| dc.description.abstract | Artificial microscale robots carrying out biomedical tasks inside the human body is an
important research goal across the entire scientific community. Fuelled by recent advancements
in nanotechnology, there has been a tremendous development in the field of micro/nanoscale
robots which are pushing the boundaries of biomedical technologies. Due to their small size,
they require special strategies to achieve locomotion in fluidic media and, in many cases, these
strategies are adopted by mimicking the motion of natural microorganisms like bacteria, algae
and spermatozoa. These artificial micro/nanorobots, as we discuss in this thesis, have
tremendous potential for future biomedical applications in sensing, targeted delivery, nanosurgery, and detoxification.
Powering an artificial micron-scale system is a challenging task, and hence, researchers have
been exploring numerous ways to induce motility in these tiny artificial systems, including
chemical, magnetic, light and electrical methods. In this thesis, we are going to focus on various
applications of magnetically actuated helical microscale objects, termed ‘microswimmers’.
Owing to their helical structure, the microswimmers can be maneuvered in any direction in 3D,
due to the rotational-translational coupling when it is actuated by a rotating magnetic field.
Fabrication of these helical microswimmers requires complex nanofabrication technologies.
We have discussed various techniques in detail that we use to fabricate the helical
microstructures. We start with a seed layer that decides the thickness of the helix, and therefore,
it is important to achieve seed layers of different sizes for varying the thickness of the helices.
On the other hand, the height can be controlled as required depending on the amount of
deposited materials. Once the helical structures are fabricated, we incorporate a thin magnetic
material to render them magnetically active for further experiments.
In this thesis, we have explored the possibility of these microswimmers to be used as a
microrheological probe. Exquisite control on positioning along with the ability to sense the
local rheological properties with high spatio-temporal resolution allows its applicability in the
mechanical mapping of a heterogeneous environment. We have utilised the dynamics of a
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microswimmer under rotating magnetic fields in fluidic media to analyse the rheological
properties. This technique can be faster than conventional passive microrheology techniques,
which can allow the study of rapidly changing environments, for example, mixing of fluids,
with very high accuracy.
Next, we have attempted to replicate the rheological studies in an intracellular environment,
which is a challenging environment undergoing constant reorganisation. We classify our
studies along three objectives. We first demonstrated spontaneous internalization and
controlled maneuverability of the microswimmers inside living cells. We find similar results
in three different cell lines, including cancerous and non-cancerous cells that we use in our
experiments confirming the generality of the observations. Thereafter, we have performed
detailed studies to understand the physical parameters involved in internalization and expulsion
of microswimmers, including direct observation of internalization and expulsion of
microswimmers by cells. Finally, we validate how one can sense in the differential intracellular
environment from the dynamics of the swimmers.
In the following section, we have shown how it is possible to manipulate the entire cell using
an internalized microswimmer using magnetic actuation, which is relevant to single-cell
manipulation applications. A microswimmer which is capable of maneuvering in an
intracellular matrix along with the potential to manipulate an entire cell can open up the
possibilities in studies related to phenotype heterogeneity like tumour metastasis, drug
resistance, stem cell differentiation, as well as, cell signalling dynamics and targeted therapy.
Finally, we have investigated the fluidic interactions of a rotating helix. Basic understanding
of fluid flow for a single helix is extremely crucial for the future studies toward many-particle
dynamics of a swarm of helices and also understanding magnetically powered artificial active
matter system. We have used particle image velocimetry to obtain the fluid profile
experimentally around a rotating helix, and the results are supported by numerical simulations
and analytical modelling using the singularity solution of Stokes equations. | en_US |
