The potential of using proteins as drugs is held back
by their
low stability in the human body and challenge of delivering them to
the site of function. Extensive research is focused on drug delivery
systems that can protect, carry, and release proteins in a controlled
manner. Of high potential are cross-linked degradable starch microspheres
(DSMs), as production of these is low-cost and environmentally friendly,
and the products are degradable by the human body. Here, we demonstrate
that DSMs can absorb the model protein lysozyme from an aqueous solution.
At low amounts of lysozyme, its concentration in starch microspheres
strongly exceeds the bulk concentration in water. However, at higher
protein contents, the difference between concentrations in the two
phases becomes small. This indicates that, at lower lysozyme contents,
the absorption is driven by protein–starch interactions, which
are counteracted by protein–protein electrostatic repulsion
at high concentrations. By applying small-angle X-ray scattering (SAXS)
to the DSM–lysozyme system, we show that lysozyme molecules
are largely unaltered by the absorption in DSM. In the same process,
the starch network is slightly perturbed, as demonstrated by a decrease
in the characteristic chain to chain distance. The SAXS data modeling
suggests an uneven distribution of the protein within the DSM particles,
which can be dependent on the internal DSM structure and on the physical
interactions between the components. The results presented here show
that lysozyme can be incorporated into degradable starch microspheres
without any dependence on electrostatic or specific interactions,
suggesting that similar absorption would be possible for pharmaceutical
proteins.