could help in the development of novel dental filling material.
1
Introduction
In the recent past, the use of polymer composites for biomedical applications has gained massive recognition. Composites are popularly used as dental filling materials for restoration of tooth caries and are much preferred over other dental restorative materials due to their user friendly properties . Properties of the composites, such as their biological and mechanical characteristics, can be tailored by changing type and ratio of the ingredients which may affect the interfacial adhesion at the interface where polymeric matrix comes in contact with the surrounding environment . Therefore, it has always been of great significance to develop a correlation between in vitro and in vivo results regarding the changes in physical and chemical properties, stability, degradation, etc. of the composites.
Polyurethane (PU) polymers are widely used as biomaterials in clinical applications. Over the years, their morphology, mechanical properties, synthesis and chemical properties have managed to gain significant attention of researchers due to their excellent and easily alterable properties . PU is comprised of different groups of polymers containing linkages of urethane within chains of each polymer. The fundamental parts of the polymer include the hard segments containing isocyanates and diols as chain extenders, and the soft segment containing polyols. All these monomers determine the properties of the resulting polymer . Degradation of PU is known to be affected by pH and temperature and it is reported that the body temperature (37 °C) along with its aqueous environment is sufficient to degrade a number of polymers . The process of hydrolysis and the rate of hydrolytic degradation for PU are related to the ester and ether linkages in the structure which are determined by the composition of hard and soft segments . Some other factors involved in affecting the biostability of PU are synthesis, processing, fabrication, surface area, physiological environment, etc. .
Hydroxyapatites (HA) due to their biocompatibility have found wide variety of uses in the field of dentistry and medicine. HA can exist in both crystalline and amorphous forms and is able to transform from one form to the other during preparation . The availability of HA in different forms has resulted in variable results in the respective fields and has caused some restrictions regarding its requirements by US Food and Drug Agency . Crystalline forms of HA are known to have better bond strength and cohesive properties whereas amorphous forms of HA are known to have high dissolution . The amorphous forms of HA when present on the outer surface of coating promote the growth of osseous tissue better than the crystalline form but do influence the stability of the implant or composite if present in excess or throughout the material . It is therefore of utmost importance to evaluate a particular form before its application.
The stability of dental composites in an aqueous environment depends upon its adequate mechanical properties with non-porous and smooth surfaces because these properties are profoundly affected by the action of water . It has also been reported that dissolution or degradation in surface layers may take place in materials that remain in contact with body fluids and some loss of unbound components is suspected may be along with fluid uptake into the basic structure. This fluid uptake in discrete zones of the material may exert unwanted residual pressure on the tissues thus resulting in softening, degradation or leakage of the materials .
The object of the present investigation is to prepare PU–HA composites and fibers with varying HA concentrations in composites and PU hard segments in fibers and study the effect of artificial saliva on their stability and solid-state properties with respect to time. This study would help in the development of composites and fibers of PU–HA with appropriate solid-state properties that could be used in the field of dentistry as novel obturating materials.
2
Materials and methods
2.1
Materials
Biomedical grade PU (Z3A1) was obtained from Biomer Technology Limited (Runcorn, UK), HA (sintered powder, Captal ® S) from Plasma Biotal Limited (Buxton, UK) and tetrahydrofuran (THF) (99.99%) from Fisher Scientific Inc. (Loughborough, UK). Versalink ® P-650 (oligomericdiamine) was procured from Air products and Chemicals, Inc. (Allentown, USA), methylene diphenyl diisocyanate from Biesterfeld (Hamburg, Germany) and Cil Release ® (1812 E) spray from Chemical Innovations Limited (Preston, UK). Artificial saliva (Saliveze™, Wyvern Medical Limited, UK) was purchased from a local pharmacy.
2.2
Preparation of PU stock solution
A stock solution of 5% PU was prepared in THF. The solution was stirred on a magnetic stirrer for 24 h at room temperature, protected from light.
2.3
Preparation of PU–HA composites by solution casting method
PU–HA composites were prepared in different ratios of 1:2, 1:4, 1:6, 1:8 and 1:10 by solution casting technique. From the stock solution, 10 ml of PU was taken out in a 25 ml conical flask and further stirred for 30 min. HA is insoluble in THF and forms a suspension when mixed with PU in THF solution. Therefore, each time HA powder was gradually added to the conical flask containing PU solution during stirring in order to obtain uniform dispersion. After adding all the HA powder of the desired ratio, the solution was further stirred for 30 min and then immediately poured in an aluminum mold. Before pouring of the suspension, the molds were sprayed with a mold releasing agent (Cil release ® ) in order to remove the composites easily from the mold. Slow evaporation of the solvent was achieved by covering the molds with a glass slide. Each set of composites was prepared in triplicate.
