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Article

Comparison of the Bioactive and Bacteriostatic Performance of Different Alginate-Based Dental Prosthetic Impression Materials with and without Zirconium Phosphate-Based Ion Exchange Resin Containing Silver: An In Vitro Study

by
Zbigniew Raszewski
1,*,
Marcin Mikulewicz
2,
Dariusz Brząkalski
3,
Daria Pakuła
3,4 and
Robert E. Przekop
3,*
1
R&D, SpofaDental, Markova 238, 506-01 Jicin, Czech Republic
2
Department of Dentofacial Orthopaedics and Orthodontics, Division of Facial Abnormalities, Medical University of Wroclaw, Krakowska 26, 50-425 Wroclaw, Poland
3
Centre for Advanced Technologies, Adam Mickiewicz University in Poznan, 61-614 Poznan, Poland
4
Faculty of Chemistry, Adam Mickiewicz University in Poznań, Uniwersytetu Poznańskiego 8, 61-614 Poznań, Poland
*
Authors to whom correspondence should be addressed.
Appl. Sci. 2023, 13(21), 11639; https://doi.org/10.3390/app132111639
Submission received: 29 September 2023 / Revised: 19 October 2023 / Accepted: 23 October 2023 / Published: 24 October 2023
(This article belongs to the Special Issue Treatment and Materials of Dental Restorations)
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Abstract

:

Featured Application

An alginate-based impression material with bacteriostatic properties for application in the production of dental prosthetics, requiring a less labour-intensive process of disinfection.

Abstract

Disinfection of alginate impressions used in dental prosthetics is a very problematic procedure, during which the dimensions of the obtained model can change, and it can be easily mechanically damaged. Therefore, it would be beneficial to create a material with bacteriostatic properties for this application. In this work, we investigated alginate materials with the addition of 0.25–1.0 wt% of zirconium phosphate-based ion exchange resin containing silver and compared them with commercial alginate impression products. The obtained materials passed the normative requirements of ISO 21563:2021 (setting time, elastic recovery, strain in compression, compatibility with gypsum, detail reproduction, tearing strength, and linear dimensional change) as well as bacteriostatic properties in relation to Streptococcus mutans. The newly developed alginate materials met all the properties required by the standards. Their setting time was comparable to the commercial materials, and their 24 h contraction did not exceed 1.3%. Furthermore, the tearing strength was 0.42–0.48 MPa, less than that of the comparative commercial materials (0.52–0.56 MPa). From the point of view of clinical practice, it will be possible to create an alginate material with bacteriostatic properties that will prevent the transmission of cross-infections.

1. Introduction

Despite the new techniques of taking impressions, in the form of intraoral scanners and various types of silicone materials, alginates are still one of the most commonly used impression materials [1]. This is the result of their properties, including ease of use, hydrophilicity and therefore good contact with the tissues to have the impression taken from, and low pressure needed to be exerted on delicate soft tissues [2,3]. Slight pressure on soft tissues during impression taking is especially important when making removable dentures, skeletal dentures, or removable orthodontic devices. It is also significant that alginate is a bioderived material and may easily decompose after use, in contrast to silicone materials [4]. Alginate powder, after mixing with the appropriate amount of water, forms an elastic sol, which, when working, quickly becomes a gel, reacting with calcium ions present in the formulation [5,6]. There are also studies on making alginate material available in the form of two separate gels, which can be mixed by extrusion from the cartridge, similar to silicone sealants or two-component adhesives.
Unfortunately, alginate materials have a number of disadvantages, including low tear resistance and release of water from the material during storage, causing shrinkage of the setting material and changes in dimensions [7,8]. Therefore, casting gypsum models from alginate impressions should be performed as soon as possible [9,10,11].
However, before making a plaster model, it is necessary to disinfect the impression, as it is a source of cross-infections due to the initial contact with the patient’s tissues and saliva [9,12,13,14]. The disinfection of the impression can be carried out using two methods: immersing the impression tray with setting material in a solution of a disinfectant or spraying the surface of the impression with a suitable preparation. It can be performed by contacting the impression with 0.15% chlorhexidine water solution [15] or placing it in a closed bag for a period of 10–15 min, the time necessary for proper disinfection [16,17]. Other disinfectants for alginate can be alcohol or aldehyde solution [18,19]. However, special attention should be paid to alcohol-based disinfectants, as they cause strong water absorption and changes in the dimensions of the impression during the disinfection process itself [9,20].
On the other hand, storing impressions in aqueous solutions of disinfectants causes water absorption through the alginate and changes in its dimensions (expansion) [20]. Furthermore, the disinfection process itself is a labour-intensive step, which requires appropriately trained personnel to ensure that no damage or distortions to the material are caused. Therefore, it would be desirable to create an alginate material formulation with inherent antibacterial properties. There are reports that describe the addition of various types of chemical substances that have antibacterial properties: ceramic negative ions [9], silver [21], 0.2% silver nitrate, 0.2% chlorohexidine solution [22], chitosan [23], and silver nanoparticles [24,25]. However, a large part of this research focusses on the addition of a germicidal substance to the water–alginate mixture, which is clinically undesirable because it complicates the preparation of the alginate sol. A much better solution is to add the disinfectant directly to the powder, thus modifying the formulation, while leaving the sol preparation procedure unchanged.
Some of the popular impression disinfectants cannot be used directly in the impression material due to their very high toxicity (aldehydes, sodium hypochlorite, iodine compounds, etc.) [26,27].
One possible substance that exhibits bactericidal properties is AlphaSan RC 2000, a commercial zirconium phosphate-based ceramic ion exchange resin. There are a number of papers in the literature describing the addition of silver to alginate materials. However, their use requires adding silver in the form of a solution [22,24] that is not desired by the end user. The addition of silver nanoparticles to the powder can change the colour of the alginate material itself towards grey. AlphaSan is a material containing silver but has a neutral white colour which allows the desired colour of the formulation to be retained. The silver content is responsible for antimicrobial activity in a wide range of applications. AlphaSan RC 2000 is used in healthcare products, food packaging, fibres, and textiles [28]. In the literature, it is possible to find information about the use of this material in combination with alginates as dressings accelerating the healing process of burn wounds [29]. However, to the knowledge of the authors, this material has not yet been used in alginate materials for dental impressions. Therefore, the objective of this work is to investigate the addition of silver-containing ion exchange resin to the alginate formulation. The tests cover both the mechanical and biological properties of the obtained material, as well as the setting characteristics. As a comparative material, the authors decided to test commercial alginate impression products. The thesis of this article is that the alginate material after modification with a bactericidal agent will have virtually the same setting or mechanical properties as commercial products available but will be significantly improved in terms of utility, based on its antimicrobial properties.

