Adhesion of Lipid Vesicles on Bio-Inspired Surfaces
by Lucia Milena Wesenberg
Date of Examination:2025-05-16
Date of issue:2025-10-08
Advisor:Prof. Dr. Marcus Müller
Referee:Prof. Dr. Marcus Müller
Referee:Prof. Dr. Stefan Klumpp
Persistent Address:
http://dx.doi.org/10.53846/goediss-11554
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Abstract
English
Many biological transport processes rely on vesicles composed of a lipid membrane enclosing a fluid volume. Examples are the neurotransmitter release at the synapse or the transport by vesicles in and between cells. The ability to transport cargo makes vesicles a promising tool for targeted drug delivery. The adaptive adhesion and desorption of the vesicles to a biological substrate are key for all these processes. Consequently, it is important to understand the factors determining whether and how a vesicle adsorbs to a substrate and what shape it will adopt. In this thesis, I employ static Fourier-mode-based simulations to systematically study the equilibrium shapes of vesicles adsorbed to planar substrates with short- and long-range interactions, with and without buoyancy. Additionally, we study the dynamics of such a process using coarse-grained Molecular Dynamics simulations. For the static equilibrium shapes of vesicles, we focus on the shape of adhered vesicles on a substrate with a finite-range membrane–substrate interaction, a relevant experimental characteristic. When comparing our theoretical predictions with the experimental measurements from the Tanaka group, we found a good qualitative agreement in the vesicle shapes. Furthermore, the interaction range of the membrane potential alters the type of the adsorption transition from a second-order transition for a contact potential to a first-order adhesion transition for finite-range potentials. In contrast, the local transversality condition that relates the maximal curvature at the edge of the adhesion zone to the adhesion strength remains rather accurate even for a finite interaction range as long as the vesicle is large compared to the interaction range. Hence, the transversality condition can be utilized to extract the interaction strength between substrate and vesicle from experimental data. We extended our model to incorporate the influence of buoyancy, a factor largely omitted in theoretical investigations. For large vesicles, even minor density differences between the inside and the outside of the vesicle give rise to significant buoyancy effects. Buoyancy leads to an offset in the transversality condition. For upward buoyancy, we find that adsorbed vesicles are at most metastable. We constructed an adsorption diagram summarizing the metastable region of upward buoyant adsorbed vesicles. Lastly, we also investigated how the interface between two vesicles adhered to one another deforms as a function of their size ratio. Further, we study permeable vesicles, adapting dynamically to substrates with a finite-range adhesion potential using coarse-grained Molecular Dynamics (MD) simulations. The membrane deforms during adhesion and induces fluid flow inside and outside the vesicle. This complex interaction between membrane deformation and hydrodynamics significantly affects the shapes observed during adhesion. We note that final shapes remain similar in the presence of hydrodynamics, whereas the transient shapes differ significantly. We find that the hydrodynamic coupling seems to set the timescale of the adhesion process. This timescale can be estimated using dimensional analysis and predicts a faster adhesion time for smaller vesicles, whereas larger vesicle adhesion takes longer. Our findings help with the interpretation of experimental data. All the explored effects might help to design optimized artificial vesicles for efficient and targeted drug delivery.
Keywords: vesicle; adhesion; wetting; biophysics; Helfrich; membranes
