Fluoride Rechargeable and Acid-neutralising Layered Double Hydroxides for Dental Materials

Objectives: To investigate the absorption and release of fluoride over two cycles, from 3 Layered double hydroxide (LDH) powders, and to determine their neutralising ability over six acid-challenges. LDH may be used to develop re-chargeable fluoride dental devices to prevent early-stage caries, via maintaining a low therapeutic fluoride concentration in the oral environment, and neutralising acid-challenges that cause demineralisation.
Methods: ZnAl, MgAl and CaAl LDH powders were synthesised using a 2:1 M²⁺:Al³⁺ ratio. Each powder (1g/L) was immersed in sodium fluoride (NaF, 230ppm) to absorb fluoride, and then release it (de-ionised water, DW, cycle 1), followed by re-absorbing fluoride (230ppm NaF) and re-releasing (DW, cycle 2). Fluoride concentration was continuously recorded for 1h at 36.8°C per cycle (ion-selective electrodes-NICO2000, precision ±2%). Separately, the neutralising ability of ZnAl-LDH, MgAl-LDH and CaAl-LDH were subjected to six acid-challenges with pH recorded every 10secs over 1h (Oakton-Instruments; control solution with no LDH). Initially, 1g/L LDH was added to 0.1M acetic acid (pH 3.9±0.1, 1^st acid-challenge) and after 10min, 1ml of 0.1M acetic acid was added to the solution (2^nd acid-challenge). Four acid-challenges were further conducted every 10mins.
Results: 2:1 CaAl-LDH absorbed the greatest amount of fluoride (159.8ppm; p<0.05), followed by MgAl-LDH and ZnAl-LDH. After 1h, all compositions continued to absorb fluoride, however the majority was absorbed within 3mins. All compositions re-absorbed fluoride (2^nd cycle), with CaAl-LDH re-absorbing the greatest (22.9ppm; p<0.05). MgAl-LDH released and re-released the most fluoride, 20.7ppm and 24.3ppm, respectively, followed by CaAl-LDH and ZnAl-LDH in both cycles. All powders repeatedly neutralised the 6 acidic conditions, with MgAl-LDH neutralising the greatest to pH 5.4, followed by ZnAl-LDH and CaAl-LDH.
Conclusions: LDH powders demonstrate potential for caries prevention via repeatedly absorbing and releasing fluoride and neutralising acid-challenges. Their quick absorbing ability make LDH incorporated in dental materials (e.g. composites) suitable for charging with fluoride toothpaste/mouthwash.