2. Materials and Methods

2.1. Materials

A number of common commercial products have been acquired for the sake of comparison of their characteristics with the formulations obtained within this study. Before use, the materials were stored in a thermostatic room at 23 °C for 24 h to reach the required temperature. For the tests, all tested products were mixed with distilled water (23 °C) in the ratio recommended by the manufacturer. In order to thoroughly mix the material, distilled water was first poured into the bowl, and then the recommended amount of powder was added. The alginate materials were mixed manually in a silicone bowl using a plastic spatula for the time recommended by the manufacturers.
In the case of materials with the addition of silver ceramics, the material preparation method was as follows, and the mixing time was 30 s. The commercial materials are summarized in the Table 1. All the materials and chemicals used to prepare the studied bacteriostatic impression formulations are summarized in Table 2. Agar for biological tests was obtained under the trade name MS-agar from Difco, Detroit, MI, USA. AlphaSan RC 2000 (Milliken, Spartanburg, SC, USA) was used as a bacteriostatic agent. The agent is claimed by the producer to be a zirconium phosphate-based ceramic ion exchange resin, in the form of a very fine powder (average particle size of 1.3 μm), containing 10 wt% of silver [30].

2.2. Preparation of Alginate Impression Materials

The composition of the alginate material prepared for testing is presented in Table 2. Raw composition of alginate impression material is widely known in the literature [5,31]. All raw materials were used without further purification. The components were added by weighing all ingredients on a laboratory scale and mixing for 10 min in a high-speed mixer at 500 rpm (Netzsch, Selb, Bayern, Germany). Finally, 0.25 wt%, 0.5 wt%, and 1.0 wt% of AlphaSan 2000 were added to the base powder prepared in this way, and mixed again for 5 min. The compositions with 0.25 wt%, 0.5 wt%, and 1.0 wt% of AlphaSan were designated as X1, X2, and X3, accordingly. For testing the commercial materials, they were mixed with water according to the recommended ratio shown in Table 1. X1–X3 formulations were mixed by spatula with ratio of 6 g powder to 16 mL water. Figure 1 shows the mixing of the new alginate.

2.3. Testing Methods

2.3.1. IR Infrared Spectroscopy

Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet iS 50 Fourier transform spectrophotometer (Thermo Fisher Scientific, Waltham, MA, USA) equipped with a diamond ATR unit with a resolution of 0.09 cm−1.

2.3.2. Thermogravimetric Analysis

Thermogravimetry (TGA) was performed using a NETZSCH 209 F1 Libra gravimetric analyzer (Selb, Germany). Samples of 4 ± 0.2 mg were placed in Al2O3 crucibles. Measurements were conducted under nitrogen (flow of 20 mL/min) in the 30–800 °C range and a 10 °C/min heating rate. Sensitivity: 0.1 μg.

2.3.3. Optical Microscopy

A Digital Light Microscope Keyence VHX 7000 with 100× to 1000× VH-Z100T lens and a VHX 7020 camera (Osaka, Japan) were used. Polarization attachment was used to minimize the negative effects of the materials’ reflectance and therefore the amount of the artefacts generated during the picture capture.

2.3.4. Material Setting Tests

The material was tested according to the requirements of the ISO21563:2021 [32] standard, determining the setting time, compressive strength, tearing strength, elastic recovery and strain, detail reproduction, and compatibility with plaster. The working time of the materials was measured with glass rods. Materials prepared in accordance with Section 2.2 were mixed with water and placed in a stainless-steel mould with a diameter of 30 mm and 16 mm thickness. The surface of the unset alginate is touched with a glass rod every 5 s. until no material adheres to it. For each of the alginate materials, five measurements were made. The description of this test was presented by Liu Zhao (2019) [9].

2.3.5. Detail Reproduction

The material after mixing the powder with the liquid was placed in a metal block having the 50 μm and 75 μm wide lines engraved and covered with a glass plate. After setting, it was taken out and analysed under an optical microscope (Karl Zeiss) at 10× magnification. On the surface of the alginate material, according to the normative requirements, a 50-micron-wide line must be reproduced (Figure 2) [33]. For each of the alginate materials, 5 measurements were conducted. Tests were performed according to ISO 21563:2021 [32].

2.3.6. Compatibility with Plaster

Because alginates are very sensitive to evaporated water, compatibility tests with plaster were performed immediately after assessing the material surface, i.e., within 10 min of mixing the powder with the liquid. After surface analysis, the alginate impression was washed under a stream of water at a temperature of 23 °C for 30 s in order to remove unreacted alginate residues from its surface, which could disturb the smoothness of the surface of the plaster cast. Then, the disc was placed in a metal ring as described in ISO 21563:2021 [32] and poured with gypsum class 3 (Mramorit Blue, SpofaDental, Jicin, Czech Republic). The plaster was mixed manually in the proportion of 30 g of water/100 g of plaster for 30 s according to the manufacturer’s instructions. Then, the silicone bowl with the mixed plaster was placed on a vibrating table (Falab, Warsaw Poland) for 30 s, stirring constantly to remove the gas bubbles contained in the material. Next, the alginate impression in the metal ring was placed on the same vibrating table, and the plaster was poured very slowly with small amount only from one side of the mould (left or right). The whole thing was subjected to vibration for 30 s.
Thirty minutes after the plaster material had set, the plaster model was released from the metal ring and analysed under an optical microscope (Karl Zeiss, Jena, Germany) at 10× magnification. A line of 50 microns must be visible on the surface of the plaster model [34]. A detailed description of this method is provided in ISO 21563:2021 [32]. Also, the number of samples in this test is defined normatively: 5 for each tested material.

