Hybrid Wood Materials with Magnetic Anisotropy Dictated by the Hierarchical Cell Structure

Anisotropic and hierarchical structures
are bound in nature and highly desired in engineered materials, due
to their outstanding functions and performance. Mimicking such natural
features with synthetic materials and methods has been a highly active
area of research in the last decades. Unlike these methods, we use
the native biomaterial wood, with its intrinsic anisotropy and hierarchy
as a directional scaffold for the incorporation of magnetic nanoparticles
inside the wood material. Nanocrystalline iron oxide particles were
synthesized in situ via coprecipitation of ferric and ferrous ions
within the interconnected pore network of bulk wood. Imaging with
low-vacuum and cryogenic electron microscopy as well as spectral Raman
mapping revealed layered nanosize particles firmly attached to the
inner surface of the wood cell walls. The mineralogy of iron oxide
was identified by XRD powder diffraction and Raman spectroscopy as
a mixture of the spinel phases magnetite and maghemite. The intrinsic
structural architecture of native wood entails a three-dimensional
assembly of the colloidal iron oxide which results in direction-dependent
magnetic features of the wood–mineral hybrid material. This
superinduced magnetic anisotropy, as quantified by direction-dependent
magnetic hysteresis loops and low-field susceptibility tensors, allows
for directional lift, drag, alignment, (re)­orientation, and actuation,
and opens up novel applications of the natural resource wood.