The work conducted has concentrated on the following:
I. the construction of energy-generating droplet networks
II. the systematic optimization of the factors that impact energy generation
III. the replacement of the oil surrounding droplet networks by aqueous solutions
We have constructed a tissue-like energy-generating droplet network. To attain this goal, we screened a library of lipids and polymers and achieved stable droplet networks with 1,2-diphytanoyl-sn-glycero-3-phosphatidylcholine (DphPC). A model system was constructed with an anode droplet and a cathode droplet. The oxidation of nicotinamide adenine dinucleotide (NADH) by NADH dehydrogenase in the anode droplet generated electrons, which were shuttled to a gold electrode by electron mediators, 9,10-Anthraquinone-2,6-disulfonic acid (AQDS). The electrons subsequently moved to the cathode droplet via the electrode to reduce oxygen. Meanwhile, to balance the charges, monovalent cations were transported across the lipid bilayer through gramicidin A (gA) channels. We have confirmed and assessed the generation of electricity using an amplifier.
Unlike the 'bioelectronic' devices composed of non-natural components (e.g. elastomers), our energy-generating droplet networks are built of delicate biological molecules. Initially, electrical outputs that were high enough to rupture the lipid bilayer in 3 mins were achieved. In control experiments, we observed droplet coalescence under either a high voltage (> 0.73 V) generated between the gold electrodes, or a high density of charges accumulated on the sides of lipid bilayer as shown by the high capacitance current. To sustain a stable generation of electricity over a long period of time, we systematically varied the components and optimized the electrical output along with the droplet stability. In particular, we included the electron mediators, AQDS and 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), in both anode and cathode droplets, respectively, to moderate the electrical potential. We found that the current was powered solely by the enzymatic oxidization of NADH in the anode droplet, which drove the oxygen reduction in the absence of a catalyst in the cathode droplet. This configuration resulted in a more stable droplet network for functional electrical output by preventing a charge overload on the lipid monolayers prior to the formation of droplet interface bilayers (DIBs).
To move droplet networks formed in oil to aqueous solutions is challenging. Our preliminary results showed that the droplet-droplet interactions are not sufficiently stable to allow this transition. We, therefore, used the agarose hydrogel to stabilize the droplets from inside. The hydrogel-supported droplet networks preserved the intact structure during the transition from oil to water. This paved the way for further applications of energy-generating droplet networks under physiological conditions at the interface with living cells.
The conclusions of the action:
1. Tissue-like energy-generating droplet networks have been constructed.
2. The components inside the droplet battery for sustainable electrical output and sufficient stability particularly have been optimized.
3. Transferring droplet network from oil to aqueous solution has been achieved.
4. The electrical potential built from the droplet battery was capable to drive the directional movement of monovalent cations and membrane-permeable pyronin Y across the droplet network.
5. The possibility of regulating cellular behavior by droplet battery was investigated preliminarily.
