Sealing Porous Nanovesicles-Solutions Inspired by Primordial Biosystems Andrei P. Sommer* and Attila E. Pavla´ th† Central Institute of Biomedical Engineering, University of Ulm, 89081 Ulm, Germany Received May 19, 2003
Microcapsules designed for slow drug release have preferably some porosity. There are, however, applications in which a hermetical sealing of the microcapsules is desired. Sealing is not a trivial problem and could be necessary to durably encapsulate toxic compounds which cannot be eliminated from the body, or to encapsulate harmful substances stored in the atmosphere. Nature may have one solution: Nanobacteria have developed surprisingly simple mechanisms to access and use primal energies, and to survive arid periods by sealing their surface. Keywords: nanovesicles • self-assembly • microencapsulation • drug delivery systems • biofilm
Self-Assembled Microcapsules Millimeter-size solid objects floating at the plane interface between two virtually immiscible liquids-perfluorodecalin (PFD) and water, self-assembled into two-dimensional symmetrical patterns.1 Considerations based upon minimization of the interfacial free energy indicated that similar self-assembly patterns could be realized via microscale objects. Indeed, selfassembly of microscale objects has been observed in various natural systems and on different scales.2 10 µm polystyrene spheres, e.g., encapsulated microscopic oil drops in an oilwater emulsion.3 A closer inspection of the scanning electron microscopy (SEM) pictures of the self-assembled shells surrounding the oil droplets (Ø ≈ 120 µm) shows the effect of the size-distribution of the material employed for encapsulation: identically sized polystyrene spheres tended to form a regular periodic pattern around the oil droplets.3 Microcapsules engineered via self-assembly of nanospheres could need additional treatment, by which the triangular gaps between the spherical elements of the skeleton could be sealed, partially or completely. Controllable permeability could be relevant for applications in a liquid environment, and could be practically realized by sintering the microcapsules.3 It is apparent that the net porosity of the shell could also depend on the sizedistribution of the beads employed: even a small number of nanospheres with a diameter larger than a certain mean diameter could disturb the spherical shell architecture, and increase the permeability of the system for material flow from both sides. Nature may offer strategies to compensate such irregularities.
Living Nanovesicles Self-assembly in association with controlled and selective permeability seems to be a paradigm of life. It is likely that the * To whom correspondence should be addressed. E-mail: samoan@ gmx.net. † Present address: U.S. Department of Agriculture, Western Regional Research Center, 800 Buchanan, Albany, CA 94710
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first self-assembled biosystems realized by nature were nanobacteria.5 Nanobacteria isolated from blood have a spherical architecture with diameters between 80 and 300 nm and possess an internal cavity that is protected by a nanocrystalline apatite envelope.5 Under environmental stress, nanobacteria produce in culture media a slimy fluidsa process exposing a transient permeability of the mineral shell. Interestingly, nanobacterial colonies can survive in air, a phenomenon suggesting an ability to seal the mineral shell hermetically, thus protecting possible biological contents from desiccation. In particular, nanobacteria isolated from human kidney stones have been revitalized in culture.4 Presumably, sealing was here achieved primarily by a film of slime closing the pores of the shell. This multifunctional film could receive substantial stabilization in a densely packed colony, a milieu providing additional conditions for growth.6 By promoting biofilm formation, the slime also protects solitary nanovesicles indirectly from hostile conditions. Atherosclerotic plaques inducing myocardial infarcts might result partly from such processes triggered by stressed nanobacteria.7 The analysis of the extremely successful survival modalities of primordial biosystems, as potentially represented by present nanobacteria, could expose powerful concepts based on a synergistic interplay of an irreducible set of uniquely simple physical and chemical principles with material properties,5 inspiring the design of novel nanotechnological routes for mimicking metabolic activities of primitive biosystems. According to current views, nanobacteria could have arrived to Earth from space, as star dust or via comets, suggesting that they have possessed some mechanism allowing them to survive the extremely dry and cold interplanetary world, and possibly also extended passages across the atmosphere with its characteristic vertical temperature profile with values approaching -60 °C at 15 km, rising to about 0 °C at 50 km, and decreasing to nearly -90 °C at an altitude of 80 km above sea level. 10.1021/pr034040o CCC: $25.00
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Sealing Porous Nanovesicles
Sealing by Water It is interesting to consider the possible survival capacity of nanobacteria in the atmosphere of the Earth. Here, nanobacteria could exploit the interplay of humidity and low temperatures for additional sealing. The presence of nanoscopic water layers on hydrophilic surfaces at room temperature, deposited by exposure to normal humidity, and on the surface of ice particles in clouds, has been demonstrated in numerous experiments.8 Surfaces consisting of apatitesa hydrophilic materialsare expected to be coated with similar water layers. By ascending to atmospheric altitudes with decreasing temperature and increasing relative humidity, nanoscopic water layers deposited on apatite particles would first continuously grow and gradually freeze, starting at the solid-liquid interface. In the case of the nanobacteria, this could result in additional sealing, thus securing their survival capability in a prebiotic planetary world. Nanobacteria potentially utilize both sealing modes (slime and ice) to protect their biological inner load from possible