Conventional method for Liposome Production

Phospholipids are dissolved in an organic solvent (used a chloroform/methanol mixture) and deposited from the solvents as a thin film on the wall of a round bottom flask by use of rotary evaporation under reduced pressure. MLVs form spontaneously when an excess volume of aqueous buffer containing the drug is added to the dried lipid film. Drug containing liposomes can be separated from non-sequestered drug by centrifugation of the liposomes or by gel filtration. The time allowed for hydration of the dried film and conditions of agitation are critical in determining the amount of the aqueous buffer (or drug solution) that will be entrapped within the internal compartments of the MLVs.

It is reported that more of the aqueous phase can be sequestered when the lipid is hydrated for 20 hours with gentle shaking, compared with a hydration period of two hours, with vigorous shaking of the flask, even though size distribution of the MLVs was unaffected. This means that slow hydration is associated with greater entrapment of aqueous volume.

Sonication method

This method is used in the preparation of SUVs and it involves the subsequent sonication of MLVs prepared by the convectional method either with a bath type or a probe type sonicator under an inert atmosphere, usually nitrogen or argon. The principle of sonication involves the use of pulsed, high frequency sound waves (sonic energy) to agitate a suspension of the MLVs. Such disruption of the MLVs produces SUVs with diameter in the range of 15–50nm.

The purpose of sonication is to produce a homogenous dispersion of small vesicles with a potential for greater tissue penetration.

The commonly used sonicators are :
  1. the bath sonication
  2. probe tip type. 

A probe tip sonicator delivers high sonic energy to the lipid suspension. But, there are some disadvantage in this method :
  1. overheating the lipid suspension which can cause degradation.
  2. probe tip also tends to release titanium particles into lipid suspension, which must be removed by centrifugation. 

Sonication with a probe results in faster breakdown of the MLVs to smaller structures than can be achieved by a bath sonication. The reduction in size of the liposomes, however, also decreases the amount of interior aqueous space, thereby limiting the amount of water-soluble drugs that can be entrapped. However, degradation of lipids, metal particle shedding from the probe tip (titanium particles) and generation of aerosols from solutions containing radioactive traces, carcinogenic chemicals or infectious agents that have been added to the preparation by probe technique could cause serious biohazards, which are less frequently encountered with bath sonication.

Bath sonication are the more widely used compare to probe tip sonicator for reasons above. Bath sonication is a closed system allowing for temperature control to minimize thermo degradation of the lipid and entrapped substance. The position of the tube and water level in the bath is also regulated for maximum efficiency. By this technique, a test tube containing the suspension is placed in the bath sonicator and sonicating for 5–10 minutes above the transitional temperature of the lipid (i.e., the temperature at which the lipid melts).

Disadvantage of Liposome Preparation with Sonication Method

The major drawbacks in the preparation of liposomes by sonication :
  1. oxidation of unsaturated bonds in the fatty acid chains of phospholipids and hydrolysis to lysophospholipids and free fatty acids. D
  2. enaturation or inactivation of some thermolabile substances (e.g., DNA, certain proteins, etc) to be entrapped. 

Read more in here : Technology of liposomes production (PART II)