Physical properties of beads have to fulfil several criteria as otherwise their microfluidic manipulation will fail. It is the size, shape and material of which beads are made that are critical to smooth moving, metering, packing and removal of beads. The particles must be spherical, their size in the range 50 to 200µm. Smaller beads will not be retained in column or in a flowcell, while larger beads may become trapped in flow channels. The traditional silica based support (A) or any type of similar chunky material should be avoided, because such particles will become interlocked and thus difficult to remove. Hard polymer made beads (OASIS HBL, Phenomerex XL) are ideal (B), while soft and transparent beads (Sephadex, Sepharose) are also suitable (C). Sepharose 4B beads are more elastic than Sepharose 6B, which are more crosslinked Cytodex beads (D), used as carriers for cell culture, can be used, but because they are fragile they need to be handled with care.
Functional groups on Sepharose and Sephadex beads offers almost limitless opportunities for development of novel assays for organic compounds, enzymes, antigens, antibodies, DNA, RNA and other biomolecules, as well as pathogens and bacteria. Sepharose is made of cross-linked, purified polysaccharide (galactose) derived from seaweed, while Sephadex is made of cross-linked microbial polysaccharide (glucose). Both beaded supports are widely used for affinity chromatography. These microspheres (size 20 to 180 µm) are available with the following immobilized functional groups:
Protein A –from S. aureus, binds mammalian immunoglobulins (IgG) by their Fc region. Protein G –from S. G148, binds a range of immunoglobulins, including rat and goat IgG. Avidin – from egg white, binds D-biotin, useful for attaching biotinylated ligands. Streptavidin – from S. avidinii, for strong attachment of biotinylated molecules. Cytodex (Hodder 1999) and Cytopore (Schultz 2002) beads were used as carriers for BI-based cellular assays
Polymer made beads have besides outstanding physical properties robust long lasting functional groups. Oasis sorbents are available in seven different solid-phase extraction (SPE) chemistries, Oasis HLB is an all purpose, strongly hydrophilic, reversed-phase, water-wetttable polymer with a unique Hydrophilic-Lipophilic Balance. Oasis HLB maintains high retention and capacity even if it runs dry after conditioning. This sorbent is ideal for acidic, basic and neutral analytes as it's stable from pH 0-14. (Waters Inc.) It also works as the traditional silica based C-18 thus making it well suited for sorbent extraction of metal chelates. Is particle size is 40micrometers. Strata XL is 100micron size , reversed phase functionalized polymeric sorbent that gives strong retention of neutral, acidic, or basic compounds under aggressive, high organic wash conditions. This sorbent relies on 3 mechanisms of retention: pi-pi bonding, hydrogen bonding (dipole-dipole interactions), and hydrophobic interaction. (Phenomerex)
Beads
3.1.4.
