Synthesis and Characterization of Gelatin-Based Magnetic Hydrogels

Abstract

A simple preparation of thermoreversible gelatin-based ferrogels in water provides a constant structure defined by the crosslinking degree for gelatin contents between 6 and 18 wt%. The possibility of varying magnetite nanoparticle concentration between 20 and 70 wt% is also reported. Simulation studies hint at the suitability of collagen to bind iron and hydroxide ions, suggesting that collagen acts as a nucleation seed to iron hydroxide aggregation, and thus the intergrowth of collagen and magnetite nanoparticles already at the precursor stage. The detailed structure of the individual ferrogel components is characterized by small-angle neutron scattering (SANS) using contrast matching. The magnetite structure characterization is supplemented by small-angle X-ray scattering and microscopy only visualizing magnetite. SANS shows an unchanged gelatin structure of average mesh size larger than the nanoparticles with respect to gel concentration while the magnetite nanoparticles size of around 10 nm seems to be limited by the gel mesh size. Swelling measurements underline that magnetite acts as additional crosslinker and therefore varying the magnetic and mechanical properties of the ferrogels. Overall, the simple and variable synthesis protocol, the cheap and easy accessibility of the components as well as the biocompatibility of the gelatin-based materials suggest them for a number of applications including actuators.

Figure 1

SANS macroscopic cross-section dΣ/dΩ versus scattering vector Q for 18 wt% gelatin in D₂O (T = 20 °C). At low Q (< 0.02 nm⁻¹) USANS data are also presented after rescaling. The solid line represents a fit of the two levels Beaucage equation.

Figure 2

Schematic representation of the ferrogel synthesis. a) Unloaded gelatin hydrogel, b) hydrogel loaded with ferrous and ferric ions, and c) magnetic nanoparticles distributed inside the hydrogel after in situ co-precipitation with NaOH.

Figure 3

TEM images of a) and b) ultramicro-cuts of an embedded ferrogel at 10 wt% gelatin concentration after 6 reaction cycles (RC) at different magnifications.

Figure 4

Morphology and pore size of two dried hydrogels a) without and b) with magnetite incorporated.

Figure 5

SANS scattering pattern of the ferrogel in pure D₂O and in a mixed D₂O/H₂O solvent of 28 vol% D₂O and 72 vol% H₂O. The solid lines represent the fitting of the Beaucage expression. The form factor of the magnetite is plotted as dashed dotted line.

Figure 6

SAXS intensity dΣ/dΩ(Q) versus scattering vector Q for a 18 and 12 wt% wet and dry ferrogel. In all cases the structure factor S(Q) was not negligible. Therefore, the data were fitted with the product of Equation S2 in the SI (structure factor) and the Beaucage equation (form factor) as shown by the solid lines. The dashed dotted lines represent the form factor of the particles.

Figure 7

Magnetic properties of the synthesized hybrid materials. a) Magnetization curves of a dried ferrogel at 2 K and 293 K. Inset: Enlargement of the low field region showing the different coercive fields for the NPs at 2 and 293 K. b) ZFC-FC curves as a function of temperature.

Figure 8

Degree of hydrogel swelling plotted as a function of the swelling time at 25 °C for different samples with a gelatin concentration of 10 wt%. The equilibrium swelling degrees Sd (%) for the plotted samples are 779.2 ± 9.6 (gelatin), 1531.4 ± 62.0 (RC 1), 684.65 ± 80.84 (RC 3) and 195.64 ± 0.26 (RC 6).

Figure 9

(left) Representative structure for Fe^III(OH)₃ coordination by collagen. Note that three carbonyl/hydroxyl groups are providing O·Fe salt bridges via one short (2.3 Å) and two weaker (2.6 Å) contacts. (right) Fe^II(OH)₂ cluster coordination by collagen leading to distorted/incomplete octahedral coordination of Fe^II (the number of coordinating water molecules from the solvent varies from 0 to 2). Atom colors: Fe (yellow), O (red/green for solvent), H (white), N(blue) and C(grey).

Attraction of ferrogel with a) no magnetic field and b) external magnetic field (ca. 1T)