2.3.7. Contraction Measurement

It is assumed that the maximal limit of impression material contraction is a linear change not greater than 2% [35]. Alginate samples after mixing were placed in a metal mould with engraved lines (from ISO 21563:2021 [32]). The distance between the lines is defined. After being set, the alginate disks were placed in a plastic bag with a wet towel. After 24 h, the distance is measured with a Carl Zeiss microscope (Jena, Germany), and the contraction factor was calculated from the following equation:
Cx = ((S − R)/S) × 100%
where S is the defined distance on the block (24.805 mm), R is the distance measure after 24 h, and Cx is the contraction. For each of the alginate materials, 5 measurements were taken. The scheme of this block is shown in Figure 2.

2.3.8. Compressive Strength

During impression taking, some of the material on the impression tray is subjected to compressive forces. Therefore, a very important parameter is the resistance of the material to compression. The test was carried out on a universal testing machine (WPM, HAIDA, Dongguan City, China). A cylinder-shaped Teflon mould was used to obtain samples with a diameter of 12.5 mm and a height of 19 mm by pouring the alginate mixture into a mould covered on both sides with laboratory glass. Thirty seconds after the material had set, it was removed from the mould and placed on the compression device. The sample was compressed by moving the upper platform at a speed of 2 mm/min. The test ended with a fracture within the material. This test is not required by the new ISO standard, but it provides important information about the setting characteristics of the impression material. The minimum value required by the previous version of ISO 21563:2013–Hydrocolloid impression materials standard is 0.35 MPa [6]. For each of the alginate materials, 5 measurements were made. Detailed descriptions of the tests performed are available [9].

2.3.9. Tearing Strength

The version of the ISO 21563:2021 [32] standard defines the need to test alginate materials for tensile forces. After mixing, the alginate material was placed in a steel mould with the 80 × 10 × 4 mm testing specimen dimensions and shape shown on Figure 3 [33]. After filling the mould, a piece of PE foil and the glass pane were placed on top. One minute after the material had set, the samples were removed from the mould and placed in the universal testing machine at 2 mm/min speed (WPM, HAIDA, Dongguan, China). The test ended when the sample ruptured. Also, the number of samples in this test is defined normatively: 5 for each tested material.

2.3.10. Elastic Recovery

This study was carried out in accordance with the ISO 21563:2021 standard [32]. A measuring device (Somet, Bílina, Czech Republic) equipped with a micrometer of 0.01 mm accuracy was used. Cylinder-shaped samples obtained according to the Section 2.3.8 were used for the test. One minute after the setting time had passed, the material was removed from the mould and placed in the instrument after another minute. The load was added until 20% deformation in height was recorded, and the load was kept for 5 s. Next, the load was removed after 30 s, the measurements were taken, and the elastic recovery calculated according to Equation (2). Five repeated measurements were taken for each tested material.
he = (1 − ((H1 − H2)/19)) × 100%
where he is the elastic recovery, H1 is the original high of the sample, and H2 is the value after 20% deformation read after 30 s.

2.3.11. Compressive Strain

The sample of an alginate impression was processed in the same method as described in Section 2.3.8 and Section 2.3.10. The compressive strain of the samples was tested analogously to the description of Section 2.3.10 and using the same equipment. The test was carried out according to the ISO21563:2021 [32] standard. The first S1 value was determined when the sample of alginate material was placed in the device and the S2 value after 40 s of the sample being under the load of a 900 g weight. The number 19 in the equation is the height of the sample in mm. The compressive strength value was calculated from Equation (3). Five repeated measurements were taken for each tested material.
E = (1 − ((S1 − S2)/19)) × 100%
where E is the compressive strain, S1 is the original high of the sample, and S2 is the value upon the deformation after 40 s under 900 g weight load.

2.3.12. Determination of the Microbiological Activity of Alginates against Streptococcus mutans

This study was performed according to the method presented by Köhler and Bratthall [36]. It involves the use of a flat wooden spatula, which, after contact with saliva, is imprinted on the agar plate. One of the authors as a volunteer was instructed to chew a paraffin block for approximately 3 min, then a wooden spatula (151 × 18 mm) is used to collect the saliva by swirling it 10 times in the saliva on the tongue and removing excess through the patient’s slightly compacted lips. Then, both sides of the spatula are imprinted immediately on the agar plate. Incubations were carried out for 48 h at 37 °C in anaerobic conditions (95% N2 + 5% CO2). In the next stage, three samples of aligning materials from each tested system were placed on the substrate prepared in this way. After 24–48 h, the zone of inhibition of the growth of microorganisms in contact with the alginate sample was examined.

2.3.13. Statistical Analysis

Statistical analysis was performed with nine repetitions for each parameter, providing a robust dataset for the study. Data were expressed as mean ± standard error of the mean. A one-way analysis of variance (ANOVA) was performed to test for significant differences between the different alginate materials, including 4 commercial alginates and 3 testing materials containing AlphaSan (X1–X3), to determine the differences between the new alginate samples with the bacteriostatic agent and materials available on the market. A p-value < 0.05 was considered statistically significant. Post hoc tests were performed following the ANOVA analysis. GraphPad Software Inc., San Diego, CA, USA, was used for the statistical analysis.

