**1. Introduction**

A nanocrystal is a particle having one or more dimensions of the order of 200 nm or less and considered to have novel characteristics which differentiate them from other materials [1]. When the size of the material is reduced to less than 200 nanometers, the realm of quantum physics takes over and five materials begin to demonstrate entirely new properties. Hence, nanodesign of drugs by various different techniques, like melting, homogenization, and controlled precipitation, is explored to produce drug nanocrystals, nanoparticles, nanosuspensions, etc. [2]. As decrease in size will increase the solubility of drugs, this technology is explored to increase oral bioavailability of sparingly water soluble drugs [3]. Development of soluble and/or permeable drug molecules using nanocrystal formulations has been proven to be successful due to their unique size range and higher surface: volume ratio, which results in enhanced drug dissolution, bioavailability and permeability [4].

Atenolol is a selective *β*1 receptor antagonist, a drug belonging to the group of beta blockers, which is used mainly in different cardiovascular diseases [5]. It often suffers from poor bioavailability after oral dosing due to stumpy permeability through GIT [6]. Approximately 50% of an oral dose is absorbed from the gastrointestinal tract, the remainder being excreted unchanged in the feces. Researchers have been endeavored to increase its permeability and bioavailability by different techniques including osmotic pump, cyclodextrin-based delivery systems, hydrophilic matrices, transdermal delivery systems, and so on [7–12].

In the present study, we had prepared nanocrystals of atenolol to improve its permeability and modify its solubility, because this method is less time-consuming, required no organic solvents or harsh chemicals like other nanodelivery systems, has a high product yield, has good product stability, and is cheap. High pressure homogenization method was employed to prepare nanocrystals [13]. In this method, high pressure was applied on liquid suspension to force it through a gap or narrow channel inside a pipe. Here, the medium was aqueous containing a hydrophilic surfactant SLS to prevent agglomeration of suspended particles and thus it helped in stabilization. The surfactant used in the study also prevented crystal growth (Ostwald ripening) that could change the dissolution and bioavailability of the drug after storage [14].
