ABSTRACT
Deep eutectic solvents, considered ionic liquid (IL) analogues, show promise for many material science and engineering applications over typical ILs because they are readily available and relatively inexpensive. Atomistic molecular dynamics simulations have been performed over a range of temperatures on one eutectic mixture, 1:2 choline chloride/urea, using different force field modifications. Good agreement was achieved between simulated density, volume expansion coefficient, heat capacity, and diffusion coefficients and experimental values in order to validate the best performing force field. Atom-atom and center-of-mass radial distribution functions are discussed in order to understand the atomistic interactions involved in this eutectic mixture. Experimental infrared (IR) spectra are also reported for choline chloride-urea mixtures, and band assignments are discussed. The distribution of hydrogen-bond interactions from molecular simulations is correlated to curve-resolved bands from the IR spectra. This work suggests that there is a strong interaction between the NH2 of urea and the chlorine anion where the system wants to maximize the number of hydrogen bonds to the anion. Additionally, the disappearance of free carbonyl groups upon increasing concentrations of urea suggests that at low urea concentrations, urea will preferentially interact with the anion through the NH2 groups. As this concentration increases, the complex remains but with additional interactions that remove the free carbonyl band from the spectra. The results from both molecular simulations and experimental IR spectroscopy support the idea that key interactions between the moieties in the eutectic mixture interrupt the main interactions within the parent substances and are responsible for the decrease in freezing point.
ABSTRACT
Ideal biomaterials for bone grafts must be biocompatible, osteoconductive, osteoinductive and have appropriate mechanical properties. For this, the development of synthetic bone substitutes mimicking natural bone is desirable, but this requires controllable mineralization of the collagen matrix. In this study, densified collagen films (up to 100 µm thick) were fabricated by a plastic compression technique and cross-linked using carbodiimide. Then, collagen-hydroxyapatite composites were prepared by using a polymer-induced liquid-precursor (PILP) mineralization process. Compared to traditional methods that produce only extrafibrillar hydroxyapatite (HA) clusters on the surface of collagen scaffolds, by using the PILP mineralization process, homogeneous intra- and extrafibrillar minerals were achieved on densified collagen films, leading to a similar nanostructure as bone, and a woven microstructure analogous to woven bone. The role of collagen cross-links on mineralization was examined and it was found that the cross-linked collagen films stimulated the mineralization reaction, which in turn enhanced the mechanical properties (hardness and modulus). The highest value of hardness and elastic modulus was 0.7 ± 0.1 and 9.1 ± 1.4 GPa in the dry state, respectively, which is comparable to that of woven bone. In the wet state, the values were much lower (177 ± 31 and 8 ± 3 MPa) due to inherent microporosity in the films, but still comparable to those of woven bone in the same conditions. Mineralization of collagen films with controllable mineral content and good mechanical properties provide a biomimetic route toward the development of bone substitutes for the next generation of biomaterials. This work also provides insight into understanding the role of collagen fibrils on mineralization.
