**1. Introduction**

The advances in the field of nanotechnology and the material sciences have been open a new horizon for the development of various drug delivery systems (DDS) for the effective and efficient delivery of therapeutic and diagnostic agents [1]. During last few decades, many new DDS have been explored in term of their structural components and medical applications. These systems enhance the application of novel approaches toward the translational medicines by improving the preparation techniques and combing the natural and synthetic polymers and materials [2]. Formulation of these DDS helps to encapsulate a variety of chemotherapeutic agents, vaccines, proteins, antibodies, nucleic acids and diagnostic agents [3]. These agents might be encapsulated inside the core of NPs or might be adsorbed on the surface individually or in combination. These formulations enhance the pharmacokinetic and pharmacodynamics properties of the NPs by increasing the solubility, dispersion, permeability and overall bioavailability of the formulation. The release of the drug might be controlled passively or actively through various stimuli such as

temperature, and pH [4]. All these factors results in the higher concentration of the entrapped drug that reached the systemic circulation that helps to attain the mean effective concentration without producing any toxic effects [5].

Among these DDS, polymeric nanoparticles (NPs), liposomes, niosomes, dendrimers and porous silicon NPs have been extensively employed in the pharmaceutical delivery. Polymeric NPs have versatility in term of their chemical composition and applications. Large type of chemical material based NPs were formulated such as polymeric NPs, Porous silicon NPs, Carbon nanotubes, Graphene NPs and quantum dot. All these DDS have their own pros and cons in term of drug loading, encapsulation, release and applicability [6]. Furthermore, all these DDS were decorated with different chemical reagents and ligands to impart the desired characteristics through modifying the physicochemical properties of the NPs including (1) enhanced means residence time and improved stability, (2) external stimuli driven drug release, (3) controlled and targeted delivery of various chemotherapeutics agents and (4) administration of various theranostic agents [7].

These nanocarriers have been explored due to their extensive potential applications and excellent in vitro performances, but these NPs still have poor in vivo properties in term of their poor solubility in various body fluids, rapid uptake and excretion by the body defense system, poor penetration among the various biological membranes and body tissues, uncontrolled fluctuations in the plasma levels of the active therapeutic components and dose related toxicity issues [8, 9].

The most important domains of these nanocarriers include the polymeric DDS and the lipid based vesicular systems. The polymeric DDS provide the variety in term of their structural materials and chemical composition [6]. Different polymers include from the synthetic and natural sources have been employed for the medical applications. These DDS include the polymeric NPs, mesoporous silicon NPs, metal coated NPs, inorganic NPs, dendrimers and the carbon nanotubes [10]. Vesicular DDS include the liposome and noisome. These were defined as the single and the multilayer lipid vesicles while the niosomes were made up of nonionic surfactants instead of the phospholipids [11]. These novel systems provide excellent compatibility with other ingredients, higher and simultaneous encapsulation of the hydrophilic and lyophilic therapeutic moieties and due to lipid nature it provide better pharmacokinetic profiles that might lead to improved therapeutic response of the encapsulated drug. But still this system might suffer from some draw back in term of drug leakage, stability problems and difficulty in the scale up of the process [12].

The above-mentioned problems associated with these DDS including liposomes and polymeric nanoparticles can be overcome by merging their structural components by formulating the lipid-polymer hybrid nanoparticles (LPHNPs). These NPs combine the potential benefits and reduce the different drawbacks of all the individual structural components [13]. These hybrid particles might be produced in different morphologies including the core shell and matrix type LPHNPs. The core or the central material may be encapsulated in single and/or multiple layers of the lipid on the polymeric core materials that also provide the site for the surface modification with different targeting moieties and ligands that help to induce the desired characteristics in the DDS [14].

In this chapter, the different structural components such as lipids and polymers were explained along with the different formulation methods to prepare the LPHNPs along with various process parameters and their pros and cons.
