Introduction: Bisphosphonates (BP) are a class of drugs with a strong affinity to the mineral phase of bone, i.e., hydroxyapatite (HAp)[1]. Bonds formed between BP and HAp are not only strong but also reversible, which allows the development of self-healing composite biomaterials[2]. Although the interaction between bisphosphonates and calcium phosphates has been widely studied, interaction of bisphosphonates with other Ca-containing inorganic phases, such as calcium silicates, has not been explored extensively yet. Bioactive glasses (BG) are typically calcium-containing silicate glasses with applications in a wide range of biomedical areas[3]. Recently, we found that the binding force between BP and BG (45S5 composition) is around 3-fold higher than that between BP and HAp[4]. In this study, we have further investigated interactions between BP and 45S5 BG. In addition, we have evaluated the feasibility of exploiting BP-BG interactions for development of novel hybrid and biologically active materials for bone regeneration.
Materials and Methods: Alendronic acid (AK Scientific, USA) was selected as BP compound. To investigate the interactions between free BP and inorganic particles, 45S5 BG (Schott, Germany) and HAp (Merck, Germany) powders were immersed in aqueous BP solutions (100 mM) and incubated at 37°C for 72h. Subsequently, the particles/precipitates were air-dried and further analyzed. BP-functionalized hyaluronan was synthesized as described elsewhere [2]. To investigate gel formation based on BP-BG interaction, suspension of 45S5 BG powder was mixed with a 2 w/v% aqueous solution of BP-free (HY) or BP-functionalized (BPHY) hyaluronan. An AR2000 rheometer (TA Instruments) was used to evaluate viscoelastic properties and self-healing ability of the hydrogels.
Results and Discussion: During immersion in BP-free solutions, both HAp and BG powders were stable. Nevertheless, during the immersion in BP-containing solutions, although HAp powder did not show any changes, BG powder experienced a complex dissolution and precipitation process. When the BG powder was immersed in BP-containing solutions, after few minutes, the powder completely dissolved. Surprisingly, after the incubation, we observed formation of needle-like precipitates (Fig.1A). Further analyses of the precipitates revealed formation of a novel hybrid crystalline phase (Fig.1B, C & D).
In gel formation experiments, we only observed gel formation when HY polymer chains were functionalized with BP (Fig.2A, B & C). The gels showed remarkable self-healing ability (Fig.2D).
Conclusion: BP exhibited a very high affinity for BG. It was observed that high concentrations of BP led to complete dissolution of BG and further precipitation of a novel hybrid phase. The BP-BG interaction could be successfully exploited for development of self-healing hybrid composite hydrogels.
We acknowledge financial support from AgentschapNL (IOP Self Healing Materials, Project no. SHM012014).; M.D. acknowledges the travel award from the ICSHM2013 conference supporting his research stay at the MPI-Potsdam.
References:
[1] Rodan GA, Fleisch HA. Bisphosphonates: mechanisms of action. The Journal of Clinical Investigation. 1996;97:2692-6.
[2] Nejadnik MR, Yang X, Bongio M, Alghamdi HS, van den Beucken JJJP, Huysmans MC, et al. Self-healing hybrid nanocomposites consisting of bisphosphonated hyaluronan and calcium phosphate nanoparticles. Biomaterials. 2014;35:6918-29.
[3] Miguez-Pacheco V, Hench LL, Boccaccini AR. Bioactive glasses beyond bone and teeth: Emerging applications in contact with soft tissues. Acta Biomaterialia. 2015;13:1-15.
[4] Diba M, Schmidt S, Harrington MJ, Jansen JA, Leeuwenburgh SCG. Quantitative evaluation of interaction forces between building blocks of colloidal hydrogels. New Frontiers Symposium on Regenerative Medicine The Netherlands 2014.