The idea of a swallowable surgeon or a fantastic voyager that can be ingested and undertakes various therapeutic activities like finding and killing cancer cells has motivated scientists to develop smart micro/nanoscale machines over the last decade. Such devices have since come a long way and have been demonstrated to deliver drugs inside the stomach of animals and to specific cancer cells.
Among the other feats achieved by these machines are artificial fertilization, cleaning polluted water and sensing and elimination of nucleic acids from a mixture of biomolecules. In a recent study, researchers from the Indian Institute of Science, Bangalore and Technion Institute of Technology, Israel have extended the capabilities of these nanomachines to perform sensitive mechanical sensing of the surrounding environment.
Local mechanical properties of fluids provide significant information about the physicochemical state of the fluid that has far reaching consequences in biophysics, microfluidics and gelation or polymerization studies. The current methods of doing these measurements, widely known as microrheology, involve a probe particle that undergoes active or passive motions in a fluid environment. This method is faced with two major challenges: there is no control on the position of the probe particle and are thus not suitable to characterize a heterogeneous fluid and secondly, the measurements take a long time because of extensive averaging techniques used.
In the present study, published in the journal Advanced Functional Materials, Ghosh, Dasgupta, Pal, et. al., used nanoscale helical machines, made of glass and a magnetic material, to do local measurements of the viscosity of a fluid mixture. The helical nanomachines could be precisely positioned using external magnetic fields in an untethered manner in a fluid medium and thus eliminates the first problem discussed above.
The swimming dynamics of these nanomachines that replicate the swimming of bacteria like E. coli, could be controlled using the external fields and also depend on the local viscosity of the medium in which it swims. Thus, a simple measurement of a swimming parameter of the nanomachine brings out the measurement of the local viscosity of the medium.
The measurement of the local viscosity could be done almost in real time owing to the instantaneous effect of the change in viscosity on the swimming parameter of the nanomachine. Not only that, the researchers also showed that these mobile viscometers could be moved around and create a viscosity map of a region which is several thousands of times larger than the nanomachine. Such measurements were performed across boundaries of fluids whose viscosities vary by almost 70 times.
The nanomachines could also perform viscosity measurements, over a wide range, in a different class of fluids, called shear thinning fluids, whose flow properties are somewhat similar to the blood flowing through our veins. Apart from that, the measurements were sensitive enough to pick up small variations in the viscosity resulting from small changes in temperature as well.
The technique of creating a viscosity map using these nanoscale probe that vary from 1 to 3 µm in size, can provide significant insight into microfluidic environments like mixing or co-flowing fluids, polymerization and gelation. It can bring out significant information of the changes in the physicochemical properties of the inside of a cell under the influence of a drug or various other physiological conditions. The technique also shows the potential of using nanomachines for in vivo detection of diseases using mechanical sensing as the measurement technique.
The group is currently working to perform these measurements in live cells, both healthy and cancerous ones and also in animal tissues. An obvious extension of the present work is to characterize other kinds of model fluids like viscoelastic fluids or gels, that are more ubiquitously found in biological systems.