Enhancing mechanical properties of glass-ionomer cement with hemp fiber: a sustainable approach for dental restorations

Abstract

Background

A novel approach to enhancing the mechanical properties of glass-ionomer cement (GIC) may incorporate natural fiber. This study aims to evaluate the effects of raw and pure hemp fiber additions on the flexural strength (FS) and surface roughness (SR) properties of glass-ionomer cement (GIC).

Methods

Hemp fibers sourced from the local population were harvested, dried, and separated, then prepared in raw and pure forms to assess their impact on the FS and surface roughness of GIC. Synthesized GICs dopped raw and pure hemp fiber were characterized using SEM and EDX for morphological and elemental analysis. The mechanical features of GICs were measured using an FS and SR testing apparatus.

Results

Raw hemp fibers, known for their sustainability and mechanical resilience, showed significant enhancements in FS, particularly at 1% addition. High FS and SR values using pure fibers at 1% and 3% concentrations were due to improving fiber-matrix interactions, reducing the cracks on GICs. SEM analysis of fracture surfaces supported these findings, showing fiber reinforcement at crack lines.

Conclusion

This approach offers a promising, eco-friendly, cost-effective alternative for dental restorations, combining mechanical durability and surface quality. These results highlight the potential of hemp fiber-reinforced GIC in advancing sustainable, restorative materials in dentistry.

Background

Composite materials are structural materials of significant importance in science and engineering, enabling the achievement of superior properties by combining different components [1]. The early use of composite materials can be traced back to ancient Egyptian and Babylonian civilizations, exemplified by the reinforcement of mud bricks with straw [2]. The development of modern composites dates back to the mid-20th century, particularly during World War II, when the demand for lightweight, high-strength materials increased significantly. This led to the widespread use of glass fiber-reinforced plastics (GFRP) and carbon fiber-reinforced polymers (CFRP) [1, 2]. In recent years, natural fibers, such as hemp, have emerged as renewable and environmentally friendly alternatives in the pursuit of environmental sustainability [3]. Hemp and other natural fibers stand out, particularly due to their low densities, high strength-to-weight ratios, and abundant availability [4]. In this context, the use of natural fibers such as hemp in dental materials like glass ionomer cement (GIC) presents new opportunities for the development of environmentally friendly and high-performance products [3]. Glass ionomer cement (GIC) is frequently used in dental practice. It possesses desirable properties as a restorative material, such as biocompatibility, chemical bonding to hard dental tissues, fluoride release, and the ability to be recharged with fluoride ions [5]. However, there are still some limitations to GICs as restorative materials, primarily related to their mechanical properties such as wear resistance, flexural strength (FS), and fracture toughness [6]. These mechanical disadvantages have limited the use of GIC as a permanent filling material, leading to its more frequent use as a liner or cementation material [7]. In response, extensive research has been conducted to improve the mechanical properties of GIC [8]. These studies have focused on adding various materials to the powder component of GIC to enhance its FS. Metallic powders, hydroxyapatite powders, bioactive glass particles, nano clay, and short glass fibers have been incorporated into GIC to improve its mechanical and physical properties [9]. As a result, metal-reinforced, resin-modified, or nanoparticle-added types of cement have been developed [10]. However, producing these synthetic materials involves high energy consumption, high costs, and environmental pollution [11]. For this reason, recent years have focused on the synthesis and properties of natural fiber-reinforced GICs [12].

Adding natural fibers to GIC has shown promising potential for improving mechanical performance [13]. These studies have primarily focused on enhancing the FS of the cement. However, there is a lack of research on surface roughness (SR) changes resulting from hemp fiber reinforcement. Hemp fiber addition is one of the methods explored for this purpose. Hemp is a plant used to treat various medical symptoms and has found applications in different forms in the medical field [14]. It has also been used in dental materials for reinforcement [15]. This study investigated the effects of natural hemp fiber on the FS and SR of glass ionomer composites. Hemp is a sustainable, eco-friendly material with a low carbon footprint, offering high yields at low costs, and has a wide range of applications. It is a miracle product for future generations [16].