2.4
Preparation of PU–HA fibers by solution casting method
The composition of each type of fiber is given in Table 1 . The fibers with varying amount of PU were prepared by thoroughly mixing Versalink ® P-650 (PU-soft segment) and methylene diphenyl diisocyanate (MDI) (PU-hard segment) with HA powder to form a homogenous mixture before casting into a custom made aluminum mold. The mold was sprayed in the similar manner as described above to easily obtain the longitudinal fibers. Each set of fibers was prepared in triplicate.
| Composition | PU | HA (g) | |
|---|---|---|---|
| Versalink ® P-650 (g) | MDI (g) | ||
| 1 | 7.0 | 3.0 | 1.0 |
| 2 | 8.0 | 2.0 | 1.0 |
| 3 | 9.0 | 1.0 | 1.0 |
| 4 | 9.5 | 0.5 | 1.0 |
2.5
Study of PU–HA composites and fibers in artificial saliva
The dried samples (both composites and fibers) were carefully taken out from the molds and individually immersed completely in 5 ml of artificial saliva (Saliveze™). The saliva solution has a neutral pH and contains calcium chloride, magnesium chloride, sodium chloride, potassium chloride, dibasic sodium phosphate, carboxymethylcellulose, sorbitol, glycerol, methyl parabens, propyl parabens and mint flavor. Each sample was kept at 37 °C for 1, 3 and 5 days, respectively, mimicking natural oral environment.
2.6
Characterization studies
The effect of artificial saliva on the solid-state properties of composites and fibers was characterized by employing X-ray diffractometry, differential scanning calorimetry, scanning electron microscopy and Fourier transform infrared photoacoustic spectroscopy. Composites and fibers with no salivary exposure were used as controls and characterized in a similar manner. All the samples were stored in a desiccator during the course of characterization.
2.6.1
X-ray diffraction (XRD) analysis
The change in the degree of crystallinity of samples was studied through XRD analysis using Philips PW 1830 diffractometer. The data were collected over the 2 θ range of 10–60° with a scan speed of 2°/min and processed by HBX software.
2.6.2
Differential scanning calorimetry (DSC)
All the samples were analysed for their thermal behavior by using DSC 6 (PerkinElmer). Samples in an amount of 3.0 ± 0.1 mg were weighed accurately in an aluminum pinhole pan covered with a lid. An empty reference was also prepared in the same manner as that of the samples. Calibration of the instrument was carried out using standard zinc and indium. The sample along with the reference was heated from 30 to 350 °C with a rate of 10 °C/min and nitrogen was used as a purge gas with a flow rate of 20 ml/min. The data collected was processed using Pyris 7.0 software.
2.6.3
Scanning electron microscopy (SEM)
In order to study the morphology of the prepared samples, SEM Inspect F (FEI, Holland) was employed with an accelerating voltage of 5 kV. The samples were mounted on 0.5 inch aluminum stubs using double-sided carbon adhesive tabs (12 mm) (Agar Scientific, UK). The stubs were made conductive with silver lining and then subjected to carbon coating using Speedivac carbon coating unit (Model 12E6/1598, UK). Several magnifications of the microscope were selected to obtain optimum details of the samples.
2.6.4
Fourier transform infrared photoacoustic spectroscopy (FTIR-PAS)
The structural properties of the composites and fibers were studied using a Thermo Nicolet Nexus FTIR spectrophotometer (Thermo Fisher Scientific Inc., USA) equipped with a photo-acoustic detector (MTEC Model 200, USA). Spectra were obtained at 8 cm −1 resolution averaging 128 scans in the range 4000–400 cm −1 . The spectral data was processed using the OMNIC software version 7.4.
2
Materials and methods
2.1
Materials
Biomedical grade PU (Z3A1) was obtained from Biomer Technology Limited (Runcorn, UK), HA (sintered powder, Captal ® S) from Plasma Biotal Limited (Buxton, UK) and tetrahydrofuran (THF) (99.99%) from Fisher Scientific Inc. (Loughborough, UK). Versalink ® P-650 (oligomericdiamine) was procured from Air products and Chemicals, Inc. (Allentown, USA), methylene diphenyl diisocyanate from Biesterfeld (Hamburg, Germany) and Cil Release ® (1812 E) spray from Chemical Innovations Limited (Preston, UK). Artificial saliva (Saliveze™, Wyvern Medical Limited, UK) was purchased from a local pharmacy.
2.2
Preparation of PU stock solution
A stock solution of 5% PU was prepared in THF. The solution was stirred on a magnetic stirrer for 24 h at room temperature, protected from light.
2.3
Preparation of PU–HA composites by solution casting method
PU–HA composites were prepared in different ratios of 1:2, 1:4, 1:6, 1:8 and 1:10 by solution casting technique. From the stock solution, 10 ml of PU was taken out in a 25 ml conical flask and further stirred for 30 min. HA is insoluble in THF and forms a suspension when mixed with PU in THF solution. Therefore, each time HA powder was gradually added to the conical flask containing PU solution during stirring in order to obtain uniform dispersion. After adding all the HA powder of the desired ratio, the solution was further stirred for 30 min and then immediately poured in an aluminum mold. Before pouring of the suspension, the molds were sprayed with a mold releasing agent (Cil release ® ) in order to remove the composites easily from the mold. Slow evaporation of the solvent was achieved by covering the molds with a glass slide. Each set of composites was prepared in triplicate.
2.4
Preparation of PU–HA fibers by solution casting method
The composition of each type of fiber is given in Table 1 . The fibers with varying amount of PU were prepared by thoroughly mixing Versalink ® P-650 (PU-soft segment) and methylene diphenyl diisocyanate (MDI) (PU-hard segment) with HA powder to form a homogenous mixture before casting into a custom made aluminum mold. The mold was sprayed in the similar manner as described above to easily obtain the longitudinal fibers. Each set of fibers was prepared in triplicate.
