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COMPONENTS OF SOLID LIPID NANOPARTICLES

Lipid

Because lipids are the main building material of SLNs, the selection of an appropriate lipid or mixture of lipids is an important element to consider when fabricating SLNs. Generally, lipids are a large and diverse group of organic substances; however, the lipid components used in the formation of SLNs have the common property of being solid both at room temperature and at body temperature. Different types of solid lipids behave differently, which consequently affects the properties and the intended use of nanocarriers themselves. The selection of a pertinent solid lipid or lipid blend for SLNs depends on several factors: (1) ability to produce particles in the submicron range, (2) biodegradability, (3) biocompatibility, (4) adequate drug-loading capacity, and (5) stability upon storage. Moreover, lipids can be classified on the basis of the lipid matrix formed, i.e., ordered, less ordered, and disordered matrices [6]. Thus the selection of lipids plays a crucial role in the formulation of stable SLNs. The purity and composition of lipids should be examined using several analytical techniques such as gas chromatography—mass spectrometry, Fourier transform infrared spectroscopy, and Raman spectroscopy. The apparent degradation of lipids can be quantified with the help of the spectra. Lipids tend to be metabolized easily. For example, a study pertaining to the lipolysis half-life of lipids revealed that Gelucire, which is a polyethylene glycol (PEG) ester, metabolizes easily, thereby exhibiting a low half-life of lipolysis. This could lead to problems such as precipitation and crystallization of drugs. Drug lipophilicity with respect to the selected lipid is an important aspect to be considered for a stable formulation. An ideal lipid phase should be sufficiently lipophilic to solubilize lipophilic drugs easily. The lipids most commonly used in the preparation of SLNs are listed in Table 12.2. For detailed information on the types of lipids used in SLNs, readers can refer to our earlier publication [7] and other references [8,9].

Table 12.2 Lipids commonly used in the preparation of solid lipid nanoparticles

Lipids

Matrix

arrangement

Examples

Literature

Triglycerides

Highly ordered

Tricaprin

Trilaurin

Trimyristin

Tripalmitin

Tristearin

Hydrogenated coco-glycerides

[154]

[49.114.155]

[49.114.156] [109,114] [15,154] [157]

Hard fat types

Witepsol W 35 Witepsol H 35 Witepsol H 42 Witepsol E 85

[15,48,158]

[15,32,44]

[32]

[44,157]

Acylgylcerol

mixtures

Less ordered

Glyceryl monostearate (Imwitor 900)

Glyceryl behenate (Compritol 888 ATO)

Glyceryl palmitostearate (Precirol ATO 5)

[87,97,156,159]

[87]

[160]

Table 12.3 Surfactants used in preparation of solid lipid nanoparticles

Emulsifiers

Literature

Poloxamer 188

[19,48,161]

Poloxamer 182

[157]

Polysorbate 20

[162,163]

Polysorbate 60

[164]

Polysorbate 80

[157]

Sodium cholate

[156,157,165,166]

Sodium glycocholate

[109,167]

Soybean lecithin

[44,49,167]

Soybean phosphatidylcholine

[165,166]

Sorbitan trioleate

[168]

Surfactant

Surfactants stabilize SLNs by decreasing the surface tension between water and lipids. The selection and concentration of surfactant used depends on the lipid and the route of administration. Surfactants are divided into three categories, depending on their charge as ionic, nonionic, and amphoteric surfactants. Table 12.3 lists a few examples of surfactants commonly used in SLNs. The toxicity of a surfactant is an important consideration, and not all surfactants can be used for preparation of all types of SLNs. For example, nonionic surfactants such as poloxamer 188 and lecithin are preferred for parenteral and ocular routes of administration.

 
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