Hemp fibers are categorized among the most potent natural fibers [17]. The composition of hemp fibers includes 70–74% cellulose, 15–20% hemicellulose, 3.5–5.7% lignin, 0.8% pectin, and 1.2–6.2% wax. Pectin, a polysaccharide derivative, has high water retention capabilities. In its water-insoluble form, pectin is calcium, magnesium, and iron salts of pectic acid. Another component in hemp fibers is lignin, which consists of units derived from phenylpropane. Lignin is the second most abundant biopolymer after cellulose. Containing aromatic and aliphatic groups, lignin is a molecule that is difficult to break down [17].

In a study conducted by Yadav et al., it was reported that reinforcing dental materials with fibers or particles significantly enhances the physical and mechanical properties of the dental resin matrix [18]. Another study highlighted that ceramics or fibers with suitable physical, mechanical, and biological properties can be readily produced through the selection of appropriate parameter conditions and processing procedures. Consequently, the physical and mechanical properties of resin-based dental materials are enhanced, making them more advantageous compared to polymers, ceramics, and metals [19]. Another literature review revealed that the addition of nanoparticles to dental materials has been found to provide enhanced physical and mechanical properties [20]. Studies aimed at reinforcing various restorative dental materials have determined that the type of matrix composition, as well as the type, size, and ratio of nanoparticles in the filler composition, influence the amount of wear exhibited by the material under masticatory forces [21].

This study is the first to investigate the incorporation of hemp fibers into GIC, as no previous studies in the literature have explored the addition of hemp fibers to GIC. Hemp fiber was selected for this study because it is biocompatible, sustainable, economical, eco-friendly, offers easy handling, and can be easily mixed with GIC. In this study, hemp fiber was added to glass ionomer cement—a material frequently used in dentistry—and its effects on the FS and SR of the cement were evaluated. The study’s null hypothesis is that adding hemp fibers will significantly affect the FS and SR of the GIC.

Methods

Preparation of hemp fiber

In this study, the hemp fiber material used was sourced from the local hemp population and cultivated in the hemp-growing areas of our university, a specialized institution in “Industrial Hemp.” The hemp was planted under the Hemp Research Institute of Yozgat Bozok University field conditions in May 2021 and harvested when the plants reached physiological maturity in August-September. After harvest, the hemp stalks were dried under appropriate conditions to equalize moisture levels. Separating the fibers from the stalks was carried out at the Faculty of Agriculture, Yozgat Bozok University. The hemp fibers to be added to the GIC were prepared in raw and chemically pre-treated fibers. The aim of setting up two different trials was to investigate whether the lignin and pectin in the hemp fibers affect the FS and SR of the GIC.

Preparation of raw hemp fiber

First, after harvesting, the hemp fibers were dried without undergoing any pre-treatment, and the woody parts were removed. The fibers were then ground into a fine powder using a ball mill available at Hemp Research Institute. The ground hemp fibers were sieved through a mesh with a pore size of 100 microns. The fibers were characterized using SEM (Scanning Electron Microscopy) for their morphological properties and EDX (Energy Dispersive X-ray) analysis to determine their elemental content.

Preparation of chemically treated hemp fiber

The study used raw fiber and two fibers that were tried to be removed by chemical treatment. In addition to cellulose, hemp’s structure contains hemicellulose, lignin, and pectin. Pectin is the structure that connects the bare fibers and keeps them together. Lignin is a component found on the outside of the fiber [22]. Chemical treatment was used to soften the hemp fiber, and lignin and pectin were removed from the fiber structure. In this process, the fibers were treated with 20% NaOH solution for 4 h, washed with pure water 4 times, and dried [23]. The dried fibers were finely ground using a ball mill and passed through a 100-micron sieve. Raw and processed fibers were characterized by SEM and EDX analyses (Figs. 1, 2 and 3).

Characterization of fibers

Scanning electron microscope analysis

SEM (Scanning Electron Microscope) analysis of the fibers was performed to examine the surface morphology and microstructure of the materials with high resolution. In the examined SEM images (Fig. 1), it was determined that the fiber bundles in the raw fiber had an integrated structure due to lignin and pectin. It was observed that the fiber bundles in the NaOH-treated fiber were separated from each other and had a more dispersed structure. Here, it was interpreted as the dispersion caused by the separation of the fibers from each other by removing the lignin and pectin that hold the fiber bundles together with NaOH.

