Current standard techniques for investigating protein-protein interactions require volumes and concentrations significantly higher than those relevant under physiological conditions. Microfluidic diffusional sizing offers an alternative method for investigating protein-protein interactions and protein assembly under physiological conditions and has been applied to investigate a wide range of protein properties, functions, and interactions. In this thesis, current approaches are further extended and their applicability to various interactions related to human diseases is shown. Specifically, recent work is described which makes use of microfluidic techniques combined with chemical kinetic and thermodynamic analyses to study the mechanism of the disaggregation of $\alpha$-synuclein fibrils with Hsc70 along with its co-chaperones DnaJB1 and Apg2 as well the interaction between BRICHOS and amyloid fibrils. These analyses provide mechanistic insight into the role of molecular chaperones in clearance of amyloid fibrils and inhibition of their formation. The second part focuses on the development of a diagnostic tool based on microfluidic diffusional sizing for detection of antibodies in pristine human serum and plasma samples. This allows to characterise affinity and concentration as well as neutralisation potential of patient antibodies. This technique is then further applied to characterisation of plasma samples from convalescent SARS-CoV-2 infected individuals, revealing that the affinities span more than two orders of magnitude while the concentrations are more constant. The thesis shows how advances physico-chemical technology enable answering emerging questions in biophysical chemistry and medicine.