*2.2. Bottom-Up Technology*

Bottom-up technology is mainly based on precipitation and evaporation [33]. The basic principle is to obtain drug nanocrystals from the supersaturated state of drugs and subsequently control the size distribution of the nanoparticles by appropriate methods [41]. Nucleation is especially important for the formation of small and homogeneous nanocrystals. Controlling the crystal growth is the best way to precisely control the particle size of drug particles. Many physical methods have been used to control the crystal growth, such as high gravity controlled precipitation. Compared with top-down methods, these methods provide better control of particle properties [42]. In conclusion, bottom-up technology is simple in principle and operation, but difficult to scale up due to poor reproducibility. The process may also use organic solvents.

#### 2.2.1. Liquid Antisolvent Precipitation

Liquid antisolvent (LAS) precipitation is the preparation of nanocrystals by mixing a solution stream (organic phase) dissolved with an insoluble drug with an aqueous antisolvent. The solution–antisolvent method of nanoprecipitation is the most commonly reported. Since this method only contains nucleation and growth steps, it is simple and costeffective [12]. The optimized nanocrystals can be prepared through two steps. However, unstable crystal particles are also recrystallized in the process, leading to the aggregation and precipitation of nanocrystals [16]. Additionally, the use of organic solvents in the preparation process leads to the problem of solvent residues, and it is unsuitable for drugs that are neither soluble in aqueous nor insoluble in non-aqueous solvents. Currently, some studies have used this method to obtain suspensions of hydrochlorothiazide [39], budesonide [43], etc.

#### 2.2.2. Precipitation Assisted by Acid-Base Method

The carbon dioxide-assisted precipitation method using acid-base reactions usually involves dissolving the drug in a weak acid solution as the acid phase, and weak base in a solution containing stabilizer as the base phase. The acid phase is slowly added to the base phase to produce carbon dioxide, then the drug nanocrystals are precipitated by vapor effervescence [44]. This method avoids the addition of organic solvents, which is more friendly to the environment. However, it is only applicable to insoluble drugs whose solubility is related to pH and is stable to acids-bases [19]. Wang et al. [45] prepared tacrolimus nanocrystal suspensions using this method. *In vivo* pharmacokinetic results indicated that tacrolimus nanosuspensions significantly increased the oral bioavailability compared with commercial hard capsules.

#### 2.2.3. High Gravity Controlled Precipitation

High gravity controlled precipitation (HGCP) is [46] the improvement of the precipitation method using gravity control to obtain more uniform and smaller drug nanocrystals. Reactant concentration, rotational speed, and volumetric flow rate are effective factors influencing particle size. In this process, the drug suspension in the device can be circulated for long-term mixing and reaction. However, local oversaturation of the feed stream at the turbulent edge during mixing leads to continuous nucleation, thus limiting the industrial application of this method [25]. To date, this method has been successfully used on the laboratory scale to prepare salbutamol sulfate [47] and sorafenib [48].