Fig. 1

SEM analysis of raw fiber (a) and treated fibers (b)

Component analysis (EDX)

The elemental composition analysis of the fibers was performed using energy dispersive X-ray spectroscopy (EDX). In addition, impurities in the material and the homogeneity of the elements were determined.

Fig. 2

EDX analysis of raw fiber

When the EDX analysis of the raw fiber was examined, it was revealed that it contained 65.2% C, 34.7% O and trace amounts of 0.1% Na (Fig. 2). When the EDX analysis of the chemically treated fiber was examined, it was found that it contained 56.1% C, 40.3% O and 3.6% Na elements (Fig. 3). When the EDX analyzes of the raw fiber and the chemically treated fiber were compared, it was seen that the percentage C and O values ​​changed significantly. This change can be interpreted as the removal of lignin and pectin from the structure as a result of the chemical treatment. Apart from this, although the fibers were thoroughly washed with pure water 4 times as a result of the chemical treatment, it was seen that 3.6% Na element remained in the EDX analysis. This impurity is due to the NaOH used during the chemical treatment (Fig. 3).

Fig. 3

EDX analysis of the treated fiber

Preparation of groups

The hemp fibers were weighed using a precision balance (A&D GR 300, Japan) and added to the GIC (Voco; Meron, Cuxhaven, Germany). Experimental groups were formed for our study based on the different forms of hemp fiber (raw and chemically pre-treated) and the fiber added. No hemp fiber was added to the control group of GIC. Ten samples were prepared for each experimental group.

Group 1: Control group (no fiber).

Group 2: 1% by weight untreated raw hemp fiber added to GIC.

Group 3: 3% by weight untreated raw hemp fiber added to GIC.

Group 4: 5% by weight untreated raw hemp fiber added to GIC.

Group 5: 1% by weight chemically pre-treated hemp fiber added to GIC.

Group 6: 3% by weight chemically pre-treated hemp fiber added to GIC.

Group 7: 5% by weight chemically pre-treated hemp fiber added to GIC.

Flexural strength tests

For the FS test, molds were prepared for the samples according to the ASTM E 399 − 90 standard, with dimensions of 25 × 2.5 × 5 mm and a notch in the center measuring 0.5 mm wide and 2.5 mm long [24]. The powdered hemp fibers were added to the cement powder at concentrations of 1%, 3%, and 5% by weight. The powder-to-liquid ratio of the cement was mixed according to the manufacturer’s instructions and condensed into the molds. After removing the molds, the samples were measured with calipers and stored in distilled water at 37 °C in an incubator for 24 h. The samples were subjected to an FS test using a universal testing machine (Shimadzu AGS-X, Shimadzu Scientific Instruments, Columbia, North Carolina, USA) at a 0.5 mm/min crosshead speed. The results were recorded in MPa.

Surface roughness tests

For the SR test, a standard metal mold with a diameter of 10 mm and a height of 2 mm was used. The metal mold was placed on a glass surface covered with acetate. After adding the hemp fibers to the GIC powder at the appropriate concentrations (1%, 3%, and 5% by weight), the GIC was mixed according to the manufacturer’s instructions and overfilled into the mold. A strip band was placed on the top-facing surface of the mold, and a second glass slide was pressed onto the metal mold to allow the excess cement to escape from the sides. After the setting time, the samples were removed from the mold, and any excess material was cleaned. All samples were ultrasonically cleaned in deionized water (Pro-Sonic 600; Sultan Healthcare, NJ, USA) for 10 min and then dried with compressed air. The thickness of the samples was checked using a digital caliper (Absolute Digimatic, Mitutoyo, Japan). The samples’ average SR (Ra) was analyzed using a contact profilometer (Taylor Hobson Surtronic 25, Leicester, UK) with a 0.25 mm cutoff value. A constant measurement speed of 0.5 mm/s was used to determine an average roughness profile (Ra) in micrometers. The profilometer was calibrated before the measurements of each group. All SR measurements were taken at the center of each sample. Five measurements were taken for each sample, and the mean value was recorded as the Ra parameter. A lower Ra value indicates a smoother surface [25].

