Liquid Delivery Devices

The liquid delivery or propulsion device is a basic piece of equipment in all flow analysis systems. However, certain characteristics of FIA call for specific requirements in its propulsion systems. FI systems are in essence low pressure systems, with seldom more than a few bars pressure developed in the conduits. Therefore, expensive high pressure liquid delivery devices such as those used for HPLC are not necessary. On the other hand, FIA is a technique based on highly reproducible timing,— a feature which demands pulseless and reproducible flow-rates in liquid delivery. The high versatility of FIA also demands easily manageable propulsion devices which will not depreciate the flexibility of the technique. HPLC pumps, expensive as they are, do not always meet all these requirements. The main features of an ideail liquid delivery device for FIA may perhaps be sunmiarized as follows  

Unfortunately, none of the presently available liquid delivery devices can meet all these requirements. Those described in this chapter are the ones whose performance matches closest with the requirements of FI systems, particularly for separation purposes, and are used most broadly in such applications. 

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Before discussing the three categories of delivery device, the nature of the emitted aerosol will be considered. Droplet formation may be characterized in terms of the nature of the propulsive force and the liquid being dispersed, and this topic is dealt with for specific situations in the following sections. However, dry particles, which are delivered from suspension in pMDIs or from DPIs alone or from a blend, must be prepared in respirable sizes. The production of respirable aerosol particles has traditionally been achieved by micronization of the drug [25]. This... 

Demand pacemakers are very low current devices, requiring only 25-50 jiW for sensing and 60-100 pW for stimulation. In contrast, implanted ventricular defibrillators (Fig. 1.3) must be able to deliver short electric pulses of 25-40 J (e.g. 2 A at 2 V for 10 s) which can shock the heart into normal rhythm, and hence require a much higher rate battery. The most common system is a lithium-silver vanadium oxide cell with a liquid-organic based electrolyte. More than 80 000 such units have been implanted. Implanted drug delivery devices also use lithium primary batteries, as do neurostimulators and bone growth stimulators. 

Solutions (lavages) and foams are liquid preparations. The foams differ from the solutions in the presence of a suitable propellant, in the formulation, and the type of container, a pressurized delivery device. Plasma concentration profiles obtained after solution administration are characterized by a burst effect followed by a rapid decrease below therapeutic levels, due to the low residence time of the formulation in the vaginal cavity. Such preparations are designed to achieve a local effect particularly in case of inflammations or infections caused by bacteria or yeasts (anaerobic bacteria or Candida species). Nonoxynol-9 (N-9) foam is used as a contraceptive and against sexually transmitted diseases [19]. 

Design of delivery device Shape/geometry Cylinder or rod Microparticles (microsphere, microcapsule, nanoparticle) Film or sheet Viscous gel or liquid... 

The size of the delivery system varies with the type of formulation, i.e., a buccal tablet may be approximately 5-8 mm in diameter, whereas a flexible buccal patch may be as large as 10-15 cm in area. Mucoadhesive buccal patches with a surface area of 1-3 cm are most acceptable. It has been estimated that the total amount of drug that can be delivered across the buccal mucosa from a 2-cm system in 1 day is approximately 10-20 mg.f The shape of the delivery system may also vary, although for buccal drug administration, an ellipsoid shape appears to be most acceptable. The thickness of the delivery device is usually restricted to only a few millimeters. The location of the delivery device also needs to be considered. A mucoadhesive retentive system is preferred over a conventional dosage form. A bioadhesive buccal patch would appear to be the most appropriate delivery system because of its flexibility and the area of the buccal mucosa available for its application. The maximal duration of buccal drug retention and absorption is approximately 4-6 h because food and/or liquid intake may require removal of the delivery device. 

The microsealed delivery device is a variation of the matrix-type transdermal system in which the drug is dispersed in a reservoir phase which is then immobilized as discrete droplets in a cross-linked polymeric matrix. Release can be further controlled by inclusion of a polymeric microporous membrane. This system therefore combines the principles of both the liquid reservoir and matrix-type devices. Rate of release of a drug from a microsealed delivery system is dependent on the partition coefficient between the reservoir droplets and the polymeric matrix the diffusivity of the drug in the reservoir, the matrix and the controlling membrane and on the solubility of the drug in the various phases. There are, obviously, many ways to achieve the desired zero-order release rate, but only nitroglycerin has been commercially formulated into this type of delivery device (Karim 1983). 

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