The long-standing goal in membrane development is creating materials with superior transport properties, including both high flux and high selectivity. These properties are common in biological membranes, and thus mimicking nature is a promising strategy towards improved membrane design. In previous studies, we have shown that artificial water channels can have excellent water transport abilities that are comparable to biological water channel proteins, aquaporins. In this study, we propose a strategy for incorporation of artificial channels that mimic biological channels into stable polymeric membranes. Specifically, we synthesized an amphiphilic triblock copolymer, poly(isoprene)– block –poly(ethylene oxide)– block –poly(isoprene), which is a high molecular weight synthetic analog of naturally occurring lipids in terms of its self-assembled structure. This polymer was used to build stacked membranes composed of self-assembled lamellae. The resulting membranes resemble layers of natural lipid bilayers in living systems, but with superior mechanical properties suitable for real-world applications. The procedures used to synthesize the triblock copolymer resulted in membranes with increased stability due to the crosslinkability of the hydrophobic domains. Furthermore, the introduction of bridging hydrophilic domains leads to the preservation of the stacked membrane structure when the membrane is in contact with water, something that is challenging for diblock lamellae that tend to swell, and delaminate in aqueous solutions. This new method of membrane fabrication offers a practical model for making channel-based biomimetic membranes, which may lead to technological applications in reverse osmosis, nanofiltration, and ultrafiltration membranes.
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Design Considerations for Artificial Water Channel–Based Membranes
Aquaporins (AQPs) are naturally occurring water channel proteins. They can facilitate water molecule translocation across cellular membranes with exceptional selectivity and high permeability that are unmatched in synthetic membrane systems. These unique properties of AQPs have led to their use as functional elements in membranes in recent years. However, the intricate nature of AQPs and concerns regarding their stability and processability have encouraged researchers to develop synthetic channels that mimic the structure and properties of AQPs and other biological water-conducting channels. These channels have been termed artificial water channels. This article reviews current progress and provides a historical perspective as well as an outlook toward developing scalable membranes based on artificial water channels.
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- NSF-PAR ID:
- 10355492
- Date Published:
- Journal Name:
- Annual Review of Materials Research
- Volume:
- 48
- Issue:
- 1
- ISSN:
- 1531-7331
- Page Range / eLocation ID:
- 57 to 82
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Water channels are employed by nature to move pure water across cell membranes while selectively rejecting salts. At present, synthetic channels successfully mimic water permeation, yet even the best channels, such as carbon nanotubes (CNTs) and graphene oxide stacks, still fall short of the selectivity target. The present paper analyzes factors that may help to enhance and control salt rejection based on the lessons learned from conventional membranes and CNTs. First, it highlights the importance of raising the ion self-energy (dielectric mechanism), which suggests that having the channels both narrow and surrounded by a low-dielectric environment is key to high selectivity. In contrast, pore charge alone is insufficient, yet it may help to enhance and tune ion rejection, provided that non-mean-field effects enhanced in low-dielectric pores, such as ion association and sorption, especially of H + and OH − ions, are properly understood and addressed in the channel design. Second, the role of concentration polarization (CP) is analyzed, which shows that the CP level is apparently low in isolated channels or microscopically small membranes. However, the geometry of the diffusion field should change and CP should increase drastically in macroscopic membranes incorporating densely spaced channel arrays. If not properly addressed in membrane design, the increased CP level in scaled-up channel-based membranes may significantly compromise the observed selectivity and require that target of selectivity be re-set to an even more challenging value. These points may help guide the future development of high-performance artificial water channels and their scale-up towards utilization in next-generation water purification membranes.more » « less
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Peptide appended pillar[5]arene (PAP) is an artificial water channel resembling biological water channel proteins, which has shown a significant potential for designing bioinspired water purification systems. Given that PAP channels need to be incorporated at a high density in membrane matrices, it is critical to examine the role of channel–channel and channel–membrane interactions in governing the structural and functional characteristics of channels. To resolve the atomic-scale details of these interactions, we have carried out atomistic molecular dynamics (MD) simulations of multiple PAP channels inserted in a lipid or a block-copolymer (BCP) membrane matrix. Classical MD simulations on a sub-microsecond timescale showed clustering of channels only in the lipid membrane, but enhanced sampling MD simulations showed thermodynamically-favorable dimerized states of channels in both lipid and BCP membranes. The dimerized configurations of channels, with an extensive buried surface area, were stabilized via interactions between the aromatic groups in the peptide arms of neighboring channels. The conformational metrics characterizing the orientational and structural changes in channels revealed a higher flexibility in the lipid membrane as opposed to the BCP membrane although hydrogen bonds between the channel and the membrane molecules were not a major contributor to the stability of channels in the BCP membrane. We also found that the channels undergo wetting/dewetting transitions in both lipid and BCP membranes with a marginally higher probability of undergoing a dewetting transition in the BCP membrane. Collectively, these results highlight the role of channel dynamics in governing channel–channel and channel–membrane interfacial interactions, and provide atomic-scale insights needed to design stable and functional biomimetic membranes for efficient separations.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1146/annurev-matsci-070317-124544
