Tannic and boric acid crosslinked glycerol–citrate polyesters: Structural, mechanical and biological performance

• Glycerol-citrate polyester was modified by tannic and boric acid crosslinkers. • Modified polyesters show denser networks via borate–ester and tannic acid links. • Higher thermal stability and slower degradation were achieved in tuned polymers. • Incorporation of both crosslinkers enhanced the mechanical properties of glycerol-citrates. • Crosslinked polyesters showed lower cytotoxicity and similar antibacterial effects. Functionalization of biodegradable polymers with naturally derived bioactive compounds is highly attractive in biomedical science and engineering to enhance the chemical, mechanical, and biological properties of existing polyesters. Here, we report a simple synthesis of glycerol–citrate (GCA) polyesters dual-crosslinked with tannic (TA) and boric acids (B), with GCA/TB1 and GCA/TB2 containing 1 and 2 wt% of each crosslinker, respectively. A detailed investigation was conducted to examine their structural features, underlying reaction mechanisms, and thermal, mechanical, and biological properties, as well as biodegradation. Structural (ssNMR, FTIR, XRD), thermal (DSC/TG), and microscopic (SEM) analyses revealed that the crosslinked systems form an amorphous, more densely connected polymer network, providing evidence of borate–ester bonds and tannic acid incorporation. Mechanical properties were assessed using three complementary methods—macromechanical tensile testing, nanoindentation, and ultrasound—and showed that GCA/TB1 exhibited the highest Young’s modulus, yield stress, hardness, and tensile strength, associated with a more uniform surface, smoother fracture morphology, and effective crosslinking. The modified polyesters also displayed enhanced hydrophilicity and slower, more controlled hydrolytic degradation, accompanied by milder pH changes, beneficial for improved tissue response and biocompatibility. In vitro tests demonstrated that 50 % extracts of GCA/TB1 and GCA/TB2 supported osteoblast viability above 75 %, compared to ∼ 10 % for pure GCA, while retaining measurable antibacterial activity, particularly against Staphylococcus aureus. This work highlights how molecular-level modifications can effectively tune the performance of simple, bio-based citrate polyesters, providing a versatile platform for sustainable and adaptable scaffold materials with potential biomedical applications, including drug delivery.