velocity, the more likely the particle will penetrate the cell; this is particularly important when using small particles. The chamber vacuum affects the velocity of the microparticles by reducing the drag on the particles as they travel toward the target tissue: the higher the chamber vacuum, the less the particles will be decelerated. The distance between the stopping screen and the target tissue has several effects on the transformation efficiency. The longer the travel distance, the greater the spread of the particles over the target and the less pronounced is the helium shock wave striking the cells. On the other hand, a longer travel distance results in reduced particle speed and a decreased likelihood that a particle will penetrate the target.

Because the likelihood of a subcellular-sized particle penetrating a cell is proportional to the kinetic energy of the particle, materials of high density have generally been preferred for use in biolistic technology. These compounds include tungsten, platinum, and gold. Of these, gold is favored because it is biologically inert and because spherical gold particles can be produced in a narrow size range. Physical parameters of gold particles are presented in Table 1. Note the nearly five-fold increase in the number of particles per gram for the 0.6 gold relative to the 1.0 gold. The importance of particle size and density on transient expression has been demonstrated for a variety of cell systems (Sanford, et al., 1993). Here we present data to demonstrate that for stable expression in several biological systems, particle bombardment is more efficient using 0.6 gold particles than using particles of larger size.

Methods and R
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