Using Hydrogels to Achieve an Antibacterial, Injectable, and Retrievable Drug Delivery Vehicle for Dental Regeneration

Endodontic treatment failures, commonly caused by Enterococcus faecalis, pose a significant challenge in dental treatment. While calcium hydroxide remains the predominant medicament, it lacks sufficient antimicrobial efficiency against E. faecalis, is caustic, and has poor injection properties. This study demonstrates the potential of a mixture of calcium hydroxide and salicylic acid (CASA) encapsulated in thermoreversible hydrogels (F127-DMA) as an injectable and retrievable intracanal medicament with strong antibacterial efficacy and controlled drug release. 

We tested different ratios of calcium hydroxide to salicylic acid [1], and a 1:6 ratio proved to have optimal water solubility. This mixture was then added to F127-DMA and UV-crosslinked using Lithium phenyl (2,4,6-trimethylbenzoyl) phosphinate (LAP). The FT-IR and RAMAN spectroscopy of CASA indicated that calcium salicylate had been formed. The optimal formulation exhibited a pH of ~7, superior crosslinking efficiency (80.9%), and a swelling ratio after a 14-day immersion in DI water of <16%. The addition of CASA decreased the contact angle of F127-DMA, from 81° to ~62°, leading to better absorption of the inflammatory exudate into the root canal.

Encapsulation of the CASA (1:6) in F127-DMA, decreased the viscosity of the original paste by nearly three orders of magnitude, enabling facile injection into root canal models, which demonstrated that it was also easily retrievable. Liquid suspension tests indicated that F127-DMA loaded with only 5% CASA was effective against planktonic E. faecalis resulting in at least a six-log reduction in bacterial count. E. faecalis is one of the most difficult organisms to eliminate due to its protective biofilm. We therefore infected bovine anterior tooth slices with E. faecalis suspension and allowed a biofilm to mature for three weeks. The slices were examined with scanning electron microscopy (SEM), demonstrating penetration of the bacteria deep into the dentin tubules. CFU analysis was performed on suspensions obtained from the tooth slices to determine the bacterial activity following treatment with 10% F127-DMA/CASA, calcium hydroxide, and the empty hydrogel vehicle. The results indicated a log reduction of 3 logs relative to the controls and no significant difference relative to the endodontic gold standard of Ca(OH)2. 

In order to probe the regenerative potential of F127-DMA/CASA, human dental pulp stem cells were cultured on the filled crosslinked hydrogel for 28 days. Proliferation and adhesion were observed within the first four days and were not found to be significantly different from the control on TCP. Rt-PCR demonstrated upregulation of OCN and DSPP, while RAMAN and SEM indicated Hydroxyapatite biomineralization, consistent with odontogenic differentiation. This aspect is in sharp contrast to calcium hydroxide, whose high pH is known to cause severe tissue damage, preventing regeneration of surrounding tissue. Hence the superior rheological properties of F127-DMA/CASA, which permit injectability and retrieval from the confined space of the root canal, together with superior antibacterial properties, and regenerative potential make it a promising new alternative to standard intracanal treatments.