Seeding

Abstract: The size specifications for suitable tracer particles for particle image velocimetry (PIV), particularly with respect to their flow tracking capability, are discussed and quantified for several examples. A review of a wide variety of tracer materials used in recent PIV experiments in liquids and gases indicates that appropriately sized particles have normally been used. With emphasis on gas flows, methods of generating seeding particles and for introducing the particles into the flow are described and their advantages are discussed.

Abstract: Saltation is important in the transport of sand-sized granular material by wind and in the ejection of dust from the bed both on Earth and on Mars. The evolution of the saltating population and all its characteristic profiles is calculated from inception by pure aerodynamic entrainment through to steady state. Results of numerical simulations of single-grain impacts into granular beds are condensed into analytic expressions for the number and speeds of grains rebounding or rejected (splashed) from the bed. A model is combined with (i) this numerical representation, (ii) an expression for the aerodynamic entrainment rate, and (iii) the modification of the wind velocity profile by saltating grains. Calculated steady state mass fluxes are within the range of mass fluxes measured in wind tunnel experiments; mass flux is nonlinearly dependent on the shear velocity. Aerodynamically entrained grains in the system are primarily seeding agents; at steady state, aerodynamic entrainment is rare. The time for the entire system to reach steady state is roughly 1 second, or several long-trajectory hop times.

Abstract: Tissue engineering of 1- to 5-mm-thick, func- tional constructs based on cells that cannot tolerate hyp- oxia for prolonged time periods (e.g., cardiac myocytes) critically depends on our ability to seed the cells at a high and spatially uniform initial density and to maintain their viability and function. We hypothesized that rapid gel- cell inoculation in conjunction with direct medium per- fusion through the seeded scaffold would increase the rate, yield, viability, and uniformity of cell seeding. Two cell types were studied: neonatal rat cardiomyocytes for feasibility studies of seeding and cultivation with direct medium perfusion, and C2C12 cells (a murine myoblast cell line) for detailed seeding studies. Cells were seeded at densities corresponding to those normally present in the adult rat heart ((0.5-1) × 10 8 cells/cm 3 ), into collagen sponges (13 m m×3m mdiscs), using Matrigel® as a vehicle for rapid cell delivery. Scaffolds inoculated with cell-gel suspension were seeded either in perfused car- tridges with alternating medium flow or in orbitally mixed Petri dishes. The effects of seeding time (1.5 or 4.5 h), initial cell number (6 or 12 million cells per scaffold), and seeding set-up (medium perfusion at 0.5 and 1.5 mL/ min; orbitally mixed dishes) were investigated using a randomized three-factor factorial experimental design with two or three levels and three replicates. The seeding cell yield was consistently high (over 80%), and it ap- peared to be determined by the rapid gel inoculation. The decrease in cell viability was markedly lower for per- fused cartridges than for orbitally mixed dishes (e.g., 8.8 ± 0.8% and 56.3 ± 4%, respectively, for 12 million cells at 4.5 h post-seeding). Spatially uniform cell distributions were observed in perfused constructs, whereas cells were mainly located within a thin (100-200 µm) surface layer in dish seeded constructs. Over 7 days of cultiva- tion, medium perfusion maintained the viability and dif- ferentiated function of cardiac myocytes, and the con- structs contracted synchronously in response to electri- cal stimulation. Direct perfusion can thus enable seeding of hypoxia-sensitive cells at physiologically high and spatially uniform initial densities and maintain cell viabil-