Kathrin Hielscher of Hielscher Ultrasonics discusses the preparation of ultrasonic liposomes for pharmaceuticals and cosmetics.
Liposomes (liposome-lipid-based vesicles), transferosomes (ultradeformable liposomes), ethosomes (ultradeformable vesicles with high alcohol content) and niosomes (synthetic vesicles) are microscopic vesicles that can be artificially prepared as globular carriers into which active molecules can be encapsulated.
These vesicles, with diameters between 25mm and 5,000nm, are often used as drug carriers for topical purposes in the pharmaceutical and cosmetic industries, such as for drug delivery, genetherapy and immunisation.
Ultrasound is a proven method of liposome preparation and the encapsulation of active agents into these vesicles.
Liposomes are unilamellar, oligolamellar or multilamellar vesicular systems and are composed of the same material as a cell membrane (lipid bilayer).
Regarding their composition and size, one differs between multi-lamellar vesicles (MLV, 0.1-10um) and unilamellar vesicles, which are distinguished between small (SUV, <100nm), large (LUV, 100-500nm) or giant (GUV, >=1um) vesicles.
The composite structure of liposomes consists of phospholipids.
Phospholipids have a hydrophilic head group and a hydrophobic tail group, which consists of a long hydrocarbon chain.
The liposome membrane has a similar composition as the skin barrier, so that it can be easily integrated into the human skin.
As the liposomes fuse with the skin, they can unload the entrapped agents directly to the destination, where the actives can fulfil their functions.
Therefore, the liposomes create an enhancement of skin penetrability/permeability for the entrapped pharmaceutical and cosmetic agents.
In addition, liposomes without encapsulated agents, the vacant vesicles, are potent actives for the skin, as the phosphatidylcholin incorporates two essentials that the human organism cannot produce by itself: linoleic acid and choline.
Liposomes are used as biocompatible carriers of drugs, peptides, proteins, plasmic DNA, antisense oligonucleotides or ribozymes, for pharmaceutical, cosmetic and biochemical purposes.
The huge versatility in particle size and physical parameters of the lipids affords an attractive potential for constructing tailor-made vehicles for a range of applications.
Liposomes can be formed by the use of ultrasonics.
The basic materials for liposome preparation are amphilic molecules derived from or based on biological membrane lipids.
For the formation of small unilamellar vesicles (SUVs), the lipid dispersion is sonicated gently - for example with the handheld UP50H ultrasonic device (50W, 30kHz), the Vialtweeter or the UTR200 ultrasonic reactor - in an ice bath.
The duration of such an ultrasonic treatment lasts for around five to 15 minutes.
Another method to produce small unilamellar vesicles is the sonication of the multi-lamellar vesicle liposomes.
Dinu-Pirvu et al (2010) reports the obtaining of transferosomes by sonicating MLVs at room temperature.
Hielscher Ultrasonics offers various ultrasonic devices, sonotrodes and accessories to provide the appropriate sonicator in terms of power.
Liposomes work as carriers for active agents.
Ultrasound is an effective tool to prepare and form the liposomes for the entrapment of active agents.
Before encapsulation, the liposomes tend to form clusters as a result of the surface charge-charge interaction of phospholipid polar heads (Mickova et al, 2008).
In addition, they have to be opened.
By way of example, Zhu et al (2003) describes the encapsulation of biotin powder in liposomes by ultrasonication.
As the biotin powder was added into the vesicle suspension solution, the solution was sonicated for approximately one hour.
After this treatment, biotin was entrapped in the liposomes.
To enhance the nurturing effect of moisturising or anti-ageing creams, lotions, gels and other formulations, an emulsifier is added to the liposomal dispersions to stabilise higher amounts of lipids.
However, investigations have shown that the capability of liposomes is generally limited.
With the addition of emulsifiers, this effect will appear earlier and the additional emulsifiers cause a weakening on the barrier affinity of phosphatidylcholine.
Nanoparticles - composed of phosphatidylcholine and lipids - are the answer to this problem.
These nanoparticles are formed by an oil droplet that is covered by a monolayer of phosphatidylcholine.
The use of nanoparticles enables formulations that are capable of absorbing more lipids and that remain stable so additional emulsifiers are not needed.
Ultrasonication is a proven method for the production of nanoemulsions and nanodispersions.
Highly intensive ultrasound supplies the power needed to disperse a liquid phase (dispersed phase) in small droplets in a second phase (continuous phase).
In the dispersing zone, imploding cavitation bubbles cause intensive shock waves in the surrounding liquid and result in the formation of liquid jets of high liquid velocity.
In order to stabilise the newly formed droplets of the disperse phase against coalescence, emulsifiers (surface-active substances or surfactants) and stabilisers are added to the emulsion.
As coalescence of the droplets after disruption influences the final droplet size distribution, efficiently stabilising emulsifiers are used to maintain the final droplet size distribution at a level that is equal to the distribution immediately after the droplet disruption in the ultrasonic dispersing zone.
Liposomal dispersions, which are based on unsaturated phosphatidylchlorine, lack in stability against oxidation.
The stabilisation of the dispersion can be achieved by antioxidants, such as by a complex of vitamins.
C and E Ortan et al (2002) achieved good results in a study concerning the ultrasonic preparation of Anethum graveolens essential oil in liposomes.
After sonication, the dimension of liposomes was between 70nm and 150nm, and for MLV between 230nm and 475nm; these values were approximately constant after two months, but increased after 12 months, especially in SUV dispersion.
The stability measurement, concerning essential oil loss and size distribution, also showed that liposomal dispersions maintained the content of volatile oil.
This suggests that the entrapment of the essential oil in liposomes increased the oil stability.
Liposomal formulations were stored at 4+/-1C.
Hielscher ultrasonic processors are suitable devices for applications in the cosmetic and pharmaceutical industry.
Systems consisting of several ultrasonic processors of up to 16,000W each provide the capacity needed to translate this laboratory application into an efficient production method to obtain finely dispersed emulsions in continuous flow or in a batch - achieving results comparable to that of today's high-pressure homogenisers such as the new orifice valve.
In addition to this high efficiency in the continuous emulsification, Hielscher ultrasonic devices require low maintenance and are said to be easy to operate and to clean.
The ultrasound supports the cleaning and rinsing.
The ultrasonic power is adjustable and can be adapted to particular products and emulsification requirements.
Special flow cell reactors meeting the advanced clean-in-place and sterilise-in-place requirements are also available.
