Microfluidic Fabrication of Gadolinium-Doped Hydroxyapatite for Theragnostic Applications

Abstract

Among the several possible uses of nanoparticulated systems in biomedicine, their potential as theragnostic agents has received significant interest in recent times. In this work, we have taken advantage of the medical applications of Gadolinium as a contrast agent with the versatility and huge array of possibilities that microfluidics can help to create doped Hydroxyapatite nanoparticles with magnetic properties in an efficient and functional way. First, with the help of Computational Fluid Dynamics (CFD), we performed a complete and precise study of all the elements and phases of our device to guarantee that our microfluidic system worked in the laminar regime and was not affected by the presence of nanoparticles through the flow requisite that is essential to guarantee homogeneous diffusion between the elements or phases in play. Then the obtained biomaterials were physiochemically characterized by means of XRD, FE-SEM, EDX, confocal Raman microscopy, and FT-IR, confirming the successful incorporation of the lanthanide element Gadolinium in part of the Ca (II) binding sites. Finally, the magnetic characterization confirmed the paramagnetic behaviour of the nanoparticles, demonstrating that, with a simple and automatized system, it is possible to obtain advanced nanomaterials that can offer a promising and innovative solution in theragnostic applications.

Keywords: Computational Fluid Dynamics (CFD); microfluidics; nanomaterials; theragnostic; tissue engineering.

Figure 1

Computational fluid dynamics (CFD) from a simple chip (a) and the system of two connected in series (b). In the first image a) the flow velocity inside the channels of the second microchip can be seen. As expected, a greater magnitude is found in the center of the section, decreasing as it moves away from it, radially. The second image (b) depicts the pressure changes, and it can be concluded that there are no large pressure changes throughout the system that could influence the correct mixture of the components.

Figure 2

From (a–c). Particle position at different time points. Each particle describes equidistant trajectories. (d) Close-up of the end of the output tube. (e,f) Pressure and velocity simulations of the inlet channel assuming a grid of 25 elements on each panel. Results confirm that the presence of a small solid dragged by the flow does not affect the homogeneity.

Figure 3

SEM images of the different samples. Images from the upper row, (a–c), depict the Surface of pure HAp nanorods. The central row, (d–f), corresponding to the HAp:GD1 samples and the lower row, (g–i) represents the results of the HAp:GD10 material. It can be seen in the untreated images that the incorporation of gadolinium causes a softness in the appearance of protuberances and a general change in the complete surface.

Figure 4

(a) X-ray diffraction obtained for the resultant material from the hydrolysis reaction of GdCl₃. Major peaks correspond to Gadolinium Oxychloride (ICDD Ref: 00-012-0798), corroborating the chemical reactions predicted by the bibliography. (b) FTIR spectra obtained for the four studied samples. In the inset, the critical range of 1600–1400 is zoomed in. (c) X-ray diffraction curves for the different samples. As expected, the Sample with a higher concentration of Gd in the initial step of the synthesis, produces a higher peak at 2θ = 32.547° (lattice parameters: a = 3.614 Å, b = 3.614 Å, c = 5770 Å and indices (h k l) = (1 0 1)), Probing a successful substitution of Gd in the Ca sites. (d) Raman spectra in the 0–4000 cm⁻¹ wavenumber range of HAp:Gd1. Spectra (i) and (ii) represent the two main bands detected by the software. Figure (iii) shows the optical images of the compound distribution.

Figure 5

Temperature (a,b) and magnetic field (c,d) dependences of the magnetic moment of samples Gd10 (126.3 mg) and Gd1 (86.1 mg). The lines are fits of a Curie-Weiss function, Equation (13). Demagnetization effects are negligible (the correction would be ~2–3% at 5 K and strongly decreases on increasing the temperature) and were not taken into account.