Lipid based structures such as biomembranes and liposomes are widely used in several fields of research, for example, in the design and development of new biosensor systems, biomaterial coatings and drug delivery systems, where these structures serve as inert surfaces, biocompatible surfaces, cell membrane mimics or transport vessels, to mention a few examples.
In studying biomembranes, there are two approaches that can be utilized. It is possible to form a floating biomembrane model structure on the air-water interface that enables the you to mimic the properties and conditions of the cell environment.
Another approach is to form a supported biomembrane or lipid-based structure on a solid substrate. Supported lipid bilayers are lipid layers deposited on surfaces and are constituted by predefined ratios of selected lipids, potentially tagged with different molecules or with embedded membrane proteins. These layers can aid both the understanding of biological processes and serve as key elements in the creation of biomaterials. They can also be involved in more complex structures such as in biosensor design and in interaction with various biological or synthetic molecules such as ligands, DNA, nanoparticles, polymers or another lipid structure.
In the design and development of nano-medicines, lipid-based nanostructures can serve as vessels for drug transportation and targeted delivery in the body. Embedding the drug of interest in vesicular or micellar structures, tailored for certain environmental conditions, enables enhanced drug protection in terms of stabilization and reducing toxicity, extended circulation time, controlled release rate and improved targeting to tissue.
Most biochemical reactions take place at membranes composed of phospholipid bilayers surrounding or inside cells. The membrane affects protein folding and creates specific microenvironments where the reactions take place. To understand and mimic actual biological systems, it is essential to study these interactions in an environment that closely mimics natural conditions. Langmuir monolayers of membrane phospholipids have been verified as excellent model systems for biological membranes.
A Langmuir monolayer of lipids is analogous to half a biomembrane. Such layers can be used as model membranes and have been verified to be excellent models for biological systems in the literature. In a freely floating monolayer, the molecular diffusion and dynamics are close to what they are in actual systems. In nature, most biochemical reactions take place at biomembrane interfaces, and freely floating model membranes allow natural diffusion and migration of molecules. A Langmuir trough can be integrated with a variety of sensors and instruments in addition to the Langmuir film balance in order to study membranes. Additional techniques include BAM, SPOT, fluorescence microscopy and traditional microscopy, which make it possible to study interactions, molecular orientation, packing and domain formation in the monolayer.
Pulmonary surfactants cover the alveoli of the lungs and have a vital function in making the process of breathing easy. During inhalation, the surfactant reduces the surface tension of tissue by a factor of about 15 making it much easier to inflate the alveoli. During exhalation, the surface area of the alveoli decreases making the surfactant even more concentrated on the surface. This yields a near-zero surface tension at the end of exhalation, which prevents the alveoli from collapsing.
Dipalmitoylphosphatidylcholine (DPPC) is one of the phospholipids present on the alveoli surface. It is known that the highly ordered solid phase of DPPC sustains the near-zero surface tension on the alveoli during exhalation. In order to model the actual surfactant behavior in the alveoli, measurements at near-zero surface tensions are needed. It has been shown that a KSV NIMA Langmuir Ribbon Barrier Trough can be used to measure near-zero surface tensions of DPPC.Application note: Reaching high monolayer surface pressures using a ribbon barrier trough: the case of near-zero surface tension pulmonary surfactants
Langmuir-Blodgett (LB) and Langmuir – Schaefer (LS) dipping are two methods for preparing supported bilayers of phospholipids with layers of different lipid composition. Using a combination of LB and LS dipping, it is also possible to create a supported bilayer with lipid composition that is asymmetric between the sides. For example, it is possible to prepare biochemical sensors for use in surface plasmon resonance spectroscopy, quartz crystal microbalance measurements and x-ray photoelectron spectroscopy. A third method for preparing supported bilayers of phospholipids is via vesicle rupture and fusion, directly on to the surface, in a QCM-D setup.
Irrespective of whether we are dealing supported membranes, liposomes or other lipid-based structures, the characterization and verification of the respective related uptake and release processes at the surface, enabled by the QSense® QCM-D, are key to understanding, tailoring and optimizing the lipid-based system of interest. For example, it is possible to monitor the formation kinetics of the supported lipid membrane at the surface, and to assessing the quality of the formed bilayer. It is also possible to monitor subsequent interaction with the lipid membrane, such as uptake or binding to membrane-incorporated molecules, or verification of a lack thereof. In the context of nanomedicines, it is possible to characterize the uptake, delivery, and release processes for the lipid-based nanostructures serving as vessels for the targeted drug delivery.