Osmotic pressure differentials

For osmotic drug delivery systems, Eq. (2) is of critical importance. This equation demonstrates that the quantity of water that can pass a semipermeable film is directly proportional to the pressure differential across the film as measured by the difference between the hydrostatic and osmotic pressures. Osmotic delivery systems are generally composed of a solid core formulation coated with a semipermeable film. Included in the core formulation is a quantity of material capable of generating an osmotic pressure differential across the film. When placed in an aqueous environment, water is transported across the film. This transported water in turn builds up a hydrostatic pressure within the device which leads to expulsion of the core material through a suitably placed exit port. 

The value of Cs is the most critical parameter in determining the overall release rate from a given osmotic system. Indeed, its value will determine whether or not it is feasible to utilize an osmotic system to deliver a particular drug for a specified duration. The maximum release rate achievable is likely that seen with KC1. The relevant values for the parameters in Eq. (6) for OROS [10] are as follows A = 2.2 cm2, h = 0.025 cm, LpO = 2.8 X 10 6 cm2/(atm hr), Us = 245 atm, and Cs = 330 mg/mL. This translates to about 20 mg/hr or about 250 mg over a 24 hr period. This is for a highly water soluble drug with a high osmotic pressure differential. For drugs of moderate solubility—for example,... 

Pumps are intended for implantation. An osmotic pressure differential triggers drug release from a reservoir embraced by a semipermeable membrane [26]. 

Mechanism of Gel Dilation by Photoirradiation The swelling of gels with fixed charges in the network can be quantitatively understood by the osmotic pressure differentials obtained from Donnan equilibrium(22,23). The equilibrium value of the volume... 

E is directly proportional to the osmotic pressure differentials as stated in the van t Hoff equation. 

Two solutions are isotonic solutions if they have identical osmotic pressures. In that way the osmotic pressure differential across the cell membrane is zero, and no cell disruption occurs. 

The flow of water through a reverse osmosis membrane is primarily dependent on the applied pressure differential and the osmotic pressure differential across the membrane. The osmotic pressure is directly dependent on the salt concentration of the process stream. As a rule of thumb, each 100 mg/ of dissolved solids is roughly equivalent to one psi of osmotic pressure. Since the product stream usually has a very low salt content, the osmotic pressure of that stream is negligible. In addition, the product stream normally leaves the reverse osmosis pressure vessels at near atmospheric pressure so that the applied pressure differential is the feed pressure. Consequently, the term "net applied pressure" has come to mean the applied pressure minus the feed osmotic pressure. 

Figure 1.5 Schematic representation of the situation giving rise to depletion flocculation. Polymer molecules are excluded from the space between particles causing an osmotic pressure differential between the excluded region and the continuous phase and giving rise to a net attractive depletion force.

The timescales of biochemical reactions and the preservation conditions applied to the organism play crucial roles in determining the success of preservation. For example, the ratio of the timescale of water diffusion, td, across the ceU membrane (td = r/SIpAIl, where r. Ip, and AO are the cell radius, membrane permeability, and osmotic pressure differential, respectively) to the timescale of cooling the ceU experiences, xq xq = (2cp/or A T)where Cp, p, q", and A T are the specific heat, mass density, heat flux, and temperature differential, respectively) determines the fate of a cell during freezing such that (Figure 41.1) ... 

Forward osmosis relies on the osmotic pressure differential across a membrane to drive water through the membrane RO relies on the hydraulic pressure differential to drive water through the membrane. A draw solution is used on the permeate side of the membrane to osmotically drive water from the feed side of the membrane into the draw solution, which becomes more dilute. The draw solution is then treated (sometimes by heating followed by membrane distillation or by RO) to recover the water and to regenerate the draw solution for reuse. 

---