2.1.2. Liposomes

Liposomes were discovered in the mid-1960s by A. D. Bangham [82]. The vesicle of the liposome is easily fabricated in a laboratory and made of one or more phospholipid bilayers [83] (Figure 3). These are self-assembled versatile particles with diameters ranging from nanometer to micrometer scale [84]. Resembling lipid cell membranes, the nature of phospholipid depends on the length of fatty acid chains [48]. They have the ability to encapsulate and carry hydrophobic aqueous agents [82]. They exhibit many advantages over other carrier systems [85,86].

**Figure 3.** Fabrication of liposome loaded scaffold.

Bone morphogenetic protein-2 (BMP-2) is one of the most potent proteins in bone regeneration [87]. For this reason, encapsulation of BMP-2 in nanomaterials has attracted great interest. BMP-2-loaded liposomal-based scaffolds may possess better osteoinductivity and bone formation ability [88].

Liposomes can carry drugs directly to the site of action and sustain their levels without causing toxicity for long periods [89]. By changing the composition of lipids, liposome properties can change. Some liposome preparations for anticancer drugs have successfully released on the market by acquiring FDA's approval [83]. Gentamycin- and vancomycin-integrated liposome-loaded particles are employed for manufacturing of scaffolds [90]. The integration of bioactive aspirin into a liposome delivery system would have a beneficial impact on stem cell osteoblast differentiation [91]. The initial drug amount and the chemical and physical drug properties are considerable factors for the encapsulation efficiency [92]. DOXIL®, the first FDA-approved nanodrug, which consists of liposomes encapsulating doxorubicin, was prepared by this remote loading method [93]. This method can also be used for preparing liposomes encapsulating other drugs such as daunorubicin and vincristine [94]. Liposomal systems are highly used despite being the oldest of the non-viralgene-delivery vehicles [95]. Scaffolds used as delivery vehicles for bioactive agents offer many advantages such as enhanced and extended gene expression, and the ability to control a localized delivery of cargo [96] (see Figures 3 and 4).

**Figure 4.** Schematic depiction of the role of tissue-engineered scaffolds in gene delivery [96] (adapted with permission from Elsevier © 2009).
