A framework for accurate evaluation of the promise of aquaporin based biomimetic membranes

Aquaporin based membranes (ABMs) are considered a promising biomimetic desalination technology and have been intensively studied over the last few years. The most common strategy to synthesize ABMs is to deposit the aquaporin incorporated lipid or block copolymer (BCP) vesicles onto porous substrates or more recently to integrate them within the active layer of polyamide membranes. However, ABMs with orders of magnitude improvement in permeability and perfect salt rejections proposed in initial work have not been realized. Early results were based on materials and methods that were rudimentary, especially considering the progress that has been made in this field. In particular, low signal to noise ratios (SNRs <50) of stopped flow measurements for vesicle-based assays have led to large inaccuracies in permeability estimation. We show that such low SNRs can result from using vesicle samples with a high concentration of micelles and provide a connection between morphology and data quality. We have conducted a comprehensive evaluation of the true promise of these membranes using improved methods for polymer synthesis, self-assembly, experimental evaluation as well as calculations that more directly compare the outcome of biophysical evaluations to those used in the desalination membrane industry. We propose these as standard methods for use in ABM research. The role of concentration polarization in introducing error into vesicle based permeability measurements is identified. We further describe a simple technique to calculate the expected flux from a membrane synthesized using vesicle immobilization on a permeable substrate that can be used to estimate realistic membrane fluxes from stopped flow data. These calculations show that it is possible to achieve permeabilities one to two orders of magnitude higher than current membranes using ABMs but several innovations will be needed to reach this potential.