Reproducibility and Batch-Dependent Variability in Polymer-Based Nanocarriers

Abstract Polymers are essential functional constituents in most nanomedicines. While frequently treated conceptually as defined compounds in this respect, polymers are intrinsically heterogeneous materials, as they arise from statistical chain growth. Consequently, batch-to-batch variability is inherent to polymers. While this aspect is often overlooked, it potentially impacts reproducibility in downstream processes, particularly in systems that heavily rely on polymers, such as polymeric micelles. To systematically capture the impact of batch-to-batch variability in such systems, we compare five batches each of three different amphiphilic poly(2-oxazoline)/poly(2-oxazine) (POx/POzi) block copolymers synthesized in three independent laboratories. In aqueous solution, all batches formed micelles. Minor differences in hydrodynamic size and critical micelle concentration, however, could not be readily deduced from standard physicochemical data. Drug solubilization up to a certain level proved to be an intrinsic and reproducible property of a given POx/POzi copolymer. However, individual batches displayed enhanced loading, which was not representative of the reliably achievable solubilization capacity. Cell viability assays indicated mostly consistent profiles across batches, yet rare nonrepresentative batches with altered responses did occur. Overall, our findings demonstrate reproducible baseline behavior alongside an inherent and sometimes pronounced batch-dependent component, which can critically confound interpretation and overstate reliability when only single batches are examined.