Statistical analysis

In this study, the data were analyzed using SPSS 17. Normality tests were performed with the Shapiro-Wilk test, and variance analysis was conducted using Levene’s test. Since the data were normally distributed and there were differences in variances, One-way ANOVA was used for comparisons between groups, and Duncan’s test was applied for post-hoc analysis. To determine differences between groups, the Independent Samples T-Test was used. A significance level of 0.05 was adopted, with p < 0.05 indicating a significant difference and p > 0.05 indicating no significant difference.

Results

Flexural strength tests

The FS test results for the groups with untreated hemp fiber additions and the control group are shown in Table 1. According to the results, Group 2 exhibited a significantly higher average FS value than the other groups (p < 0.05). Statistical pairwise comparisons were conducted between the groups with untreated hemp fiber additions and the control group regarding flexural strength. The test revealed that Group 2 had a significantly higher average FS value than the control group (p = 0.000).

Table 1 FS of the groups (Groups with untreated hemp Fiber Added)

Table 2 FS of the groups with chemically treated hemp Fiber added

The FS test results and pairwise comparison values for the groups with chemically treated hemp fiber additions and the control group are shown in Table 2. It was found that Group 5 had a significantly higher average FS value compared to the other groups (p < 0.05). In the pairwise comparisons, the control group’s average FS value was significantly lower than Groups 5 and 6 (p = 0.000).

Surface roughness tests

The SR test results for the groups with untreated hemp fiber added and the control group are shown in Table 3. The control group’s average Ra value was significantly higher than the average Ra values of Groups 2, 3, and 4 (p < 0.05). A pairwise comparison was performed to statistically compare the average SR values between the control and groups with untreated fiber added. According to the results, the control group’s average Ra value was significantly higher than those of Group 2 and Group 3 (p = 0.000). However, there was no significant difference between the control group and Group 4 regarding average Ra values (p = 0.026).

Table 3 SR of the groups (Groups with untreated hemp Fiber Added)

The SR test results for the groups with chemically treated hemp fiber added and the control group are shown in Tables 4 and 5. Group 7 exhibited a significantly higher mean Ra value than the other groups (p < 0.05). A pairwise comparison of SR values between the control group and the groups with treated fiber was performed. According to the analysis, the mean Ra value of the control group was significantly higher than that of Group 5 and Group 6 (p = 0.000). No significant difference was observed between the mean Ra values of the control group and Group 7 (p = 0.127).

Table 4 SR of the groups (Groups with chemically treated hemp Fiber Added)

Table 5 All group profilometer data of SR (µm)*

Scanning electron microscopy (SEM) images of the fracture surfaces of the samples obtained after the FS test were captured. The fracture surfaces of the groups with untreated fibers and those with treated fibers were displayed in Fig. 4 at 130x magnification. Figure 5 shows SEM images of the groups at 5kx magnification. Upon examining the SEM images of the treated fiber-added groups (at both 130x and 5kx magnifications), it was observed that in the groups containing 5% hemp fiber by weight, regardless of the form of the hemp fibers, there was a more significant amount of fibers visible at the fracture line compared to the other groups. In some images, cracks were observed in the GIC; however, the hemp fibers appeared to prevent the propagation of these cracks. It can be seen that hemp fiber is not evenly distributed in GIC at concentrations greater than 1%, and individual clustering of fibers can be distinguished. This is demonstrated by the fact that the groups containing 1% fiber concentration showed the best performance in structural analyses, such as FS and SR, in agreement with SEM.

