Dental caries is a widespread preventable chronic disease globally. Caries is a multifactorial disease associated with acid-producing bacteria leading to the imbalance between demineralisation and remineralisation. The increase in demineralisation leads to the development of a cavitated carious lesion requiring restorative treatments [1]. The major limitation of the current restorative treatment is the risk of developing secondary caries or recurrent infection around the restoration [2]. This can fail the filling, which may lead to a severe infection or tooth loss. Hence, a restorative material with a high potential for antibacterial and remineralisation may be needed to prevent secondary caries.

Glass ionomer cement (GIC) is the most commonly used fluoride-releasing material, particularly in patients at high risk of caries. It was suggested that the use of GIC may provide beneficial effects for preventing secondary caries [3]. The current GICs have been produced with extremely high production temperatures resulting in loss of composition and air pollution. Recently, GIC was successfully produced using a low-temperature process known as the sol-gel method [4], [5], [6]. The glass network structure and related physical properties of a newly produced sol-gel derived GIC (SGIC) have been intensively evaluated, including an attempt to improve the biological properties of this new SGIC.

Recently, the SGIC modified with strontium-containing fluorinated bioactive glass nanoparticles (SrBGF) was developed [7]. Strontium was employed as a main composition in this SrBGF instead of calcium due to its ability to improve the bioactivity and stability of apatite formation. The addition of fluoride further improved the fluoride release and uptake ability of the material. Some literatures has also reported that the synergistic effect of strontium and fluoride ions could effectively inhibit bacterial growth [8]. The previous study successfully incorporated both strontium and fluoride into bioactive glass by the sol-gel method, yielding a SrBGF nanoparticle with an amorphous structure and SrF2 crystals [7]. The incorporation of SrBGF into SGIC showed comparable setting times, mechanical properties, and enhanced fluoride release and uptake abilities in comparison to the unmodified SGIC. Despite these promising results, the antibacterial and remineralisation properties of this SGIC-modified SrBGF have never been fully investigated.

Besides the BGF, the strontium-containing fluorapatite nanoparticle (SrFA) is another interesting candidate for use as an additive to modify SGIC. Various research has shown the positive impact of incorporating fluorapatite (FA) powder into restorative dental materials such as GICs. This is due to their biocompatibility and the ability to form fluorapatite, which offers greater resistance to acid penetration compared to hydroxyapatite in human dental structures [9], [10]. In addition, FA doped with strontium (SrFA) was also investigated in order to enhance biocompatibility and antibacterial activity. The previous study successfully prepared SrFA using a hydrothermal method, resulting in a highly crystalline nanoparticle with promising antibacterial properties against Streptococcus mutans [11].

Both SrBGF and SrFA are well-known for their use as an additive for biomaterial to enhance the ability to antibacterial, bioactivity, and biocompatibility properties. However, the differences in crystal structure and chemical composition between SrBGF and SrFA can alter the levels of effectiveness in improving these biological properties. To date, there has been no direct comparison of the efficacy of these two materials as additives in dental materials. The amorphous structure of SrBGF is expected to offer superiority over SrFA in terms of remineralisation ability and antibacterial properties, as it allows for easy dissolution and release of ions that promote apatite formation and antibacterial activity.

This study aimed to compare the effectiveness of two additives, fluoride-containing strontium-based bioactive glass (SrBGF) and strontium-containing fluorapatite (SrFA), as additions to SGIC. SrBGF was developed in our previous study with a composition of 4.5SiO2-0.5 P2O5-3.5SrO-1.5SrF2 [7]. SrFA with a composition of (Ca5Sr5)(PO4)6F2 was modified from a previous study [11]. Both additives were added to SGIC in an attempt to enhance its biological properties which were antibacterial properties, remineralisation ability, and in vitro cytocompatibility. The surface roughness and ion-releasing behavior of the material were also determined to clarify the effect of the material on these biological properties.