Energy Conversion from Gradients across Bio-Inspired Membranes

Abstract

Membranes are fundamentally important barriers that enable the processes of life by slowing the dissipation of gradients and transducing usable energy from entropic driving forces. This dissertation presents three investigations of gradient equilibration across bio-inspired membranes, with a focus on permeability and geometric considerations in membrane-based energy transduction systems.

The first study models the dynamics of pressure generation from osmotic gradients in an expandable compartment based on mass transport principles. Using an osmotic working fluid composed of aqueous polyethylene glycol inside commercially available dialysis cassettes whose membranes exclude polymer solutes, we validated this model and explored the importance of cassette geometry, restrictions on expansion, and membrane porosity characteristics on pressure generation rates over time. The model made it possible to predict the kinetics of nastic motions caused by osmotically-induced shifts in turgor pressure in plants such as Mimosa pudica; the model’s projections based on plant cell dimensions agree well with published time scales. These cassettes are “waterable” pressure generators available to the general public; we demonstrated their utility by actuating a soft robotic gripper and published our characterization algorithm as an open-source tool.

The second study investigates the relationship between the chemical structures of a class of tethered membrane-spanning lipids found in hyperthermoacidophilic archaea and the proton/hydroxide permeability of self-assembled monolayer membranes that the lipids form. We determined permeability values by measuring the fluorescence of solutions of liposomes containing pH-sensitive dyes over time after a step change in the external pH. The presence of isoprenoid methyl groups led to reductions in permeability values, and the length of the transmembrane tether unexpectedly displayed a direct correlation with the permeability, leading us to speculate about the importance of hydrophobic crowding in the membrane interior. Surprisingly, the presence of a transmembrane tether had no significant effect on the permeability at room temperature. We observed a strong positive correlation between the permeability of a membrane and the probability of observing water molecules spontaneously clustering inside the hydrophobic region of a simulated membrane of identical composition using molecular dynamics, providing a predictive parameter obtainable without any “wet” experimentation that may be useful for the design of membrane compositions with specific permeability characteristics.

The third study presents a novel hydrogel-based reverse-electrodialytic energy transduction scheme inspired by the electric eel. As in the biological system (but unlike traditional batteries), the “artificial electric organ” presented here is a soft, flexible, potentially biocompatible means of generating electricity using only a repeating arrangement of ionic gradients across selective membranes. The artificial electric organ generates numerous additive voltages at the same time in order to produce a large transient electric signal; while the biological system accomplishes this synchronicity through neural signaling, the artificial setup uses geometries that allow simultaneous mechanical registration of the gels. This scheme is flexible enough that we were able to implement it in three distinct ways: fluidically, through the printing of large arrays (enabling the generation of over 100 V), and using a Miura-ori folding geometry that assembles planar films, which achieved a power density of 27 mW m^-2.

The work presented here draws inspiration from biological systems and earlier industrial efforts to extract electrical power from salinity gradients. Membrane-based strategies are well-suited for the local generation of usable energy on small scales and may be useful in the microelectromechanical systems and implantable devices of the future.