Fig. 4

SEM images of groups with treated fiber additives (130x)

Fig. 5

SEM images of groups with treated fiber additives (5kx)

Discussion

Glass-ionomer cement (GICs) are widely used in various clinical dental applications, such as full crown restorations, cavity liners and bases, luting agents, and fissure sealants [26]. However, GICs exhibit brittleness, reduced transparency, and technical sensitivity, as well as being susceptible to dehydration and wear [5]. Therefore, to improve the mechanical properties of GICs, metallic fillers such as titanium, silver, graphene, and carbon, or bioactive materials such as hydroxyapatite particles and bioactive glass, have been added [27]. Incorporating natural fibers into GICs has been a promising initiative for enhancing their mechanical performance [28]. In many studies on the reinforcement of GICs with natural fibers, improvements in the mechanical properties of the cement have been observed [29]. These studies primarily investigated the effect of natural fiber additions on the mechanical durability of the cement but did not explore their impact on SR. Therefore, in our study, hemp fiber was added to the structure of glass-ionomer cement to evaluate its effects on FS and SR. Since hemp fiber is one of the strongest natural fibers, it was hypothesized that its addition to GIC could enhance the FS of the cement. Additionally, three different weight percentages of hemp fiber were selected to assess the effects of fiber content on FS and SR [30]. A total of seven experimental groups were created based on the form and amount of hemp fiber, and SR and FS were compared with the control group (GIC without fiber). Our study found that adding hemp fiber significantly affected the FS and SR of GIC. Therefore, the null hypothesis of our study was accepted.

In this study, different results emerged in terms of the effect of untreated and chemically treated hemp fiber addition to GIC on FS. Additionally, it was observed that the added amounts of untreated and chemically treated hemp fiber had an effect on the average FS values. In our study, adding 1 wt% untreated hemp fiber to the GIC significantly increased its FS (p = 0.000). Adding 3 wt% and 5 wt% untreated fiber also improved the FS of the cement, but these increases were not statistically significant. The higher FS observed in the group with 1 wt% hemp fiber compared to the control group is believed to be due to the chemical structure of the hemp fiber. Because of this property of hemp fibers, when subjected to pressure, the fibers can quickly elongate and exhibit high tensile strength. Considering the results of Group 2, the presence of flexible hemp fibers may absorb the energy or stresses required for crack propagation, dissipating these stresses or energy into the surrounding GIC matrix, thus suppressing stress localization. This can be associated with a significant increase in the stiffness of the GIC due to adding hemp fibers. In a study by Abou et al. (2017), it was reported that flax fiber reinforcement significantly increased compressive and yield strength at higher weight percentages [31]. Low fiber content has been found to increase the compressive strength of flax fibers produced through silanization. In our study, it is significant that when 1 wt % raw hemp fiber was added, the average FS value was found to be higher than the other raw hemp fiber added groups (3 wt% and 5 wt%) and the control group. However, the fact that group 3 and group 4 had higher average FS values ​​than the control group was not found to be statistically significant. This may suggest that the hemp fibers did not sufficiently interact with the GIC matrix at higher concentrations and could not effectively suppress stress localization.

Similar to the results in the raw fiber added groups, the average FS value in the 1% chemically treated hemp fiber added group was found to be higher than the other chemically treated fiber added groups and the control group. It is significant that the average FS value in both the 1% and 3% chemically treated fiber added groups (group 5 and group 6) was higher than the control group. However, adding 5% fiber did not have a statistically significant effect on the FS. According to these results, it can be said that the amount of fiber added to the GIC different results in the effect of raw fiber and processed fiber addition on FS. Furthermore, considering the results from Groups 5 and 6, it can be stated that processed hemp fibers absorbed the energy or stress required for crack propagation, redistributing these stresses or energy to the surrounding GIC matrix and thus suppressing stress localization.

In conclusion, it can be said that increasing the concentration of hemp fibers and the presence of lignin and pectin in the fibers significantly impact stress localization. In the study by Silva et al., it was reported that the incorporation of cellulose microfibers into the GIC matrix did not significantly improve the mechanical properties of the GIC, but the addition of a small amount of cellulose nanocrystals to the GIC showed significant improvements in all evaluated mechanical properties. In this study, compressive strength increased by 110% compared to the control group, elastic modulus rose by 161%, diametral tensile strength increased by 53%, and mass loss decreased from 10.95 to 3.87% [32]. It was also reported that adding physically processed cellulosic fibers to GIC increased compressive and diametral tensile strengths and exhibited acceptable elastic modulus and hardness. In contrast to our study, better mechanical durability was observed with higher concentrations of cellulose fiber (7.24%) in this study [33].

