Biennial Report 2016/2017

Selected Results
High Energy Electron Irradiation of Gelatin Ferrogels for Bioapplications
E. Wisotzki, A. Weidt, S. G. Mayr
Ferrogels are emerging responsive materials that
consist of a hydrogel matrix embedded with
magnetic nanoparticles (MNP). Through coupling
of the particles to the polymer network, ferrogels
can deform, change rheology and alter swelling
behavior in applied magnetic fields. With
significant development in biocompatible
hydrogels and MNP, ferrogels have gained
attention for potential bioapplications such as soft
actuators, drug delivery devices and tissue
engineering scaffolds.
In [1], high energy electron irradiation was
demonstrated to crosslink gelatin hydrogels,
resulting in thermal stability and a range of
available mechanical properties, dependent on
the irradiation dose. This reagent-free technique
has the additional advantage of not requiring any
(potentially toxic) chemical crosslinkers and has
demonstrated low cytotoxicity in cell culture [2]. 10
MeV electron beam irradiation was extended to
gelatin ferrogels to investigate the ability to locally
stabilize and confine such nanostructures into the
gel network [2]. Using emerging methods of
magnetic particle spectroscopy (MPS) and
magnetorelaxometry (MRX) at the PTB Berlin, the
thermal stability of these ferrogels was examined.
In Fig. 1 at point c), the magnetic response of the
nanoparticles in untreated gelatin showed strong
changes near 34˚C in both MRX and MPS. This
response clearly stemmed from the macroscopic
sol-gel transition in native gelatin, as illustrated by
the vanishing storage modulus in Fig. 2. In
general, as the network transitioned from a (a) gel
to (d) solution, it was easier to magnetize the
particles in the applied magnetic fields. Therefore,
Figure 2: (left) Storage modulus (G’) showing macroscopic
changes to the viscoelasticity across the native sol-gel
transition. (right) Effective relaxation times from 0 kGy and 5
Ȃ
kGy ferrogels. was magnified by 6 and shifted right by
2.8˚C. (bottom) Four distinct regimes observed in the native
sol-gel transition (0 kGy) correspond to a) magnetization of
small particles, b) onset of local softening, c) magnetization
of larger aggregates now possible in softening matrix, d)
sharp decrease in magnetization times due to liquefied
network.
relaxation times decreased Ȃ
(̀ ), while
magnetization (M 3 ) increased. Even a small dose
of 5 kGy irradiation largely stabilized this thermal
transition, indicating confinement of the particles
in the network. However, MRX was sensitive to
changes in the nanorheology of the gelatin,
shown in Fig. 2. Nanoparticles appeared sensitive
to a local softening across the sol-gel transition,
leading to reduced effective relaxation times at
higher temperatures. With a dose of 5 kGy, this
effect was approximately 1/6 th of that observed in
native gelatin and occurred several degrees later,
as a direct result of the irradiation crosslinking.
Overall, high energy electron irradiation is a viable
method to sufficiently stabilized gelatin ferrogels
for a wide range of future bioapplications.
Figure 1: (left) Effective relaxation times determined by MRX
with increasing temperatures, for unirradiated (0 kGy) and
5 kGy irradiated ferrogels. (right) Temperature dependence
of the nonlinear magnetization (3 rd harmonic, M 3) using MPS
for untreated (0 kGy) and 20 kGy irradiated ferrogels.
Literature
[1] E. I. Wisotzki, M. Hennes, C. Schuldt, F. Engert, W.
Knolle, U. Decker, J. A. Käs, M. Zink, S. G. Mayr, J. Mater.
Chem. B 2 (2014) 4297.
[2] E. I. Wisotzki, R. P. Friedrich, A. Weidt, C. Alexiou, S. G.
Mayr, M. Zink, Macromol. Biosci. 16 (2016) 914.
[3] E. I. Wisotzki, D. Eberbeck, H. Kratz, S. G. Mayr, Soft
Matter 12 (2016) 3908.
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