## 2 Nanotechnology and Nanomedicine
### Synthesis of CaCO\({}_{3}\) Aragonite from Cockle Shell  
Several methods of synthesizing nanoparticles were developed, adopted and improved to modify the properties and minimise cost of production. In addition, several efforts were also made to improve the old existing methods to obtain more desired qualities of the nanoparticles through enhanced physicochemical properties [111]. Aragonite exhibits unstable thermodynamic properties while its size and shape produced in the laboratories are mostly dependent on the condition and methods used for its preparation [11]. From the early stage of micron sized CSCaCO\({}_{3}\)NP fabrication up to the recent synthesis of desirable nanosized particles, many studies adopted the bottom-up approach through the precipitation process, either by carbonation [99] or solution methods [102]. The carbonation method is sometimes faced with difficulties in the control of the shape and surface modifications [112]. Further, the bottom-up methods also failed to produce appropriate sizes and shapes required for pure aragonite nanoparticles [98; 99], because it is often in combination with other polymorphs e.g., calcite and vaterite [113]. Other bottom-up methods which were initially thought to be useful and productive in industries and also considered to be environmentally friendly, were later found to be quite tedious, expensive, time consuming and complicated. For example, the carbonation method is associated with strict temperature regulations, strenuous gas bubbling phase (mixture of CO\({}_{2}\) and N\({}_{2}\) or CO\({}_{2}\) alone) and the use of purified raw materials. Another limitation of the bottom-up method is that various toxic impurities are added to the final products which is toxic to normal cells [103]. The use of chemical additives in the bottom-up approach for the production of crystalline CaCO\({}_{3}\)NP has a disadvantage because it affects the final chemical composition of the nanoparticle produced [98]. An example of such a method is the formation of needle-like crystalline vaterite CaCO\({}_{3}\)NP in the presence of gold nanoparticles and Mg\({}^{2+}\) and the formation of calcium carbonate amplified with organic thiols, fatty acids and polymer [114; 115], thus this combination is usually not suitable for biomedical use due to the associated non-biocompatibility and toxicity reports [116]. Hence, the bottom-up approach is regarded not appropriate for some specific biomedical research. Scientists from different fields aspiring to synthesise great quality aragonite nanoparticles with biomedical importance have adapted the top-down approach with continuous modifications and improvements, which recently attracted attention due to the tremendous striking properties of aragonite nanoparticles. As synthesis of CaCO\({}_{3}\)-aragonite from cockle shells are mainly performed using top-down (destructive) method perhaps this method allows CaCO\({}_{3}\)-aragonite to be obtained in their natural forms by retaining and protecting their great unique features [11] since the method involves diminution of bulk material to micron size and finally to the desired nanoscale size particles [40]. Mechanically synthesized CaCO\({}_{3}\)-aragonite nanoparticles have superior advantages over the chemically synthesized material since they are more biocompatible with cells with little or no toxic and immunogenic effect as earlier reported in previous studies [10; 20; 91]. This promotes their wide biomedical applications such as; drug delivery as a carrier, in diagnostic imaging as demonstrated by [PERSON] et al. [22] and as a scaffold in bone tissue remodelling by [PERSON] et al. [117]. Mechanical milling among all types of top-down method is the most extensive and widely used approach for the production of various nanoparticles, others are laser ablation, sputtering, nanolithography, and thermal decomposition [40].