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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The data that support the findings of this study are available from the corresponding authors on request.
Skou, J. C. & Esmann, M. The Na,K-ATPase. J. Bioenerg. Biomembr. 24, 249–261 (1992).
Shang, L. & Zhao, Y. Droplet-templated synthetic cells. Matter 4, 95–115 (2021).
Rodríguez-Arco, L., Li, M. & Mann, S. Phagocytosis-inspired behaviour in synthetic protocell communities of compartmentalized colloidal objects. Nat. Mater. 16, 857–863 (2017).
Rideau, E., Dimova, R., Schwille, P., Wurm, F. R. & Landfester, K. Liposomes and polymersomes: a comparative review towards cell mimicking. Chem. Soc. Rev. 47, 8572–8610 (2018).
Dinsmore, A. D. et al. Colloidosomes: selectively permeable capsules composed of colloidal particles. Science 298, 1006–1009 (2002).
Sugimoto, T., Khan, M. M. & Muramatsu, A. Preparation of monodisperse peanut-type α-Fe2O3 particles from condensed ferric hydroxide gel. Colloids Surf. A 70, 167–169 (1993).
Palacci, J. et al. Light-activated self-propelled colloids. Philos. Trans. R. Soc. A 372, 20130372 (2014).
Cheng, K. L., Sheng, Y. J. & Tsao, H. K. Brownian escape and force-driven transport through entropic barriers: particle size effect. J. Chem. Phys. 129, 184901 (2008).
Grigoriev, I. V., Makhnovskii, Y. A., Berezhkovskii, A. M. & Zitserman, V. Y. Kinetics of escape through a small hole. J. Chem. Phys. 116, 9574–9577 (2002).
Wadhams, G. H. & Armitage, J. P. Making sense of it all: bacterial chemotaxis. Nat. Rev. Mol. Cell Biol. 5, 1024–1037 (2004).
Tweedy, L. et al. Seeing around corners: cells solve mazes and respond at a distance using attractant breakdown. Science 369, eaay9792 (2020).
Taylor, J. W., Eghtesadi, S. A., Points, L. J., Liu, T. & Cronin, L. Autonomous model protocell division driven by molecular replication. Nat. Commun. 8, 237 (2017).
Zhu, T. F., Adamala, K., Zhang, N. & Szostak, J. W. Photochemically driven redox chemistry induces protocell membrane pearling and division. Proc. Natl Acad. Sci. USA 109, 9828–9832 (2012).
Zhu, T. F. & Szostak, J. W. Coupled growth and division of model protocell membranes. J. Am. Chem. Soc. 131, 5705–5713 (2009).
Yewdall, N. A., Mason, A. F. & van Hest, J. C. M. The hallmarks of living systems: towards creating artificial cells. Interface Focus 8, 20180023 (2018).
Yang, Z., Wei, J., Sobolev, Y. I. & Grzybowski, B. A. Systems of mechanized and reactive droplets powered by multi-responsive surfactants. Nature 553, 313–318 (2018).
Qiao, Y., Li, M., Booth, R. & Mann, S. Predatory behaviour in synthetic protocell communities. Nat. Chem. 9, 110–119 (2017).
Yin, Y. et al. Non-equilibrium behaviour in coacervate-based protocells under electric-field-induced excitation. Nat. Commun. 7, 10658 (2016).
Fujii, S., Matsuura, T., Sunami, T., Kazuta, Y. & Yomo, T. In vitro evolution of α-hemolysin using a liposome display. Proc. Natl Acad. Sci. USA 110, 16796–16801 (2013).
Li, G., Wang, L., Ni, H. & Pittman, C. U. Polyhedral oligomeric silsesquioxane (POSS) polymers and copolymers: a review. J. Inorg. Organomet. Polym. Mater. 11, 123–154 (2001).
van der Wel, C. et al. Preparation of colloidal organosilica spheres through spontaneous emulsification. Langmuir 33, 8174–8180 (2017).
Okubo, M., Kobayashi, H., Huang, C., Miyanaga, E. & Suzuki, T. Water absorption behavior of polystyrene particles prepared by emulsion polymerization with nonionic emulsifiers and innovative easy synthesis of hollow particles. Langmuir 33, 3468–3475 (2017).
Shi, H., Huang, C., Liu, X. & Okubo, M. Role of osmotic pressure for the formation of sub-micrometer-sized, hollow polystyrene particles by heat treatment in aqueous dispersed systems. Langmuir 35, 12150–12157 (2019).
Kim, S. H., Park, J. G., Choi, T. M., Manoharan, V. N. & Weitz, D. A. Osmotic-pressure-controlled concentration of colloidal particles in thin-shelled capsules. Nat. Commun. 5, 3068 (2014).
Silletta, E. V., Xu, Z., Youssef, M., Sacanna, S. & Jerschow, A. Monitoring molecular transport across colloidal membranes. J. Phys. Chem. B 122, 4931–4936 (2018).
Datta, S. S. et al. Delayed buckling and guided folding of inhomogeneous capsules. Phys. Rev. Lett. 109, 134302 (2012).
Opdam, J., Tuinier, R., Hueckel, T., Snoeren, T. J. & Sacanna, S. Selective colloidal bonds via polymer-mediated interactions. Soft Matter 16, 7438–7446 (2020).
Kim, S. H., Shim, J. W., Lim, J. M., Lee, S. Y. & Yang, S. M. Microfluidic fabrication of microparticles with structural complexity using photocurable emulsion droplets. New J. Phys. 11, 075014 (2009).
Hyuk Im, S., Jeong, U. & Xia, Y. Polymer hollow particles with controllable holes in their surfaces. Nat. Mater. 4, 671–675 (2005).
Qiu, J. et al. Encapsulation of a phase-change material in nanocapsules with a well-defined hole in the wall for the controlled release of drugs. Angew. Chem. Int. Edn Engl. 58, 10606–10611 (2019).
Sacanna, S. et al. Shaping colloids for self-assembly. Nat. Commun. 4, 1688 (2013).
Kar, A., Chiang, T. Y., Ortiz Rivera, I., Sen, A. & Velegol, D. Enhanced transport into and out of dead-end pores. ACS Nano 9, 746–753 (2015).
Kuijk, A., van Blaaderen, A. & Imhof, A. Synthesis of monodisperse, rodlike silica colloids with tunable aspect ratio. J. Am. Chem. Soc. 133, 2346–2349 (2011).
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.
The authors declare no competing interests.
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
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.
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.
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.
a–c, 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.
SEM images of cell-mimics displaying micropores in a wide range of sizes. Scale bar, 2 μm.
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.
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.
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.
SI Video 1: . Video microscopy illustrating the self-inflation process. TPM emulsion droplets transform into vesicles upon exposure to NaOH
SI Video 2: . Video microscopy showing cell mimics ingesting, holding and expelling PS tracers. (The video has been sped up by ~2 times)
SI Video 3: . Video microscopy showing cell mimics ingesting E.coli. (The video has been sped up by ~2 times)
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