In a study where silk fiber was added to glass ionomer cement (GIC), adding 1% fiber showed the highest fracture strength [34]. Similarly, in another study where wood pulp fibers were added to the cement, the highest fracture strength was observed with 1% fiber addition [29]. In our study, although the type of natural fiber added differed, the best FS was observed with adding 1% untreated hemp fiber.

Regarding changes in SR resulting from the reinforcement of GIC with natural fibers, there is no similar study with which we can compare the results of our study. However, in the study by Öznurhan et al., artificial polypropylene fibers were added to GIC, and while no significant change in SR was observed up to 5% fiber addition, an increase in SR was noted with 5% fiber addition [30]. Our study observed that the addition of untreated and treated hemp fibers at concentrations of 1% and 3% significantly reduced SR compared to the control group. Moreover, the lowest roughness value was obtained with 1% untreated hemp fiber, while the highest SR was observed with 5% treated hemp fiber. The results of the roughness test indicate that SR varies depending on the chemical interaction between GIC and hemp fibers, fiber concentration, and the proportion of lignin and pectin in the structure.

The differential effects of untreated and chemically treated hemp fibers highlight the importance of fiber modification in tailoring the mechanical properties of GICs. Exploring alternative natural fibers and hybrid reinforcement strategies may further enhance the durability and functional properties of GICs in restorative dentistry.

Based on the findings of this study, it was determined that the effect of hemp fiber added to GIC on FS and SR varied depending on the form and amount of fiber. In our study, it was observed that the addition of 1% fiber (both untreated and treated) improved FS and reduced SR. In future studies, the fiber addition ratio could be reduced to 1% or lower to evaluate its effects on FS and SR. One of the main limitations of this study is the lack of knowledge about the performance of fiber-reinforced cement in the oral environment. The cytotoxicity and biocompatibility of the cement in oral tissues should be assessed in future research.

Future aspect

In the future, further studies could be conducted to explore the effects of both natural fibers and different types of nanoparticle fillers on the performance characteristics of dental materials such as glass ionomer cement (GIC). Specifically, analyzing the effects of nano-hydroxyapatite fillers on the tribological, mechanical, and thermal properties could open avenues for investigating alternative material combinations for GIC applications [35]. Similarly, the optimization and ranking of dental restorative composites using the ENTROPY-VIKOR and VIKOR-MATLAB methods could provide a more systematic approach in material design and selection processes [36]. In further research, a detailed investigation of the tribological, mechanical, and thermal effects of nano-tricalcium phosphate and silver particle fillers on composite resins could provide new insights into achieving a balance between biocompatibility and mechanical performance [37]. Additionally, the manufacturing, evaluation, and performance ranking of tricalcium phosphate and silica-filled dental composites could contribute to the development of potential materials that will ensure success in both clinical and commercial applications [38]. Furthermore, the manufacturing, characterization, and analysis of selection processes using the FAHP-TOPSIS technique for dental restorative composites reinforced with zirconia, titanium oxide, and marble powder could help optimize durability and aesthetic properties [39]. Ranking and selection studies utilizing FAHP-FTOPSIS and hybrid Entropy-VIKOR methods could expand the application areas of multi-criteria decision-making techniques in the selection of dental restorative materials [40, 41]. Finally, the development and optimization of ceramic particle-reinforced dental restorative materials could provide cost-effective and performance-driven solutions for restorative applications [42].

Conclusion

According to this study:

• The addition of 1% untreated hemp fiber to GIC exhibited the highest FS.

• The addition of 1% and 3% treated hemp fiber to GIC showed better FS compared to the control group.

• When 1% and 3% untreated hemp fiber were added to GIC, lower SR was observed compared to the control group.

• When 1% and 3% treated hemp fiber was added to GIC, lower SR was observed compared to the control group.

Based on our results, adding natural and robust hemp fiber in appropriate amounts and forms can be preferred to improve the FS of GIC and reduce SR. Our study suggests that hemp fiber may be a viable alternative to other synthetic fibers currently added to GIC with its sustainable, cost-effective, eco-friendly, and easy-to-process qualities.