3. Results and Discussion

3.1. FT-IR Spectroscopy

At first, the composition of the studied impression materials was studied with FT-IR and TGA. FT-IR allowed for qualitative identification of the materials’ components, while the TGA allowed for quantitative analysis of the organic (alginate) fraction within the formulations (Figure 4). As the alginate salts are a common ingredient present in all the formulations, both commercial and those prepared within this study, sodium alginate and potassium alginate were used as spectral standards for the sake of comparison and to identify the absorption bands associated with the functional groups of the alginate component (Figure 4A). From the compositions prepared within this study, only X3 was shown, as their spectra were virtually identical; it was compared with the commercial samples (Figure 4B). The FT-IR spectra for pure potassium and sodium alginates exhibited characteristic signals in the range of 3400–3200 cm−1, which originated from the stretching vibrations of hydroxyl groups (OH) and carboxyl groups (-COOH) present in alginate molecules. The bands at 2933 cm−1 can be attributed to stretching vibrations of C-H groups occurring in polysaccharides. In the range of 1428–1400 cm−1, there are bands originating from C-OH deformation vibrations and symmetric O-C-O stretching vibrations. The signals located at 1025 cm−1 correspond to the stretching vibrations of the C-O carboxyl group. The presence of vibrations of the carbonyl group (-C=O) confirms the signal at 1593 cm−1. The signals at 1123 cm−1 and 947 cm−1 are assigned to the C-O stretching vibrations occurring in the pyranosyl ring. Commercial alginate samples contain other substances, such as calcium sulphates, celite, phosphates, oxides, and colouring and flavouring substances. FT-IR-ATR spectra show characteristic signals coming from silica, which is the main component of celite. The intense band at 1068 cm−1 corresponds to asymmetric Si-O-Si vibrations, while 790 cm−1 corresponds to symmetric stretching. Signals at 1620–1615 cm−1 confirm the presence of vibrations of the carbonyl group (-C=O), while those at 1415–1405 cm−1 correspond to C-OH deformation vibrations and symmetric O-C-O stretching vibrations. The range of 3400–3200 cm−1 is assigned to the stretching vibrations of hydroxyl groups (OH). The Tropical IQ sample exhibits additional signals at wavelengths of 1416 cm−1 and 874 cm−1, which originate from the vibrations of inorganic carbonates [37]. The presence of these carbonates has been confirmed by TGA analysis, as described in Section 3.2.

3.2. Thermogravimetry

As mentioned in the previous section, TGA allowed for additional insight into the composition of the impression compositions, mostly the organic fraction content. On the basis of thermogravimetric analysis, onset temperatures (Tonset1 and Tonset2), temperature of maximum mass loss rate (Tmax1 and Tmax2, determined from the first derivative of mass loss curve, DTG), as well as residual mass were determined (Table 3). For comparative analysis, X3, containing the highest loading of AlphaSan, was presented, as all the materials showed very similar thermal characteristics. As materials undergo heating, moisture loss can occur both on their surface and within the material. Furthermore, the chemical bonds present in the material may break down, leading to its degradation. Thermogravimetric curves of sodium and potassium alginates demonstrate two stages of decomposition, as illustrated in Figure 5A.
The initial step of the process begins at approximately 47–48 °C and reaches the maximum mass loss rate at 66.3 °C as the water present on the surface undergoes evaporation. At higher temperatures (Tonset2 = 230 °C), the molecules undergo thermal degradation, causing the breakdown of polysaccharide chains into smaller fragments, including monomers and by-products. During this stage, alginate decarboxylation reactions occur along with the cleavage of the glycosidic bond [38]. As a result of these processes, the weight loss is >50%. With an increase in temperature, the remaining fragments of alginates are susceptible to degradation, resulting in the release of gaseous by-products. The residual mass, approximately 30%, consists of ash and/or mineral residues, which is typical of organic substances.
For commercial samples, the first stage of decomposition starts at a higher temperature (Tonset1 between 85–96 °C) compared to pure alginates. This indicates that the water is evaporating from the materials. The second stage begins above 225 °C, which is similar to pure Na/K alginates. This stage is responsible for breaking down the polysaccharide chains. The weight loss is lower than in the case of pure materials, due to the fact that alginates constitute a smaller percentage of the total composition. Kromopan and Elastic Cromo have the highest residual mass of 88.5% and 87.7%, respectively, which proves that they have the lowest content of pure alginates, due to containing additional components. The TGA curve of the Tropical IQ sample shows the third stage of decomposition decomposition, which reaches the maximum temperature Tmax3 at 692.6 °C, which matches the thermal decomposition event of the inorganic carbonates and indicated their presence in the material, as mentioned earlier in Section 3.1 [39]. The course of the X−1% curves with the addition of AlphaSan RC 2000 is analogous to commercial materials such as Kromopan or Elastic Cromo. The remaining materials showed an analogous decomposition route.

3.3. Optical Microscopy

The microscopy imaging provided information on the shape and size of particles constituting the studied formulations. Figure 6 presents the base materials, i.e., pure alginates and AlphaSan RC 2000, while Figure 7 presents the commercial formulations and one of the ones obtained on the example of X3. X1 and X2 showed no visible changes in comparison to X3 and were omitted.
Sodium alginate particles (Figure 6(A1,A2)) are flaky in shape and large in size, some particles exceeding 100 μm. Objects of such size were not visible in any of the formulations, which might be the effect of components grinding, while the flaky shape of the particles is visible in Tropical IQ, Kromopan, Elastic Cromo, and the obtained formulations (Figure 5B). KromaFaze, on the other hand, was characterized by a more granular shape, with a significant amount of cylindrical-shaped particles and the remaining ones being irregular (Figure 7(A1,A2)). The granular shape is a characteristic feature of potassium alginate; however, in its raw form, the material’s particle shape is more irregular and usually below 100 μm (Figure 6(B1,B2)). AlpaSan RC 2000 is a very fine powder, the single particles being barely visible and of the size of single microns (Figure 6(C1,C2)). Due to its size, AlphaSan was barely visible within the obtained formulations. The X1–X3 formulations are morphologically very similar to that of KromaFaze, as X1–X3 are also based on potassium alginate.

