From Passive Vesicles to Cell-Like Microrobots

Figure 1.  (a) Microscopic image of a GUV and lipid membrane structure. (b) Schematic illustration of the experimental setup and the application of AC electric fields across ITO-coated glasses. (c) Schematic illustration of the system components: (i) Bare GUVs and (ii) bare particles exhibit no motion individually, while (iii) GUV-particle assemblies spontaneously form under an electric field, resulting in persistent motion. 

Giant unilamellar vesicles (GUVs) are widely used as model systems for biological cells. They share the same lipid membrane structure, are soft and deformable, and can encapsulate cargos such as drug molecules. Despite these advantages, GUVs are typically passive and lack a mechanism for sustained, directed movement.

In our work, we introduce a cell-like microrobot design built from two passive components: GUVs and micrometer-sized silica particles. When these components are placed together under an alternating current (AC) electric field, they spontaneously assemble without any chemical modification or fuel uptake. While the vesicles and the particles remain passive when considered individually, their interaction with each other and with the surrounding fluid under the applied field gives rise to persistent, self-propelled motion.

From passive vesicles to active systems

When an electric field is applied, silica particles are attracted to the vesicle surface and form vesicle-particle assemblies. When the particle distribution on the vesicle surface is asymmetric, this assembly becomes motile. The applied electric field also generates fluid flows around the vesicle, and the asymmetric particle arrangement disrupts these flows in a way that drives the entire structure forward.

The resulting motion exhibits several distinctive characteristics:

• High propulsion speed: The assemblies move at relatively high speeds compared to other reported soft vesicle-based systems.
• Field-controlled activity: Motion is maintained only while the electric field is applied, allowing the system to be switched on and off in a fully reversible manner. The speed of the vesicle can be adjusted by changing the amplitude and frequency of the applied field, enabling smooth transitions between different motile states.
• Adaptive motion in complex environments: The soft, deformable nature of the vesicle and the dynamic particle attachment allow the assembly to rearrange and maintain motion even in the presence of obstacles or environmental heterogeneities.
• On-demand release of biological cargo: At higher field strengths, the same setup can trigger controlled membrane rupture, enabling the release of encapsulated cargo such as nanoparticles or motile bacteria, which remain active after release.

Figure 2.  (a) Schematic depiction of flow patterns and particle attachment. Symmetric particle attachment generates symmetric flows, resulting in no net motion, while asymmetric particle attachment induces asymmetric flows, leading to vesicle motion. The inset shows vesicles in the top view. (b) Demonstration of cargo release via vesicle bursting and the release of encapsulated particles (top) and encapsulated bacteria (bottom).

Why it matters for soft microrobotics

Our findings demonstrate that persistent motion can arise without motors, chemical fuels, or complex internal machinery. Instead, directed movement emerges from the coupling of soft membranes, simple particles, and externally applied electric fields, accompanied by fluid flows at the microscale.

This minimal and fully reversible strategy provides a new framework for designing cell-like microrobots that are adaptive, reconfigurable, and responsive to their environment. Beyond microrobotics, the system offers a versatile platform for studying particle-membrane interactions and for building simplified physical models that capture key aspects of active biological matter. More broadly, the work highlights how lifelike functionality can emerge from basic physical principles, blurring the boundary between passive materials and active systems.

Our cover image design has been selected for the front cover of Advanced Materials.

Figure 3. Two moving giant unilamellar vesicles (GUV) (one in front and one in the back) are shown with silica particles asymmetrically attached to their surface, representing the self-assembled GUV-particle system. 

Publikation:

Emergent Motility of Self-Organized Particle-Giant Unilamellar Vesicle Assembly

Selcan Karaz, Gaurav Gardi, Mertcan Han, Saadet Fatma Baltaci, Mukrime Birgul Akolpoglu, Metin Sitti

Advanced Materials

https://doi.org/10.1002/adma.202512036