A Multimodal Assessment of a New Self-Adhering Flowable Composite: Dental Pulp Stem Cell Viability and Bond Strength

Self-adhering flowable composites (SAFCs) were developed to simplify restorative procedures by eliminating separate adhesive steps; however, their bonding performance, mechanical properties, and biological properties remain controversial. This study evaluated a newly developed SAFC (SA-100R) compared with three commercial SAFCs (Fusio Liquid Dentin, Constic, and Vertise Flow) and a conventional composite (Clearfil AP-X used with a two-step self-etch adhesive). Microtensile bond strength (μTBS) to dentin, failure modes, Knoop microhardness, resin–dentin interfacial morphology (SEM), elemental composition (EDX), and cytocompatibility (CCK-8, LIVE/DEAD staining) were assessed. The conventional composite system exhibited significantly higher µTBS than all SAFCs (p < 0.05). Among SAFCs, SA-100R showed the highest bond strength and no pre-testing failures. SEM revealed abundant resin tag formation only in the conventional system, while all SAFCs showed interfacial gaps, with SA-100R exhibiting the least pronounced gaps. SA-100R showed no significant cytotoxic effects across all tested concentrations. Within the limitations of this short-term in vitro investigation, SA-100R demonstrated improved bonding performance and favorable biocompatibility compared with existing SAFCs, suggesting its potential as a simplified restorative material. However, the conventional adhesive/composite system remains the reference standard for dentin bond strength, and long-term clinical performance remains to be established through further studies.