Some bacteria can act as catalysts to oxidize (or reduce) organic or inorganic matter with the potential of generating electrical current. Despite their high value for sustainable energy, organic compound production and bioremediation, a tool to probe the natural biodiversity and to select most efficient microbes is still lacking. Compartmentalized cell culture is an ideal strategy for achieving such a goal but the appropriate compartment allowing cell growth and electron exchange must be tailored. Here, we develop a conductive composite hydrogel made of a double network of alginate and carbon nanotubes. Homogeneous mixing of carbon nanotubes within the polyelectrolyte is obtained by a surfactant assisted dispersion followed by a desorption step for triggering electrical conductivity. Dripping the mixture in a gelling bath through simple extrusion or a double one allows the formation of either plain hydrogel beads or liquid core hydrogel capsules. The process is shown to be compatible with the bacterial culture (Geobacter sulfurreducens). Bacteria can indeed colonize the outer wall of plain beads or the inner wall of the conductive capsules’ shell that function as an anode from which electrons produced by the cells are collected.

This work focuses on the creation of a new biocompatible reactor allowing the encapsulaion and the study of electroactive bacteria. Made by co-extrusion, this millimeter bioreactor is a liquid core capsule with a conducting hydrogel membrane. To create such an object, we formulate a composite hydrogel of alginate/carbon nanotubes in two steps. The first step is rapid and creates the hydrogel matrix by diffusion of divalent ions inside the alginate/carbon nanotubes mix. The second step is slower and permits the dialysis of the surfactant used to stabilize the nanotubes. During this last step, the carbone nanotube network percolates, creating a conducting network in the hydrogel for sufficient nanotube contents (above 0.5 wt%). This composite material has a macroscopic conductivity around 0.1 S.m^-1. An electrochemical study of this material allows to follow the nanotube connection inside the hydrogel. Bacteria can adhere on this composite hydrogel. Then, we demonstrate that the electroactivity of a biofilm developped on the inner side of the conductive capsule shell can be measured. This new biocompatible and electron-conducting compartment opens the way towards the development of a screening tool for the selection of electroactive bacteria but also brings innovative perspectives in the field of microbial fuel cells fabrication.
 

The local functionalities of biocompatible objects can be characterized under conditions similar to their operating ones by scanning electrochemical microscopy (SECM). In the case of alginate beads entrapping carbon nanotubes (CNTs), SECM allows evidencing the local conductivity, the organization, and the communication between CNTs. It shows that the CNTs network is active enough to allow long range charge evacuation, enabling the use of alginate/CNT beads as soft 3D electrodes. Direct connection or local interrogation by microelectrodes allows visualizing their communication as a network and eventually studying them individually at the nanoscale.

Capsules having a thin alginate hydrogel membrane and an aqueous core can be obtained by a process that involves a co-extrusion step in air followed by a sol–gel transition of the shell after immersion into a gelling bath. The possibility to encapsulate cells that further grow in these biocompatible compartments, and thus offer a versatile tool for cell culture, led us to investigate the physicochemical properties of the capsules. A cut-off pore size of the semi-permeable membrane is extrapolated from the release of polymers out of the capsule. When polymers cannot diffuse through the membrane, the osmotic pressure mismatch between the core and the surrounding medium triggers an inflation of the capsule. The swelling may reach a steady state that allows the determination of the elastic features of the hydrogel shell. On the other hand, the capsule membrane may rupture and then contract. From this stress-relaxation process, a critical deformation of the hydrogel shell above which plasticity occurs can be deduced. Finally, thanks to the physical nature of the hydrogel, the core content can be released by dissolving the membrane with the help of small electrolytes. The shell life is shown to vary inversely with the ionic strength of the solution.

