hydrophobicity that limits their solubilization in vivo); (2) fraction of the solubilized

concentration which is supposed to be absorbed through the gastrointestinal mem-

brane and enter into the systemic circulation (in this factor, the permeability factor

plays a key role that determines the level of absorption); and (3) after absorption

through oral route, the absorbed fraction undergoes biotransformation and then

transferred to the site of action.

Therefore, several novel drug delivery approaches have been concurrently

performed and developed to maximize the therapeutic efficacy of herbal bioactives.

Apart from solubility, permeability, and stability issues of phytopharmaceuticals,

improvising the biological half-life is of great concern among the formulation

scientists. As biological half-life directly determines the bioavailability and potency

of herbal bioactives at the site of action. Low biological half-life signifies low

bioavailability and high biological half-life signifies high bioavailability (Mukherjee

et al. 2015).

To curb all aforesaid problems associated with herbal bioactives, nano-sized

emulsions are pioneering among the novel drug delivery systems (NDDS) in context

to industrial viability and providing promising uniform therapeutic results to

clinicians. The herbal bioactives are usually lipophilic or have poor aqueous solu-

bility which generally limits their bioavailability. Hence, nanoemulsions are the best

suitable drug carrier system as the hydrophobic bioactive substance is dissolved in

oil phase, whereas the hydrophilic bioactive is dissolved in aqueous phase. After

solubilizing the bioactive in selective phase (oil/water) along with appropriate

surfactant and cosurfactant are mixed into the continuous phase. The colloidal

dispersion formed is subjected to either high-energy processes or low-energy pro-

cesses for nanoemulsification or size reduction. In some cases co-solvents are also

added as an adjuvant as they have been shown to increase the solubility of

compounds and penetration into biological tissues as well. In such cases,

transforming the bioactive substance into a crystalline form can resolve the issue

(Chen et al. 2011; Shegokar and Müller 2010). It has been observed that most of the

herbal bioactives are usually poorly soluble in oil and water as discussed in

Table 19.2. Under such circumstances, the route of administration plays a key role

in defining the formulation considerations. For instance, if the hydrophobic bioactive

is intended to deliver through oral route, then the oily phase is preferred as it tends to

digest rapidly in the gastrointestinal medium and forms mixed micelles also known

as swollen micelles (Rana et al. 2017). In the case of topical or transdermal route,

certain percutaneous absorption enhancers like ethanol or essential oils are reported

as an adjuvant to nanoemulsion (Shen et al. 2011). Also, for topical applications, the

increased concentration of surfactants or cationic surfactants can be employed for

improvising the low solubility and permeability issues with clinical safety (Dario

et al. 2016).

Selection of oil also plays a major role particularly for drug loading and ultimately

bioavailability of lipophilic herbal actives. The oils with long-chain triglycerides

comprise comparatively more lipophilic character due to long nonpolar chains. This

property allows accommodation of more drug into the mixed micelle formed and

nanoemulsification technology facilitates the dissolution and permeability through

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Nanoemulsions: A Potential Advanced Nanocarrier Platform for Herbal Drug. . .

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