Nanoporous Silicified Phospholipids and Application to Controlled Glycolic Acid Release
© to the authors 2008
Received: 23 June 2008
Accepted: 25 August 2008
Published: 9 September 2008
This work demonstrates the synthesis and characterization of novel nanoporous silicified phospholipid bilayers assembled inorganic powders. The materials are obtained by silicification process with silica precursor at the hydrophilic region of phospholipid bilayers. This process involves the co-assembly of a chemically active phospholipids bilayer within the ordered porosity of a silica matrix and holds promise as a novel application for controlled drug release or drug containers with a high level of specificity and throughput. The controlled release application of the synthesized materials was achieved to glycolic acid, and obtained a zero-order release pattern due to the nanoporosity.
Liposomes, or phospholipid vesicles, are good models for biological membranes, which are found in all species from bacteria to mammals [1–3]. Phospholipids are anionic, cationic, or zwitterionic, and the polar head groups, such as choline, ethanolamine, and serine, differ from each other in size and type of functional groups. The net charge of phospholipid vesicles can be adjusted through the use of different phospholipids. The binding of molecules to the phospholipid membranes also varies in relation to the net charge of phospholipid vesicles. Besides affecting the composition of the phospholipids, the preparation methods affect the structure and characteristics of the liposomes [4, 5]. Lecithin is an important component, especially with respect to membrane penetration. It has been widely used as a biosurfactant in the cosmetic industry for a long time. In addition, the chemical and physical properties of lecithin are largely dependent on its composition of different kinds of phospholipids. The commercial use of lecithin is increasing in the fields of biological membranes, skin-care formulations, and drug delivery. In modern cosmetic products, liposomes can encapsulate the active ingredients required by the skin; as a result, they can be applied directly to the skin cells. Most cosmetic and pharmaceutical liposomes are composed of various types of lecithins of natural, semi-synthetic, and synthetic origins; the major component is usually phosphatidylcholine (PC). The stability of the liposome is closely related to the composition of unsaturated fatty acids and the PC content.
Silica-based surfaces of phospholipids on inorganic surfaces have been formed by the spreading or physicochemical adsorbing of vesicles from solution such as supported lipid bilayers (SLBs). A detailed image of the structural intermediates during the formation of SLBs is emerging from both experimental and theoretical studies [6–10]. Even though many issues regarding the formation of SLBs have been reported recently, such as the role of calcium and the influence of vesicle size on rupture, the role of the inorganic solid support during vesicle formation remains poorly understood [11–13].
Metal cations have a greater influence on the membranes of anionic lipids than on neutral or zwitterionic membranes because of the stronger attractive Coulombic forces. In particular, divalent cations such as Mg2+, Ca2+, and Ba2+ affect the stability and structure of phospholipid bilayers [14–16]. Magnesium is an essential mineral in vertebrates and is the fourth most abundant cation in the body within the cell second only to potassium. Furthermore, a large number of enzymes, especially those involving phosphate compounds such as ATPases, kinases, and phosphatases, have long been known to require the Mg2+ cation for activation. The magnesium cation is involved in several physiological and biochemical processes including the synthesis of RNA, DNA, or protein and the stabilization of membranes . Recent research has confirmed that the Mg2+ cation plays an important role in the regulation of membrane channels as well as excitation–concentration coupling in skeletal muscle.
The phospholipid vesicles consisted of 2 wt% of hydrogenated soybean lecithin from 70% PC. The hydrogenated lecithin (2 g) was added to the deionized (DI) water (98 g) and homogenized at 80 °C. The hot phospholipid vesicle solutions were passed through high-pressure homogenizer three times. The nanoemulsion of the phospholipid vesicle was dropped to the magnesium-grafted talc surface; the magnesium-grafted talc contained enough deionized water and the various concentrations of magnesium sulfate were as follows: 0.35, 1.75, 2.5, 3.5, 7, and 10 mM. The mixtures were aged for 24 h, and the powder was filtered, and then dried in air. Using a Quantachrome Nova e-4000 system with a sample pretreated at 100 °C overnight in a vacuum line, the Brauner–Emmet–Teller (BET) results were obtained by the Barrett–Joyner–Halenda (BJH) method from nitrogen adsorption and desorption isotherms. The distribution of the pore size was calculated from analysis of the adsorption branch of the isotherm. The pore volume was taken at the five points ofP/P 0. The electron microscopic measurements such as TEM and SEM were achieved by a JEOL JEM-4010 TEM (400 kV) and a JEOL JSM 6700F, respectively. Infra-red (IR) and ultraviolet-visible (UV–Vis) spectra were obtained from a JASCO V-460 and JASCO V-550 including an attenuated total reflectance Fourier transform IR (ATR-FTIR) technique and diffused reflectance UV–Vis spectroscopy, respectively.
The glycolic acid release experiments were achieved from the overnight immersion of nanoporous silicified phospholipid-coated talc powder in a saturated drug solution, the mixture of which was dried over a period of 3 days at room temperature. The dried mixture was then immersed in 10 mL of a phosphate buffer (PBS, pH 7.4, 10 mM) and kept in a temperature-controlled shaker for stepwise temperature changes. The drug released concentration of the solution was measured with a UV–Vis spectrophotometer at different time intervals. After each measurement, 10 mL of the PBS buffer was replaced.
Results and Discussion
To elucidate the effect of silica precursor on the formation of silica coated phospholipids, we demonstrated the quantification of Si element in the nanoporous silicified phospholipids by the inductively coupled plasma-optical emission spectrometer (ICP-OES) with axial viewing configuration. This elemental analysis was achieved by the various concentration of silica precursor such as 0.5, 1.0, and 1.5 M. The results on Si element showed the 30.5, 58.5, and 89.3 wt% corresponding to 0.5, 1.0, and 1.5 M of silica precursor concentration, respectively. These results provided that the use of higher concentration of silica precursor was caused to strong cross-linking within the silica matrix to enhance the silica density as shown in molecular simulation images. However, the structural changes of phospholipids do not occur because this cross-linking interaction is just motivated by the hydrophilic interaction.
In summary, the nanoporous silicified phospholipids-coated inorganic powders were systematically synthesized and characterized. The silicification process involves co-assembly of a chemically active phospholipid bilayer within the ordered porosity of a silica matrix. The concentration of the Mg-chelator was optimized to fix the phospholipids and activate the surface. The controlled release application of the synthesized materials was achieved to glycolic acid, and obtained a zero-order release pattern due to the nanoporosity. Consequently, the nanoporous silicification process on the phospholipids is very useful for bio-related fields such as cosmetics and drug delivery systems. We expect the basic results to lead to a general and simple approach for preparing a wide range of controlled releasing materials such as encapsulation with cosmetics or drugs.
This work was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea.
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