3.4. Material Setting Properties and Detail Reproduction

The results of the standard setting tests, i.e., setting time and contraction after 24 h, are presented in Table 4. Because the setting reaction of the alginate formulation consists of the transformation of the material from the liquid phase (sol) into a gel, it can be characterized by the working time within which the material remains in the liquid phase and can change its shape, adapting to the oral soft tissues and teeth when taking an impression. The setting time of the alginate mass should not be too long, as it would unnecessarily extend the time needed to take an impression and cause additional discomfort to the patient. The material after its working time will cause voids in the impression. The commercial materials can be characterized by a short gel setting time of less than two minutes (Tropical IQ at 107.4 s) at room temperature, and by a normal setting time of less than 3 min (the three remaining products). Therefore, the commercial products’ setting time is in the 107–147 s timeframe. The obtained formulations fit within this range, as the setting time was found to be from 127 s for X2 to 131 s for X1 and X3, making these materials considered to be of normal setting time.
The slight increase in the setting time for samples containing a bacteriostatic agent, as compared to Tropical IQ, may be due to the reaction of the alginate with either the silver or zirconium ions contained in the AlphaSan powder instead of the calcium ions that are needed to gel the alginate. However, this process can be easily corrected at the stage of material production by increasing the loading of K2TiF6, which is a catalyst that accelerates the crosslinking process [40].
Alginate materials lose water over time, which causes their contraction. However, this value cannot be greater than 2% after 24 h. The tested materials had a dimensional change from 1.08% for the formulation X3 to 1.22% for X2, which is larger than that of most of the commercial products, but comparable with Tropical IQ. Material shrinkage can greatly vary depending on the manufacturer and grade of the material, which Sharif et al. [34] presented in their research; when testing Tropicalgin and Hydrogum 5 materials, they obtained values of 0.4% after the first 24 h of material storage.
The shrinkage of alginate materials under storage is a phenomenon widely described in the literature. Recently, a group of materials has been introduced on the market that can be stored for a longer period of time (in an environment of 100% humidity). Examples of such materials are Kromopan and KromaFaze, tested in this study. After 24 h, they had dimensional changes of 0.83% and 1.01%, respectively.
This phenomenon can be minimized to some extent by rinsing the impression with water and storing it in a sealed plastic container that is maintained in an environment of 100% humidity [31]. The research carried out showed that the surfaces of the gypsum casting poured after storage of the impression in a plastic bag and covered with a wet towel were better than those casted immediately after rinsing. This phenomenon can be explained by the higher calcium/sodium ratio and the lower organic and water content of the extended-pour alginates compared to the conventional ones [33]. A reduction in the concentration of calcium ions in the composition of alginate powder was found to increase the swelling capacity of alginate chains and, consequently, cause expansion of the material [22].
According to the recommendations of the ISO standard, alginate materials are to duplicate the lines of 50 microns width on a metal block. In addition, after casting the plaster model from the alginate impression, this line should be visible on the plaster casting surface as well. All the obtained materials (X1–X3) have passed both tests. The examples obtained with X3 are presented in Figure 8 and Figure 9.
The detailed reproduction of the alginate impressions shows that the alginate material is able to reproduce the 50-micron line, but it is not able to reproduce the 20-micron line [31,33]. This may be attributed to the sufficient flow and the non-altered viscosity of the alginate impression before gelation. The obtained results are consistent with Singer et al., 2023 [22]. This may be due to the fact that the powder particles in this type of material have a granularity of less than 100 microns.

3.5. Mechanical Properties

The materials were analysed in accordance with the ISO standard to verify their compliance with the requirements given for the impression materials. The results are collected in Table 5.
The mechanical resistance of commercial materials to compression is from 0.84 to 1.07 MPa. The obtained new alginate materials X1–X3 all have a resistance of ~0.85 MPa, which is comparable to commercial materials and meets the normative requirements of 0.35 MPa. Elastic recovery is the ability of the material to return to its original shape and, according to ISO 21563:2021 [32], must be greater than 95%. The alginate masses have typical elastic recovery at the level of 95–97%, which is a lower value than, for example, silicone materials, where the value of this parameter is 96–99.5% [40]. The results obtained indicate that all of the studied materials meet these requirements and are comparable to those of the commercial samples. This value can be variable and depends on the type of sulphate used for the alginate material and the concentration of sodium alginate itself in the material [22,24], the authors of which studied the modification of alginates by means of nanosilver particles in water. The addition of a bacteriostatic agent as an intertwining material may reduce the value of elastic recovery [5].
The compression strain according to the above-mentioned standard must be less than 20%. The tested samples have these values in the range of ~11–12%, exceeding the performance of Kromopan and Elastic Cromo. The tearing strength of alginate materials according to the previous ISO standard must be greater than 0.35 [MPa]. All tested materials have this value at the level of 0.44–0.57 MPa, passing the test; however, the addition of AlphaSan reduces the tearing strength of alginate materials along with increasing loading of the additive. These values are lower than those obtained in the study by Singer et al. 2023, who investigated the addition of silver nanoparticles to the alginate material and obtained values of 1.11 [MPa]. In the literature, such values vary from 0.4 to 1.2 N/mm [33,41]. It has been hypothesized that tear strength could be affected by certain factors, including the degree of crosslinking of the set alginate [42]. An increase in tensile strength can be achieved by reducing the amount of water used to decrease the alginate mass by 50% [41], without compromising the properties of the impression obtained.
In the case of compressive strength, no differences were observed when increasing the content of the bacteriostatic agent. Other authors Liu et al. [9], when adding more ceramic-based bactericidal agents, observed an increase in compressive resistance. The difference in the results may be due to the composition of the ceramic itself, the content of monovalent silver ions present in AlphaSan, which may react with the alginate carboxylic groups competitively with the divalent calcium ions responsible for the crosslinking reaction. This effect would explain the increasing drop in the tensile strength of the formulations accompanying the increasing loading of AlphaSan. The gelling process of the alginate mass itself occurs when approximately 10% of the alginate carboxyl groups are crosslinked with calcium ions [43].

