Engineered Living Materials (ELM) are an emerging class of biomaterials combining polymeric matrices with living cells, offering novel approaches such as in biomedicine and additive manufacturing. Besides mammalian and plant cells or fungi, bacteria render the most popular biological building block in the fabrication of ELM. They can be used as drug eluting sponges, wearable biosensors or living skin grafts. Besides their huge potential, the biggest challenges include maintaining long-term functionality and nutrient supply inside the ELM (e.g. a hydrogel matrix) as well as preventing their outgrowth. In this respect, their physico-mechanical interaction with the hydrogel matrix is crucial, yet simple 3D models to study these interactions are not yet established.

In this study, we investigated fluorescently labelled latex particles as a model system to explore hydrogel-bacteria interactions, focusing on properties such as pore size dependent bacterial motility while excluding complexities like nutrient supply and sterility.

Agarose, native collagen and in-house methacrylated collagen (ColMA) were used as hydrogel matrices in concentrations of 0.1 – 2 % (w/v). Mechanical properties such as microstructure, pore size and stiffness were tailored by varying hydrogel concentration and crosslinking density, while PEGDA concentration as spacer was adjusted for ColMA. The hydrogels were characterized regarding their mechanical behavior and microstructure by rheology and swelling tests. Fluorescently labeled latex particles (1 µm) modeled the bacteria in the hydrogel surrounding. A fluorescence microscope coupled to a load cell examined the hydrogel and latex particle deformation upon defined stress.

Agarose and native collagen were investigated due to showing iso-static compression and fluid shear stress, respectively, under mechanical load. Higher concentration led to reduced pore size (e.g. 100 – 200 nm at 2.0 % w/v agarose) in both hydrogels, decreasing swelling behavior. We observed the washing out of particles during mechanical load tests from, whereas agarose retained the particles. The yield stress increased with higher concentration for both gels, making them more resistent to stress.

ColMA, as photo-chemically crosslinkable hydrogel showing tensile stress under mechanical load, was investigated further due to desirable properties such as partial water retention and stress relaxation. Its microstructure depended on methacrylation degree and PEGDA concentration. Also the yield stress point increased with higher methacrylation degree as well as PEGDA concentration. This was an accordance with the microstructure, which also influenced the swelling behavior upon change. Furthermore, methacrylation prevented washing out of particles from the gel, in comparison to native collagen.

The authors acknowledge financial support by Deutsche Forschungsgemeinschaft (DFG, project 541299811) as part of the Priority Program “Engineered Living Materials” SPP 2451.