desiccation. This dual strategy may inspire the development of practicable techniques for a hermetical encapsulation of harmful aerosols in the atmosphere. The process could be vital for permanently sealing hazardous chemical and biological agents that could be localized as a condensed cloud of aerosols in the atmosphere, and could be subsequently captured via seeding of adequate nanospheres, allowed to selfassemble around the target nuclei.9 Practically equiradial nanospheres can be made from different materials. In combination with suitable liquids, selfassembled nanospheres may serve for both controlled release and durable storage of encapsulated compounds. In a recent experimental paper, we theorized how durable storage could work.9 It was shown that in the presence of 60-nm polystyrene nanospheres, a short interfacial contact between water and PFD was sufficient to create stable nanoemulsions in both phases: microscopic PFD droplets on the aqueous side, and microscopic water droplets on the PFD side. In both cases, the minority phase of the emulsion was stabilized by self-assembly of the nanospheres at the liquid interface. The result indicated that suitable nanospheres could be used to collect chlorofluorocarbons (CFC) from the atmosphere.9 CFCs are physically and chemically very similar to PFD. Prerequisite for continuous encapsulation of volatile liquids condensed in the atmosphere (or other environment) is the appropriate functional performance of the sealing system. Notably, microcapsules, self-assembled from 60 nm nanospheres surrounding the volatile PFD droplets, were stable in liquid over several hours.9 10 µL drops of a PFD based emulsion, containing microscopic water droplets encapsulated by the nanospheres, were allowed to evaporate on a mirror-polished titanium disk. After complete evaporation, a number of nanosphere-encapsulated water droplets were visible on the disk. Similar stable changes have been observed in self-organized patterning produced on titanium surfaces after evaporation of an aqueous emulsion containing microscopic PFD droplets encapsulated by nanospheres.9 Self-assembly of nanoparticles at curved liquid interfaces seems to be a rapid energy minimizing process, strongly depending on the interaction between capillary waves, the nature and the size of the nanoparticles. Failure to localize some 60-nm polystyrene nanospheres at a plane water-PFD interface by the sensor of an atomic force microscope (AFM), operated in the shear-force mode, a system allowing to image nanoscale objects in liquid,10 was probably due to fluctuations driving the nanospheres out of the interfacial zone. Clearly,
large-scale application of nanospheres, e.g., to surround and neutralize the ozone killing CFCs stored in the atmosphere,11 would probably require the use of nanospheres of a suitable polarity and nearly equiradial distribution. It is possible that in the atmosphere, after encapsulation of organic compounds by self-assembly of such nanospheres, the resulting cages would be subsequently sealed by deposition supercooled water vapor. Water deposition on analogous agglomerates has been described for combustion aerosols at room temperature. Combustion aerosols are of the same size as the nanospheres employed to encapsulate the PFD droplets. Remarkably, the size of the combustion aerosols was reported to be significantly bigger when determined by AFM, compared to values obtained in vacuum by SEM.12
Sealing by Chemical Deposition In addition, when passing the atmosphere, microcapsules could interact with clouds. By acting as cloud condensation nuclei (CCN) they may become eventually coated with soluble substances, present in the cloud. Measurements of aerosol composition in the Mediterranean revealed that most aerosol particles were coated with a sulfate.13 The precise processes responsible for the coating of the solid aerosols with soluble components are unknown. One probable coating mechanism discussed in the literature is the cloud processing of dust particles. In it, mineral dust particles are collected by cloud drops that have been originally nucleated by sulfate or other soluble aerosol particles. After cloud evaporation, mineral dust particles coated with soluble material are released.13 Microcapsules passing clouds may collect additional sealing layers by a mechanism similar to the processing of dust particles. The mechanism might apply to the development of novel selfassembled and self-sealing microcapsules for biomedical applications. In body fluids, sealing could be realized by proteins chemically attached to the surface of locally self-assembled capsules.
Conclusions As we recently reported,9 nanospheres might be instrumental in encapsulating microscopic liquid droplets via self-assembly. Depending on the practical application, self-assembled systems could be partly porous, or virtually totally sealed. Porous microcapsules are, in principle, useful as drug delivery systems. Sealed microcapsules have several practical applications reaching from durable encapsulation of toxic substances in body fluids to collection of atmospheric CFCs, and require suitable sealing mechanisms. Powerful sealing methods are inspired by nature, e.g., by the survival modalities of living nanovesicles.5 Analyzing the principles of these methods can have beneficial impacts in life-sciences, in particular in the development of nanotechnology-assisted processes.
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letters (6) Sommer, A. P.; Kajander, E. O. Cryst. Growth Des. 2002, 2, 563. (7) Rasmussen, T. E.; Kirkland, B. L.; Charlesworth, J.; Rodgers, G.; Severson, S. R.; Rodgers, J.; Folk, R. L.; Miller, V. M. J. Am. Coll. Cardiol. 2002, 39 Suppl. 1, 206. (8) Sommer, A. P.; Levin, Z. Atmos. Res. 2001, 58, 129. (9) Sommer, A. P., Franke, R. P. NanoLett. 2003, 3, 321.
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Sommer, A. P., Franke, R. P. Micron 2002, 33, 227. New Scientist 2003, 177 (2381), 24. Lafon, V.; Paris, E.; Henriet, A. Veeco Eur. Newslett. 2001, 4, 2. Wurzler, S.; Reisin, T. G.; Levin, Z. J. Geophys. Res. 2000, 105, 4501.
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