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Transmembrane transport in inorganic colloidal cell-mimics

Abstract

A key aspect of living cells is their ability to harvest energy from the environment and use it to pump specific atomic and molecular species in and out of their system—typically against an unfavourable concentration gradient1. Active transport allows cells to store metabolic energy, extract waste and supply organelles with basic building blocks at the submicrometre scale. Unlike living cells, abiotic systems do not have the delicate biochemical machinery that can be specifically activated to precisely control biological matter2,3,4,5. Here we report the creation of microcapsules that can be brought out of equilibrium by simple global variables (illumination and pH), to capture, concentrate, store and deliver generic microscopic payloads. Borrowing no materials from biology, our design uses hollow colloids serving as spherical cell-membrane mimics, with a well-defined single micropore. Precisely tunable monodisperse capsules are the result of a synthetic self-inflation mechanism and can be produced in bulk quantities. Inside the hollow unit, a photoswitchable catalyst6 produces a chemical gradient that propagates to the exterior through the membrane’s micropore and pumps target objects into the cell, acting as a phoretic tractor beam7. An entropic energy barrier8,9 brought about by the micropore’s geometry retains the cargo even when the catalyst is switched off. Delivery is accomplished on demand by reversing the sign of the phoretic interaction. Our findings provide a blueprint for developing the next generation of smart materials, autonomous micromachinery and artificial cell-mimics.

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Fig. 1: Self-inflating droplets.
Fig. 2: Tunable mechanical properties.
Fig. 3: Micropore fabrication.
Fig. 4: Active transport.

Data availability

The data that support the findings of this study are available from the corresponding authors on request.

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Acknowledgements

This research was primarily supported by the US Army Research Office under award number W911NF2010117. S.S. acknowledges additional support from Kairos Ventures. W.T.M.I. acknowledges additional support from the Packard Foundation and the Brown Science Foundation.We thank T. Islam, M. Youssef, R. Bahn and M. Weigl for exploratory synthetic work, and L. Mahal for providing the E. coli sample.

Author information

Affiliations

Authors

Contributions

Z.X. designed the cell-mimic synthetic protocol, synthesized all the colloidal systems and performed the active transport experiments. T.H. discovered the self-inflation mechanism and performed the preliminary synthetic work. S.S. conceived the study. S.S. and W.T.M.I. supervised and directed research. All authors analysed data, discussed the results and commented on the manuscript.

Corresponding authors

Correspondence to William T. M. Irvine or Stefano Sacanna.

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Competing interests

The authors declare no competing interests.

Additional information

Peer review information Nature thanks Yuanjin Zhao and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data figures and tables

Extended Data Fig. 1 Functional emulsions.

a, Silsesquioxanes-based emulsions form via hydrolytic condensation of functionalized trialkoxysilane molecules (oil precursor). b, Monodispersed droplets nucleate upon the addition of ammonia and grow over time until the precursor is fully consumed. The cross-linking density of the newly formed oil-phase increases with the emulsion age τ, which is defined as the time passed from the addition of ammonia to the sample (nucleation). Ageing manifests with an increase of the oil-air contact angle formed by oil droplets resting on a clean silicon substrate (SEM images, right). c, The droplets can be fully cured by UV polymerization, resulting in a suspension of solid microspheres. Scale bars, 1 μm.

Extended Data Fig. 2 Self-inflation process.

a, b, Chemically induced osmotic pressure Πi transforms primed emulsion droplets (a) into expanding vesicles (b). The osmotic pressure is generated by charged monomers produced by the reaction between NaOH and the TPM oil phase. c, Expanding vesicles can be fixed via UV polymerization, resulting in solid capsules. Scale bars, 2 μm.

Extended Data Fig. 3 Full synthetic roadmap to self-inflating microcapsules.

a, b, The schematic shows an overview of the microcapsule fabrication process and highlights key steps. NaOH can be added to the emulsion droplets at any time τ during their ageing process (b). The droplets’ response, however, depends on both τ and the NaOH concentration, as shown in Fig. 1c. Panels a and b are described in detail in Extended Data Figs. 1 and 2, respectively.

Extended Data Fig. 4 Buckling behaviour.

ac, SEM images showing the three characteristic responses that we observed during the osmotic stress experiments in Fig. 2e. In Fig. 2, we refer to these responses as (a) intact, (b) mode 2 and (c) mode 1. We studied 11 different capsule geometries each tested against 9 different osmotic pressures. Within each sample, >90% of the capsules responded in the same manner. Scale bars, 2 μm.

Extended Data Fig. 5 Tunable micropores.

SEM images of cell-mimics displaying micropores in a wide range of sizes. Scale bar, 2 μm.

Extended Data Fig. 6 Growth and ageing of TPM droplets.

We characterize the emulsions by measuring the diameter of the droplets and the oil–air contact angle θ of the droplets on a silicon substrate. The graph shows a typical emulsion behaviour where θ increases monotonically with the droplet age τ, while their diameter reaches a maximum within 1 h from nucleation. Error bars, ±1 s.d.

Extended Data Fig. 7 Quantitative model of self-inflation.

The extent of the inflation for a forming vesicle can be predicted by balancing osmotic pressure and surface tension (solid line). The model is built assuming that the vesicle have a constant volume of oil and a fixed number of molecular species inside contributing to Πi. The experimental points are measurements of the vesicles size at different external osmotic pressures Πe. Radii are measured by SEM after polymerization and corrected for the relative density change (7% increase after polymerization21). Πe is adjusted using NaCl. R0, radius of the oil droplets measured before inflation. Error bars, ±1 s.d. Scale bars, 3 μm.

Extended Data Fig. 8 Ingestion of nanoparticles.

Optical microscopy time-lapse showing cell-mimics ingesting 450 nm (a) and 300 nm (b) PS tracers. In both experiments, the cell-mimic has a micropore of 1.1 μm. Scale bars are 1 μm.

Supplementary information

Self-inflation of emulsion droplets

SI Video 1: . Video microscopy illustrating the self-inflation process. TPM emulsion droplets transform into vesicles upon exposure to NaOH

Ingestion of PS spheres

SI Video 2: . Video microscopy showing cell mimics ingesting, holding and expelling PS tracers. (The video has been sped up by ~2 times)

Ingestion of E.coli

SI Video 3: . Video microscopy showing cell mimics ingesting E.coli. (The video has been sped up by ~2 times)

Ingestion of silica rods

SI Video 4: . Video microscopy showing cell mimics ingesting silica rods. (The video has been sped up by ~2 times)

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Xu, Z., Hueckel, T., Irvine, W.T.M. et al. Transmembrane transport in inorganic colloidal cell-mimics. Nature 597, 220–224 (2021). https://doi.org/10.1038/s41586-021-03774-y

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