3.6. Antimicrobial Activity

The new formulations under study have passed all normative tests, being suitable for dental impression materials. At this point, all qualified for the last stage of the study, i.e., the antimicrobial activity investigation. For this purpose, the Streptococcus mutans strand was used, a strain of bacteria commonly known for naturally habituating the oral cavity and being the main reason for Caries dentium.
Three samples of alginate material containing 0.25–1% silver-based ceramics were made for testing. These concentrations were determined based on the information provided by the material manufacturer, who recommends using this raw material at a concentration of 0.05–1%, and the work of Qin [29].
Figure 10 shows the degree of antibacterial activity that changes with increasing AlphaSan loading in the alginate material, compared to Kromopan. When the additive content was 1%, the antibacterial inhibition zone for S. mutans was the largest and reached 4.5 mm. For the control sample (agar only), there was no inhibition zone present. For alginate Krompan, this value was 1 mm. This effect may be related to the ingredients contained in alginates, phosphates, and potassium fluorotitanate, which decomposes in contact with water, releasing small amounts of hydrofluoric acid. For the composition studied, a higher AlphaSan loading causes an increase in the concentration of silver ions, resulting in a higher ion gradient between the alginate gel and agar, silver ion migration, and an increase in the inhibition zone, where the growth of microorganisms is observed.
There are many scientific reports that are based on the effectiveness of silver in various forms as a bactericidal or bacteriostatic agent. Because of a large surface-to-volume ratio, silver nanoparticles exhibit remarkable antimicrobial activity, even at a low concentration. In addition, they are inexpensive and have shown low cytotoxicity and immunological response. The mechanism of the bacteriostatic action of silver ions has been presented in the literature. This is due to the fact that silver cations are attracted by the removal of charged sulphide anions present in the structure of proteins. The adhering ions are then responsible for increasing the permeability of the cytoplasmic membrane, which consequently leads to the disruption of the bacterial envelope. In the next stage, free silver ions are absorbed into the cells, which deactivates respiratory enzymes, producing reactive oxygen species, and disrupts the production of adenosine triphosphate. Reactive oxygen may be the main factor causing disruption of the cell membrane and modification of deoxyribonucleic acid (DNA). Since sulphur and phosphorus are important components of DNA, the interaction of silver ions with these two elements can cause problems in DNA replication, cell reproduction, and even result in the extinction of microorganisms. Moreover, Ag+ ions can inhibit protein synthesis by denaturing ribosomes in the cytoplasm [44,45].
Silver (and especially its nanoparticles) was also used as a bacteriostatic agent by other authors [21,25], in specific concentration ranges from 0.1% to 5%. In the case of Nia et al. [25], the concentration of nanoparticles was 0.1–0.2%. Therefore, the authors obtained smaller inhibition zones for this type of material than in the case of 0.25 CHX. According to Ginjupalli et al. [21], silver ions have a strong bactericidal effect in diffusion tests and also extend the binding time of the material. This was confirmed in our tests. However, the use of silver nanoparticles causes the alginate material to change colour to grey. In our case, the use of silver-based ceramics, which are white, does not change the colour of the impression material.
The addition of AlphaSan, described in the literature, was effective for the burn treatment material. Qin proposed that silver ions can be released from material through various mechanisms and act as bacteriostatic agent. First, there is an ion exchange between the silver ions in the zeolite material and the sodium and calcium ions in the wound fluid. Second, silver ions can be chelated by protein molecules. Third, AlphaSan particles can also be attached to the surface of algin fibres and detach from the fibres and enter the soft tissues [29,46]. In these studies, a zeolite containing silver ions was effective against Pseudomonas aeruginosa and Escherichia coli. However, this study requires confirmation in the case of alginate impression material.
In recent years, more and more emphasis has been placed on the possible undesirable effects of silver ions on the human body. The answer to this question can be found in Qin’s work, as silver is inert when deposited in human tissues [29]. It is normally present in mammalian tissue at concentrations <10 mg/dL. Furthermore, for serum, silver concentrations of up to 20 mg/dL are considered nontoxic. In the case of alginate materials, the contact of the alginate material with the patient’s mucosa is very short, a maximum of 3 min, which has been tested in these studies. The silver accumulated in the organs can usually be cleared after 8 weeks [45].

4. Conclusions

Novel alginate-based formulations with a silver-containing, zirconium phosphate-based additive, Alphasan RC 2000 have been obtained and extensively tested. The formulations pass all the normative requirements for alginate, or in general, hydrocolloid impression materials, as per the ISO 21563:2021 [32] standard in terms of all the setting and mechanical properties. The thesis placed at the very beginning of this work has confirmed that an alginate material containing a bacteriostatic agent based on silver-containing ceramics can be produced. The materials showed a bacteriostatic effect visible by the formation of an inhibition zone of the growth of Streptococcus mutans, increasing proportionally to the AlphaSan content. It shows that the strength of the bacteriostatic activity can be tuned based on the material requirements, such as the storage time of the impression.
The alginate material that was tested in this work would be a very desirable product in dental practice. Currently, after excretion, it must also be disinfected before casting a gypsum model. Before this step, the impression is a hazardous material with biological contamination that requires appropriate treatment. The addition of silver ceramics would mean that such material, after being rinsed in water, would probably not be a source of dangerous bacteria.
The second important issue is the influence of the alginate impression disinfection process on the stability of their dimensions. During this time, some materials absorb water or release it, which then affects the accuracy of dimensions [2,3,8,14]. Therefore, eliminating this step would be highly desirable.
The conducted research, however, has some limitations. Only one type of microorganism was used to test the antimicrobial effectiveness. Other species may also be found in the oral cavity. In our further work on this type of alginate materials, it will also be necessary to test other concentrations of AlphaSan materials and compare them with other materials with bacteriostatic properties. Therefore, further work is needed in this direction. In addition, these alginate materials should be tested for health safety, which requires another test palette, i.e., cytotoxicity and irritant testing.
Next, this type of materials should be subjected to ageing tests. However, before the material goes on the market, dentists should test it in terms of its functional properties. It would be necessary to carry out in vivo tests by taking a certain number of impressions using the X1 material and checking whether there are any microorganisms on their surfaces through appropriate microbiological tests. Finally, it is required to meet all legislative requirements for example in the EU or US (MDR certification). At the same time, new material from Class 1 Med Dev would have to be classified as Class 3 (containing medicinal materials).

