unsaturated some specific examples include myristylamine palmitylamine laurylamine and stearylamine dilauroyl ethylphosphocholine dlep dimyristoyl ethylphosphocholine dmep dipalmitoyl ethylphosphocholine dpep and distearoyl ethylphosphocholine dsep n 2 3 di 9 z octadecenyloxy prop 1 yl n n n trimethylammonium chloride dotma and 1 2 bis oleoyloxy 3 trimethylammonio propane dotap the negatively charged lipids which can be used include phosphatidyl glycerols pgs phosphatidic acids pas phosphatidylinositols pis and the phosphatidyl serines pss examples include dmpg dppg dspg dmpa dppa dspa dmpi dppi dspi dmps dpps and dsps phosphatidylcholines such as dppc aid in the uptake by the cells in the lung e g the alveolar macrophages and helps to sustain release of the bioactive agent in the lung the negatively charged lipids such as the pgs pas pss and pis in addition to reducing particle aggregation are believed to play a role in the sustained release characteristics of the inhalation formulation as well as in the transport of the formulation across the lung transcytosis for systemic uptake the sterol compounds are believed to affect the release characteristics of the formulation liposomes are completely closed lipid bilayer membranes containing an entrapped aqueous volume liposomes may be unilamellar vesicles possessing a single membrane bilayer or multilamellar vesicles onion like structures characterized by multiple membrane bilayers each separated from the next by an aqueous layer the bilayer is composed of two lipid monolayers having a hydrophobic u201c tail u201d region and a hydrophilic u201c head u201d region the structure of the membrane bilayer is such that the hydrophobic nonpolar u201c tails u201d of the lipid monolayers orient toward the center of the bilayer while the hydrophilic u201c heads u201d orient towards.