Clustering of Cyclodextrin-Functionalized Microbeads by an Amphiphilic Biopolymer: Real-Time Observation of Structures Resembling Blood Clots.

Colloidal particles can be induced to cluster by adding polymers in a process called bridging flocculation. For bridging to occur, the polymer must bind strongly to the surfaces of adjacent particles, such as via electrostatic interactions. Here, we introduce a new system where bridging occurs due to specific interactions between the side chains of an amphiphilic polymer and supramolecules on the particle surface. The polymer is a hydrophobically modified chitosan (hmC) while the particles are uniform polymeric microbeads (∼160 µm in diameter) made by a microfluidic technique and functionalized on their surface by α-cyclodextrins (CDs). The CDs have hydrophobic binding pockets that can capture the n-alkyl hydrophobes present along the hmC chains. Clustering of CD-coated microbeads in water by hmC is visualized in real time using optical microscopy. Interestingly, the clustering follows two distinct stages: first, the microbeads are bridged into clusters by hmC chains, which occurs by the interaction of individual chains with the CDs on adjacent particles. Thereafter, additional hmC from the solution adsorbs onto the surfaces of the microbeads and an hmC "mesh" grows around the clusters. This growing nanostructured mesh can trap surrounding microsized objects and sequester them within the overall cluster. Such clustering is reminiscent of blood clotting where blood platelets initially cluster at a wound site, whereupon they induce growth of a protein (fibrin) mesh around the clusters, which entraps other passive cells. Clustering does not occur with the native chitosan (lacking hydrophobes) or with the bare particles (lacking CDs); these results confirm that the clustering is indeed due to hydrophobic interactions between the hmC and the CDs. Microbead clustering via amphiphilic biopolymers could be applicable in embolization, which is a surgical technique used to block blood flow to a particular area of the body, or in agglutination assays.

"Killer" Microcapsules That Can Selectively Destroy Target Microparticles in Their Vicinity.

We have developed microscale polymer capsules that are able to chemically degrade a certain type of polymeric microbead in their immediate vicinity. The inspiration here is from the body's immune system, where killer T cells selectively destroy cancerous cells or cells infected by pathogens while leaving healthy cells alone. The "killer" capsules are made from the cationic biopolymer chitosan by a combination of ionic cross-linking (using multivalent tripolyposphate anions) and subsequent covalent cross-linking (using glutaraldehyde). During capsule formation, the enzyme glucose oxidase (GOx) is encapsulated in these capsules. The target beads are made by ionic cross-linking of the biopolymer alginate using copper (Cu2+) cations. The killer capsules harvest glucose from their surroundings, which is then enzymatically converted by GOx into gluconate ions. These ions are known for their ability to chelate Cu2+ cations. Thus, when a killer capsule is next to a target alginate bead, the gluconate ions diffuse into the bead and extract the Cu2+ cross-links, causing the disintegration of the target bead. Such destruction is visualized in real-time using optical microscopy. The destruction is specific, i.e., other microparticles that do not contain Cu2+ are left undisturbed. Moreover, the destruction is localized, i.e., the targets destroyed in the short term are the ones right next to the killer beads. The time scale for destruction depends on the concentration of encapsulated enzyme in the capsules.

Smart Hydrogel-Based Valves Inspired by the Stomata in Plants.

We report the design of hydrogels that can act as "smart" valves or membranes. Each hydrogel is engineered with a pore (about 1 cm long and <1 mm thick) that remains closed under ambient conditions but opens under specific conditions. Our design is inspired by the stomatal valves in plant leaves, which regulate the movement of water and gases in and out of the leaves. The design features two different gels, active and passive, which are attached concentrically to form a disc-shaped hybrid film. The pore is created in the central active gel, and the conditions for opening the pore can be tuned based on the chemistry of this gel. For example, if the active gel is made from N-isopropylacrylamide (NIPA), the actuation of the pore depends on the temperature of water relative to 32 °C, which is the lower-critical solution temperature (LCST) of NIPA. The concentric design of our hybrid provides directionality to the volumetric transition of the active gel, i.e., it ensures that the pore opens as the active gel shrinks. In turn, contact with hot water (T > 32 °C) opens the pore and allows the water to pass through the gel. Conversely, the pore remains closed when the water is cold (T < 32 °C). The gel thereby acts as a "smart" valve that is able to regulate the flow of solvent depending on its properties. We have extended the concept to other stimuli that can cause gel-swelling transitions including solvent composition, pH, and light. Additionally, when two different gel-based valves are arranged in series, the assembly acts as a logical "AND" gate, i.e., water flows through the valve-combination only if it simultaneously satisfies two distinct conditions (such as its pH being below a critical value and its temperature being above a critical value).

Capturing rare cells from blood using a packed bed of custom-synthesized chitosan microparticles.

We describe batch generation of uniform multifunctional chitosan microparticles for isolation of rare cells, such as circulating tumor cells (CTCs), from a sample of whole blood. The chitosan microparticles were produced in large numbers using a simple and inexpensive microtubing arrangement. The particles were functionalized through encapsulation of carbon black, to control autofluorescence, and surface attachment of streptavidin, to enable interactions with biotinylated antibodies. These large custom modified microparticles (≈164 µm diameter) were then packed into a microfluidic channel to demonstrate their utility in rare cell capture. Blood spiked with breast cancer (MCF-7) cells was first treated with a biotinylated antibody (anti-EpCAM, which is selective for cancer cells like MCF-7) and then pumped through the device. In the process, the cancer cells were selectively bound to the microparticles through non-covalent streptavidin-biotin interactions. The number density of captured cells was determined by fluorescence microscopy at physiologically relevant levels. Selective capture of the MCF-7 cells was characterized, and compared favorably with previous approaches. The overall approach using custom synthesized microparticles is versatile, and can allow researchers more flexibility for rare cell capture through simpler and cheaper methods than are currently employed.

A simple packed bed device for antibody labelled rare cell capture from whole blood.

We have developed a system to isolate rare cells from whole blood using commercially available components and simple microfluidics. We characterized the capture of MCF-7 cells spiked into whole human blood using this system to demonstrate that enrichment and enumeration studies give results similar to in situ surface-modified devices while reducing fabrication and operation complexity.