Deciphering the multifactorial determinants of tumor progression requires standardized high-throughput preparation of 3D in vitro cellular assays. We present a simple microfluidic method based on the encapsulation and growth of cells inside permeable, elastic, hollow microspheres. We show that this approach enables mass production of size-controlled multicellular spheroids. Due to their geometry and elasticity, these microcapsules can uniquely serve as quantitative mechanical sensors to measure the pressure exerted by the expanding spheroid. By monitoring the growth of individual encapsulated spheroids after confluence, we dissect the dynamics of pressure buildup toward a steady-state value, consistent with the concept of homeostatic pressure. In turn, these confining conditions are observed to increase the cellular density and affect the cellular organization of the spheroid. Postconfluent spheroids exhibit a necrotic core cemented by a blend of extracellular material and surrounded by a rim of proliferating hypermotile cells. By performing invasion assays in a collagen matrix, we report that peripheral cells readily escape preconfined spheroids and cell–cell cohesivity is maintained for freely growing spheroids, suggesting that mechanical cues from the surrounding microenvironment may trigger cell invasion from a growing tumor. Overall, our technology offers a unique avenue to produce in vitro cell-based assays useful for developing new anticancer therapies and to investigate the interplay between mechanics and growth in tumor evolution.

This work focuses on the study of physico-chemical properties of millimetric capsules having a liquid core and a thin alginate membrane.
First, we caracterized the diffusion of molecules through the hydrogel shell and estimated the cut-off radius of the semi-permeable membrane. Moreover, the leakage of small hydrophilic solutes can be limited by adding a hydrophobic shell between the aqueous core and the alginate membrane. We demonstrated that the characteristic time of transport is determined by the geometry of the system (hydrophobic shell thickness and capsule diameter)
and by the solubility and diffusion coefficient of the solute in the oil phase. Furthermore, swelling of a capsule under an osmotic pressure difference allowed us to identify the viscoelastic and viscoplastic regimes
of the membrane under stretching. We deduced the elastic modulus of the membrane which decreases above the plastic threshold. However, when capsules are compressed, the hydrogel is concentrated due to the irreversible evacuation of water. This leads to rigidification of its structure.
Then, we focused on the flow of oil-core capsules dispersed in water. There is a friction between capsules that we can control by adding surfactant. Thus we studied its role in the flow of capsules in the constriction of an hourglass. We showed that the flow is constant and does not depend on friction. It is established by the size of the constriction. Indeed, the flow is only determined by the hydrodynamic in fluid regime in
the hourglass' neck.

Lipid nanoparticles, based on nanoemulsion templates, are interesting candidates for contrast agent and active ingredient transport and delivery due to their small size, biodegradable nature and high versatility. An optimized nanosystem for drug delivery should present an important encapsulation ratio, as well as controlled release kinetics. In this context, the influence of the lipid nanoparticle core composition on the encapsulation and release behavior of the Nile Red fluorophore, used as a model molecule of intermediate lipophilicity, is explored. For that purpose, the lipid nanoparticle physical state, the encapsulated molecule localization, and the encapsulation/release behavior are studied. Careful characterization of lipid nanoparticles is performed by DSC and 1H NMR analysis.Nile Red localization is evaluated by complementary fluorescence spectroscopy and 1H NMR techniques. The encapsulation of Nile Red is governed and limited by its solubility in the lipids. A double localization of the fluorophore at the membrane and in the particle core is observed, resulting in a two-phase release kinetics: a quick burst release, followed by a slow prolonged release. Adding wax to the formulation increases the lipid nanoparticle internal viscosity while favoring Nile Red localization in the core, which results in slower release kinetics.

Liquid pearls designate liquid core capsules having a thin elastic shell.

We report an experimental investigation on the formation of liquid-core capsules having a thin hydrogel elastic membrane, named ‘liquid pearls’. These fish-egg-like structures are initially made of a millimetric liquid drop, aqueous or not, coated with an aqueous liquid film containing sodium alginate that gels once the double drop enters a calcium chloride bath. The creation of such pearls with a micrometer-thick membrane requires the suppression of mixing until gelling takes place. Here, we show that superimposing a two-dimensional surfactant precipitation at the interface confers a transient rigidity that can damp the shear-induced instability at impact. Based on this, pearls containing almost any type of liquid can be created. This opens the possibility to use such structures as a new tool for screening microorganism survival and growth in various three-dimensional environments.