Chapter 3 Bead Integration Technology and an LC-ESI/MS Chip
3.2 Working with Beads
In the literature, the terms “particles,” “beads,” and “microspheres” are often used.
Generally speaking, “particles” are usually irregular in shape and non-uniform in size.
“Beads” are more spherical and more uniform. “Microspheres” are the most spherical and
uniform of the three. However, the distinction is not so clear cut. Therefore, the three terms are often used interchangeably. In the field of liquid chromatography, the term
“bead” is mostly used to describe the packing material in the separation column.
There are a wide variety of beads commercially available. Their sizes can range from several nanometers to millimeters. The base material can be silica, alumina, polystyrene (PS), and many others. There are a number of choices for surface chemistries and bulk properties as well, such as hydrophobicity, fluorescence, and magnetism. The beads can also come in a rainbow of colors. Some major vendors for all general and special beads are Bangs Laboratories (Fishers, IN), Duke Scientific (Palo Alto, CA), and Polysciences (Warrington, PA). Liquid chromatographic beads are available from vendors such as Alltech Associates (Deerfield, IL), Hamilton (Reno, NV), and Grace Vydac (Hesperia, CA).
3.2.2 Aggregation
The interactions between particles, especially surface forces, are proportional to the first power of particle size. As a comparison, gravitational force is proportional to the mass, thus to the cube of particle size. Hydrodynamic forces caused by flow depend roughly on the square of particle size. Therefore, when the particle dimension reaches the micron level, surface forces become dominant. Aggregation is often seen for suspended micro-particles, which aggregate by collision and are held together by surface forces [12].
The leading cause for aggregation of polymeric particles is hydrophobic interaction. The particles are likely to form aggregates in aqueous solution to reduce their total surface area, and thus lower their surface energy. Silica beads are more hydrophilic
because of their surface hydroxyl groups. For them, the main cause of aggregation is charge interactions. Furthermore, higher temperature or particle concentration increases the likelihood for the particles to contact, thus creates more opportunities for aggregation.
To break aggregates and homogenize the suspended particles, two methods are commonly used, sonication and vortexing. In addition to these two physical methods, the addition of suitable molecules can often effectively reduce the hydrophobicity or surface charges of the particles, thus reversing the aggregation. The choice of the right solvent is often the best way to avoid particle aggregation.
3.2.3 Packing and Porosity
The packing quality and porosity of chromatographic columns are two of the most important factors for column performance. Therefore, it is worthwhile to discuss bead packing and porosity. Let us start with a simple case, packing of uniformly-sized solid spherical beads. Two types of packing are the most common, BCC (Body-Centered Cubic) and FCC (Face-Centered Cubic). For BCC packing (Figure 3-1 (a)), there are two beads per unit cell. The coordination number is eight, meaning each bead has eight closest neighboring beads. The porosity for BCC is 0.32. Examples of BCC materials are chromium (Cr) and tungsten (W). For FCC packing (Figure 3-1 (b)), there are four beads per unit cube. The coordination number is twelve. The porosity is 0.26. Examples of FCC materials are aluminum (Al), copper (Cu), and platinum (Pt). For all types of packing of solid spherical beads, only inter-particle porosity exists, and the porosity of 0.26 obtained for FCC is actually the smallest possible. Although dense packing is desired for chromatographic columns, the commercial non-porous-beads can only obtain column
porosities of about 0.4. One reason is that chromatographic beads have considerable variations in size and shape. The other reason is that the packing process is not able to replicate the uniform crystal structures.
Figure 3-1 Two common particle packing structures (a) BCC; (b) FCC.
In addition to dense packing, high porosity is also desired for HPLC columns.
Higher porosity means more surface area, more efficient separation, and less flow resistance. However, high porosity resulting from loose packing is undesirable since it creates dead volumes that cause excessive band broadening. Closely-packed porous-bead columns can solve this dilemma since inner-particle porosity is added to the inter-particle porosity. Commercial porous-bead columns have total porosities around 0.7 to 0.8, which means as little as only 20% of the volume inside the column is occupied by solid material.
3.2.4 Applications in MEMS
Figure 3-2 One-shot pump and valve using expandable microspheres.
A wide variety of interesting applications of micro and nano particles in MEMS are already available. For example, fluorescent and dyed particles are frequently used in microfluidic devices for flow visualization [13]. Magnetic micro-beads can be manipulated in micro devices for transportation of chemical reagents and cells [14].
Using thermally expandable microspheres, one-shot micro-valve and micro-pump (Figure 3-2) have been demonstrated [15]. The valve can be used for normally-open (NO) applications. The pump can be used for chemical release. They are one-shot only since the expansion is irreversible. Using silica bead-packed capillaries with electrokinetic pumping, pressures over 8000 psi have been achieved [16]. The pressure is proportional to the voltage applied, and inversely proportional to square of the bead diameter.
Moreover, microspheres with special coatings have been demonstrated for multi-analyte sensor arrays for the analysis of complex fluids (Figure 3-3) [17]. Each bead in the array acts like a taste bud. When combined, these beads form an array for detection of a variety of important classes of analytes, including acids, bases, metal cations, metabolic cofactors, and antibody reagents.
Figure 3-3 Multi-analyte sensor using specially-coated beads.
However, none of these applications directly integrates particles into the device.
Instead, the particles are usually injected into the device through access holes or placed by micromanipulators after device fabrication.
3.3 Integrating Beads into Micromachined Device