The structure of cell plasma membranes has been the subject of intense research because of its importance in various fields, such as pharmaceutical manufacturing, disease conditions, drug delivery and gene therapy. Plasma membranes are noncovalent assemblies of lipids and proteins that are critical to the proper function and viability of cells. Proteins present in the cell membrane play major roles in cellular functions by acting as receptors, energy transducers, pumps and channels. In order for the proper function of these membrane proteins, a suitable molecular environment is required. The bilayer of lipids achieves this by providing a flexible and fluid solvent for membrane proteins. In addition, the lipid bilayer also forms a rigid protective barrier in various physiological conditions.

The plasma membrane is composed of lipids, sterols, and proteins, the ratios of which alter its fluidity. The complex structure and fluidity make manipulation on a cellular level challenging. In particular, the extremes of stress and temperature exerted during the handling of cells, as seen in pharmaceutical manufacturing for example, require alterations in the membrane composition. Also, delivery of molecules into the cell, or extraction of intracellular compounds, requires extreme care especially in situations where cell viability is necessary. Techniques which attempt to inject molecules into cells using glass micropipettes may lead to membrane rupture and cell death. Therefore, the handling and immunogenic complexities that arise in using viruses as vectors in gene therapy limit the potential of a very promising field.

On the other hand, recent advances in nanotechnology have suggested the use of nanomaterials as possible vectors or delivery vehicles for cellular injection. Nanomaterials, especially carbon nanotubes (CNTs), are being investigated because of their size, shape, mechanical rigidity, functionalization and biocompatibility. However, the effect of CNTs on cell membrane is not well understood. A better understanding of the atomic level interactions is necessary in designing and developing superior drugs, cellular injection equipment and gene therapy vectors. To optimize these processes, a realistic model of the membrane and surrounding environment is needed. For example, the presence of cholesterol reduces the fluidity of the membrane at physiological temperatures. Given the dimensions of cells and their membranes, microscopic interactions play a very important role in determining the membrane structure.

Atomistic simulations have been used with great success to understand the dynamics, rupture, and healing of cell membranes. Using these simulations, we wish to develop a phospholipid bilayer representing the plasma membrane system and study the effect of composition and temperature on the membrane. In addition, we use this model to investigate the applicability of CNTs as drug delivery agents and the effects of plasma membrane structure. Specifically, we compute the forces required for the delivery of drugs through a CNT needle, and the effects this needle has on membrane structure, membrane self healing and dynamics.

Preliminary simulations have been performed with a simple bilayer system of phosphotidylcholine (POPC), CNT and water. These simulations consist of an initial equilibration phase, followed by the insertion and withdrawal of a CNT needle, as in a nanosyringe. We observe both the parting of the membrane by the needle, the forces required to puncture the membrane, and the dynamics of the lipid surrounding the carbon nanotube. The results of these simulations suggest a fast membrane healing process following rupture, minimal impact of CNT on the orientation or fluidity of the membrane lipids and that speed of insertion plays an important role in minimizing membrane damage. We are also studying the role of CNT size, and orientation on damage in the membrane as well as membrane composition, temperature, and insertion velocity.