Author Contributions

Conceptualization, Z.R.; data curation, Z.R., M.M., D.B. and D.P.; formal analysis, Z.R., M.M., D.B. and D.P.; investigation, Z.R., D.B., D.P. and R.E.P.; methodology, Z.R. and D.B.; project administration, Z.R. and R.E.P.; resources, Z.R.; software, Z.R., D.B. and D.P.; supervision, Z.R. and R.E.P.; validation, Z.R.; visualization, D.B.; writing—original draft, Z.R., M.M., D.B., D.P. and R.E.P.; writing—review and editing, Z.R., D.B. and R.E.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All the obtained results and data are available in the manuscript and on the request to corresponding authors.

Conflicts of Interest

The authors declare no conflict of interest.

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Applsci 13 11639 g001
Figure 1. (A) New alginate with ceramic silver. (B) Distilled water was poured into the silicone bowl, and then the algae powder was added. (C) Mixing was performed manually for 30 s.
Figure 1. (A) New alginate with ceramic silver. (B) Distilled water was poured into the silicone bowl, and then the algae powder was added. (C) Mixing was performed manually for 30 s.
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Figure 2. The metal block with lines for construction measuring (A-A means measuring distance).
Figure 2. The metal block with lines for construction measuring (A-A means measuring distance).
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Figure 3. The mould to prepare the tearing strength testing specimens.
Figure 3. The mould to prepare the tearing strength testing specimens.
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Figure 4. Infrared spectra of pure potassium and sodium alginates (A), and the X3 formulation compared to commercial alginate products (B).
Figure 4. Infrared spectra of pure potassium and sodium alginates (A), and the X3 formulation compared to commercial alginate products (B).
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Figure 5. TGA and DTG curves. (A) Pure potassium and sodium alginates; (B) commercial alginates and material with bacteriostatic agent (X3).
Figure 5. TGA and DTG curves. (A) Pure potassium and sodium alginates; (B) commercial alginates and material with bacteriostatic agent (X3).
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Figure 6. Microscopy images of the materials used for the study, 300× magnification. (A1,A2) Sodium alginate; (B1,B2) potassium alginate; (C1,C2) AlphaSan RC 2000. Left row captured in a light mode, right row in a dark mode.
Figure 6. Microscopy images of the materials used for the study, 300× magnification. (A1,A2) Sodium alginate; (B1,B2) potassium alginate; (C1,C2) AlphaSan RC 2000. Left row captured in a light mode, right row in a dark mode.
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Figure 7. Microscopy images of the commercial and obtained formulations, 300× magnification. (A1,A2) KromaFaze; (B1,B2) Tropical IQ; (C1,C2) Kromopan; (D1,D2) Elastic Cromo; (E1,E2) X3. Left row captured in a light mode, right row in a dark mode.
Figure 7. Microscopy images of the commercial and obtained formulations, 300× magnification. (A1,A2) KromaFaze; (B1,B2) Tropical IQ; (C1,C2) Kromopan; (D1,D2) Elastic Cromo; (E1,E2) X3. Left row captured in a light mode, right row in a dark mode.
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Figure 8. A sample of the tested alginate X3; after setting, lines of 75 and 50 microns in width are visible on the surface (A,B).
Figure 8. A sample of the tested alginate X3; after setting, lines of 75 and 50 microns in width are visible on the surface (A,B).
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Figure 9. The gypsum surface along with the 50-micron line, as results compatible with gypsum tested according to point 2.6.
Figure 9. The gypsum surface along with the 50-micron line, as results compatible with gypsum tested according to point 2.6.
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Figure 10. Size of the growth inhibition zone of S. mutans on agar in contact with the alginate impression materials studied.
Figure 10. Size of the growth inhibition zone of S. mutans on agar in contact with the alginate impression materials studied.
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Table 1. Commercial alginates used for testing.
Table 1. Commercial alginates used for testing.
Name
Manufacturer		Composition [a]Lot NumberColour Change Under Working [b]Mixing Ratio
Powder/Liquid [g/mL]
Mixing Time [s]
Kromopan
Lascod, Florence, Italy		Diatomaceous earth [60–80%], alginate, sodium phosphates, gypsum, potassium hexafluorotitanate [1–2.5%] flavours0173310184Violet to white9/20 [g/mL]
45 [s]
Tropical IQ
Lascod, Florence, Italy		Diatomaceous earth [60–80%], alginate, sodium phosphates, gypsum, potassium hexafluorotitanate [1–2.5%] flavours0172371112Red to orange to yellow10, 5/20 [g/mL]
