Role of Micro and Nano Motors in Therapeutics and Diagnostics

Introduction

Micro and nanomotors are the devices capable of converting energy into movement to propel themselves in the biological environment. The locomotion of nanomotors are due to fuel or energy (Baeza & Vallet-Regí, 2018). The integral components of nanomotors are engine and work unit which are responsible for providing kinetic energy and controlled response respectively. Based on the mechanism used for propulsion, micro and nanomotors can be classified as fuel powered motors and fuel free motors. If the energy required for propulsion from using chemicals it is called fuel powered motors. Fuel free motors are driven my magnetic, electric, light, acoustic and living organism driven. Ultrasound propelled nanomotors are the attractive alternates with high precision delivery (Kiristi et al., 2015). The nanomotors can also be specifically used for detection and sensing. A self-propelled gold/ nickel/ polyaniline/ platinum nanomotor, coupled with concanavalin A lecitin bioreceptor is proved to be useful for isolation of E. coli (Campuzano et al., 2012). A nanomotor functionalized with anti-carcinoembryogenic antigen monoclonal antibody is used for detection and identification of tumor cells, as CEA antigen is highly found antigen in gastric, colorectal and pancreatic tumour (Balasubramanian et al., 2011). Moreover, functionalized nanomachines with ssDNA can been used to clearly isolate the required molecule from biological fluid. Further development in these topics can help in the development of Lab on chip devices that can be used for super-fast screening and clinical diagnostic applications (Daniel et al., 2011). In the above cases the sensing capability of nanomotors and micromotors have been exploited to develop diagnostics and sensing tools.

Multiple nanomachines have been investigated for their properties to transport drugs, acting as carrier. Microspheres, nanospheres and lipospheres have already been subjected to studies regarding drug delivery. These nanocarriers requires fuels to propel to site of delivery. Diallyldimethyl ammonium chloride stabilized PtN’s was coated in nanorockets at the inner layer and the outer layer was loaded with doxorubicin. Potential propulsion force of the nanomotor was able to effectively travel up to 30 cm (Zhiguang et al., 2013). In the presence of hydrogen peroxide as fuel, rolled up nanojets with a sharp tip has shown corkscrew like movement (Alexander, 2012), but the toxicity to the biological system eliminates its use. Thus, magnetically controlled nanojets are now under research to develop a fuel free motor that is also non-toxic (Xi et al., 2013). The nanorockets were also found to be as efficient as to penetrate Hela cells and release doxorubicin inside. Use of nanomotors in imaging will help us to obtain images of high details using the existing imaging systems. Catalase coated silica nanospheres was used for the inflammation relating to hydrogen peroxide can be converted to oxygen microbubbles under ultrasound, thus pinpointing the location of microbial abscess.(Olson et al., 2013). Employing metallic nanomotors has proved to be efficient in imaging as high detailed images which can used to generate more data can be generated with existing imaging systems (Martel et al., 2009). The motion of micro and nano motors are greatly influenzed by their geometric structures. Nano tubular structure, microspheres, Nanorods and asymmetric branches are the commonly employed structures (Abid et al., 2011; Cameron et al., 2018; Evans et al., 2013; Li, Wu, Qin et al., 2016; Moo et al., 2016; Tian et al., 2018; Zeng et al., 2015). Microspheres vary from 0.1 to 100 micrometers. The velocity of microsphere is higher than the other geometric structures. The mechanism of nanorod is self-electrophoretic propulsion. Tubular structures follow bubble induced propulsion mechanism (Braun et al., 2018) and symmetric structures follow light driven photo-electro chemical reaction mechanism (Zhan et al., 2018).

Nanomotors can be used to differentiate and localize cell using their capacity to sense hydrogen peroxide, temperature, and water. Microspheres, nanospheres and lipospheres have already been subjected to studies regarding drug delivery. These nano carriers require fuels to propel to site of delivery. Further, nanorockets are also found to have efficient penetration ability and release drug into the targeted site. Moreover, multiple nanomachines have been investigated for their properties to transport drugs and acting as a carrier.