Lipid self-spreading

By taking advantage of this aspect, we can transport and manipulate any collection of molecules in the lipid bilayer, irrespective of their charge. By constructing a micro-channel on the substrate, we can control the direction of the self-spreading. Furukawa et al. fabricated photo-lithographic micro-channels with a width of 1-20 tm [50]. 

Figure 13.5 (a) Fluorescence micrograph of the self-spreading lipid bilayer doped with a dye molecule. The lipid bilayer spread on an oxidized silicon wafer from a deposited lipid aggregate illustrated on the left, (b) A schematic drawing of the selfspreading lipid bilayer from the lipid aggregate. Adapted from Ref [48] with permission. 

Figure 13.6 (a) Confocal micrograph of a circularly self-spreading lipid monolayer. A rhodamine-labeled lipid is doped to visualize the spreading behavior, (b) A schematic illustration of the front edge of the self-spreading lipid monolayer [51]. 

In addition to the self-spreading lipid bilayer, it was also found that a lipid mono-layer showed similar spreading behavior on a hydrophobic surface (Figure 13.6) [51]. By fabricating an appropriate hydrophobic surface pattern, the spreading area and direction can be easily controlled. For both the self-spreading bilayer and monolayer, non-biased molecular transportation is an important key concept for the next generation of microfiuidic devices. 

Figure 13.7 (a) The self-spreading distance and (b) velocity of egg-PC lipid bilayer in NaCI aqueous solutions with different concentrations, (x) 100mM, (0) 10mM, and ( ) 1 mM. Adapted from Ref [53] with permission. 

This result demonstrates that the self-spreading dynamics are controllable by tuning the bilayer-substrate interactions. The above-mentioned electrolyte dependence is an example of this fact. Considering that there are many parameters that alter the bilayer-substrate interaction, a diverse approach can be proposed. For example, Nissen et al. investigated the spreading dynamics on the substrate coated with polymetic materials [48]. They found that insertion of a hydrophilic and inert polymer layer under the self-spreading lipid bilayer strongly attenuated the bilayer-substrate interaction. 

Of course, other physical and chemical conditions also affect the self-spreading dynamics. Figure 13.9 shows the dependences of P on the temperature and lipid... 

In addition to the spreading dynamics, the stacking structure of the self-spreading lipid bilayer is also controllable via the NaCl concentration [54, 55]. Further experimental and theoretical investigations regarding the control of self-spreading are required before we will be able to easily control the self-spreading behavior in microfluidic devices. 

Molecular Manipulation on the Self-Spreading Lipid Bilayer... 

The most intriguing aspect of the self-spreading lipid bilayer is that any molecule in the bilayer can be transported without any external bias. The unique characteristic of the spreading layer offers the chance to manipulate molecules without applying any external biases. This concept leads to a completely non-biased molecular manipulation system in a microfluidic device. For this purpose, the use of nano-space, which occasionally offers the possibility of controlling molecular diffusion dynamics, would be a promising approach. 

Furukawa, K, Nakashima, H.. Kashimura, Y. and Torimitsu, K (2006) MicroChannel device using self-spreading lipid bilayer as molecule carrier. Lab on a Chip, 6, 1001-1006. 

Nabika, H., Takimoto, B., lijima, N. and Murakoshi, K (2008) Observation of self spreading lipid bilayer on hydrophilic surface with a periodic array of metallic nano-gate. Electrochim. Acta, 53, 6278-6283. 

A conventional BLM can also be formed by a modified L B technique. It uses essentially a lipid monolayer spread from lipid in a volatile solvent (e.g., -hexane) on each aqueous chamber with the aqueous solution below the hole. Raising the aqueous solution level on both sides above the hole results in the two monolayers combining into a BLM in the aperture, thereby forming a so-called solvent-free BLM.It should be mentioned, however, that there is a major difference between the BLM and multilayers formed by the L B technique. A BLM, formed either by the conventional painting method or self-assembling on substrates (e.g., freshly cleaved metallic wire and agar gel—see later sections) is a dynamic liquid-like structure which is capable of accommodating a host of modifiers such as polypeptides, proteins, oil-soluble compounds, etc. In contrast, a L B multilayer of bimolecular thickness, albeit more stable than a BLM, usually contains pinholes and is in a solid state [16,17]. 

Recently, Brzozowska et al. used NR and ex situ electrochemical techniques to characterize an innovative type of monolayer system intended to serve as a support for a bUayer lipid membrane on a gold electrode surface [51]. Zr ions were used to noncovalendy couple a phosphate-terminated self-assembled monolayer (SAM) formed on a gold surface to the carboxylate groups of negatively charged phos-phatidylserrne (PS). This tethered surface was then used for the formation of a PS hpid bilayer structure formed by vesicle fusion and spreading. NR studies revealed the presence of an aqueous environment associated with the tether layer which arises from nonstoichiometric water associated with the zirconium phosphate moieties [52]. 

Sebum secretion and filming on the hair surface is an effective self-defense mechanism to counteract cuticle damage. Spreading of sebum along hairs occur via mechanical contacts with body movement and hair handling. The lipidic layer lubricates the fibers, reduces the frictional forces, and provides a chemical protective barrier. By this and other mechanisms hair is maintained in a proper healthy state, characterized by shine, softness, easiness to comb and style, mechanical resistance, and elasticity. In the absence of external influences hair tends to remain in this conditioned state. The role of cosmetic conditioners is to help counteract damaging effects and to restore the healthy state of the hair. 

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