45 [s]
Elastic Cromo
SpofaDental, Jicin, Czech Republic		Diatomaceous earth, sodium alginate, sodium phosphates, gypsum, potassium hexafluorotitanate flavours10043390Violet to white9/20 [g/mL]
30 [s]
KromaFaze
Kerr, Orange, USA		Diatomaceous earth, potassium alginate, sodium phosphates, gypsum, potassium hexafluorotitanate flavours9888019Blue to white7, 5/16 [g/mL]
30 [s]
X1
Developed material with 0.25%
AlphSan		Table 2N/ANo colour changes green colour6/16 [g/mL]
30 [s]
X2
Developed material with 0.25%
AlphSan		Table 2N/ANo colour changes green colour6/16 [g/mL]
30 [s]
X3 Developed material with 0.25% AlphSanTable 2N/ANo colour changes green colour6/16 [g/mL]
30 [s]
[a]—Material composition based on information available from manufacturers. [b]—Colour change in accordance with the manufacturer’s manual.
Table 2. The added combination to the alginate-based impression materials for testing.
Table 2. The added combination to the alginate-based impression materials for testing.
MaterialConcentration [wt%]Producer
Mineral oil Dratex0.80Texas oil (Beamont, TX, USA)
Color N.V GWT-11 Green1.50Radiant (Houthalen, Belgium)
Kimica Algin 2G-200W Kimica (Potassium Alginate)11.20Kimica (Tokyo, Japan)
K2TiF62.37Merck (Praha, Czech Republic)
Celite 28175.25Merck (Praha, Czech Republic)
ZnO1.50Merck (Praha, Czech Republic)
Gypsum dihydrate6.25Merck (Praha, Czech Republic)
Na4P2O70.50Merck (Praha, Czech Republic)
MgO0.50Merck (Praha, Czech Republic)
Table 3. TGA results of pure and commercial alginates.
Table 3. TGA results of pure and commercial alginates.
MaterialTonset1 [°C]Tmax1 [°C]Tonset2 [°C]Tmax2 [°C]Residual Mass [%]
Sodium alginate48.166.3230.6244.931.1
Potassium Alginate47.666.3230.8242.931.8
Kroma Faze96.7105.6231.5244.686.6
Tropical IQ85.398.2224.7243.380.6
Kromopan91.8102.7229.9244.588.5
Elastic Cromo93.7104.6226.3240.287.7
X395.7107.5229.8243.586.6
Table 4. The results of the setting tests of commercial alginates and formulations with the AlphaSan bacteriostatic agent.
Table 4. The results of the setting tests of commercial alginates and formulations with the AlphaSan bacteriostatic agent.
MaterialSetting Time [s]Contraction after 24 h [%]
Kromopan132.2 ± 2.8 [a]0.83 ± 0.03 [a]
Tropical IQ107.4 ± 2.1 [a]1.14 ± 0.1
Elastic Cromo143.2 ± 2.4 [a]0.90 ± 0.09 [a]
KromaFaze147.4 ± 1.7 [a]1.02 ± 0.08 [a]
X1131.2 ± 1.31.10 ± 0.06 [a]
X2127.2 ± 2.6 [b]1.22 ± 0.08 [a]
X3131.6 ± 1.11.08 ± 0.06 [a]
[a]—Statistically significant compared to Kromopan alginate material, for p < 0.01. [b]—Statistically significant compared to Kromopan alginate material, for p < 0.05.
Table 5. Mechanical properties of the commercial alginate products and the studied materials.
Table 5. Mechanical properties of the commercial alginate products and the studied materials.
MaterialCompressive Strength [MPa]Elastic Recovery [%]Strain in
Compression [%]		Tearing Strength [MPa]
Kromopan1.04 ± 0.018 [a]97.28 ± 0.13 [a]13.96 ± 0.29 [a]0.54 ± 0.02 [a]
Tropical IQ0.87 ± 0.01696.88 ± 0.2 [a]11.22 ± 0.13 [a]0.50 ± 0.02
Elastic Cromo1.07 ± 0.028 [a]97.07 ± 0.1313.51 ± 0.3 [a]0.54 ± 0.02
KromaFaze0.84 ± 0.03396.95 ± 0.1 [a]11.76 ± 0.07 [a]0.57 ± 0.02
X10.85 ± 0.01596.92 ± 0.11 [a]12..49 ± 0.290.55 ± 0.01
X20.85 ± 0.01696.97 ± 0.11 [a]11.76 ± 0.33 [b]0.47 ± 0.02 [a]
X30.84 ± 0.008 [a]96.67 ± 0.07 [a]11.32 ± 0.260.44 ± 0.01 [a]
[a]—Statistically significant compared to Kromopan alginate material, for p < 0.01. [b]—Statistically significant compared to Kromopan alginate material, for p < 0.05.
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Raszewski, Z.; Mikulewicz, M.; Brząkalski, D.; Pakuła, D.; Przekop, R.E. Comparison of the Bioactive and Bacteriostatic Performance of Different Alginate-Based Dental Prosthetic Impression Materials with and without Zirconium Phosphate-Based Ion Exchange Resin Containing Silver: An In Vitro Study. Appl. Sci. 2023, 13, 11639. https://doi.org/10.3390/app132111639

AMA Style

Raszewski Z, Mikulewicz M, Brząkalski D, Pakuła D, Przekop RE. Comparison of the Bioactive and Bacteriostatic Performance of Different Alginate-Based Dental Prosthetic Impression Materials with and without Zirconium Phosphate-Based Ion Exchange Resin Containing Silver: An In Vitro Study. Applied Sciences. 2023; 13(21):11639. https://doi.org/10.3390/app132111639

Chicago/Turabian Style

Raszewski, Zbigniew, Marcin Mikulewicz, Dariusz Brząkalski, Daria Pakuła, and Robert E. Przekop. 2023. "Comparison of the Bioactive and Bacteriostatic Performance of Different Alginate-Based Dental Prosthetic Impression Materials with and without Zirconium Phosphate-Based Ion Exchange Resin Containing Silver: An In Vitro Study" Applied Sciences 13, no. 21: 11639. https://doi.org/10.3390/app132111639

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