Interfacial self-assembly of cell-like filamentous microcapsules.

We report herein the development of a self-assembly method to rapidly produce cell-like, filamentous microcapsules (MCs) that have high surface area and encapsulate liquids or gels. The fibrous surfaces and shell walls of the MCs can be biologically functionalized using bioactive peptide amphiphiles (PAs), and the cores can harbor biopolymers, proteins, and other macromolecules. This novel method combines the spray-based production of nebulized biopolymer microdroplets with the recently reported ultrafast self-assembly of oppositely charged, high-molecular-weight biopolymers and PAs. There are numerous techniques available for microcapsule formation such as interfacial coacervation or interfacial polycondensation, layer-by-layer (LbL) polyelectrolyte complexation and colloid-templated self-assembly, emulsification with polymer phase separation, spraydrying methods, and microfluidic emulsion droplet formation. The advantage of the method reported herein is the combination of a self-assembly process that leads to structural complexity with the very broad range of bioactivity offered by peptide amphiphiles. The bioactive filament-forming PAs are composed of a hydrophobic alkyl tail, and a b-sheet-forming peptide domain, followed by peptide sequences with charged amino acids or bioactive epitopes that can either bind to receptors or to specific proteins by design (Figure 1A). These molecules assemble into high-aspect-ratio filaments upon electrostatic screening of the charged amino acids and the formation of b sheets. Hydrophobic collapse of these filament-forming molecules under strong screening conditions leads to the display of a high density of biological signals on their surfaces (on the order of 10 signals per cm). In vivo and in vitro studies have shown that certain PA molecules that bear bioactive epitopes promote regeneration of spinal cord axons, angiogenesis, bone regeneration, cartilage repair, proliferation of bone marrow cells, and selective differentiation of neural progenitor cells into neurons. We previously demonstrated that solutions of PAs and oppositely charged biopolymers can self-assemble at the liquid–liquid interface to form hierarchically structured membranes that can be permeable to proteins to produce saclike structures on the macroscale with millisecond speeds (with size scales of millimeters). The shells of these sacs are highly structured and their surfaces are fully covered with nanoscale filaments. We have modified this approach for the production of filamentous MCs less than 100 mm in diameter (Figure 1C). These micrometer-scale objects could be created with highly bioactive properties, high surface area, and dimensions approaching those of cells. The first step in the MC formation requires generation of picoliter droplets of a biopolymer solution. We built a spraybased device that enables production of droplets with diameters (dMC) as small as 5 mm and an average production rate of 1! 10 microcapsules per second. The nebulizing device has three components: a) a pressure microinjector for the delivery of a biopolymer solution, b) a glass capillary (orifice diameter ca. 40 mm), and c) compressed gas (nitrogen or air; see the Supporting Information). We nebulized the stream of 0.25 wt% aqueous alginate (AL) solution using a high-velocity flow of nitrogen. The microdroplets of the biopolymer solution were directly ejected into a 0.1 wt% aqueous solution of C16V3A3K3 PA (Figure 1A) to induce the membrane-forming self-assembly process that occurs on the millisecond timescale. After allowing 15 min for dynamic selfassembly between the PA molecules and the biopolymer, Figure 1. A) Molecular structure of peptide amphiphile C16V3A3K3. B) Alginic acid. C) Schematic illustration of the cross-section of a PAalginate microcapsule.