Creating nano and micro-scale robots to assist biomedical interventions in humans is a relatively young research field receiving copious amounts of interest within scientific research and Sci-Fi.
Microbots in Fiction
Experiments with miniature machines have been successful in lab so far under externally controlled conditions, yet the concept of “swallowing the surgeon” as envisioned by physicist Richard Feynman (>50 years ago) remains to be realized. The far-reaching role of synthetic micromachines as biosensors, drug distributors or microsurgeons inside the human body is, therefore, a 20th-century dream that engineers are currently ready to tackle. Scientists have so far designed and engineered microbots, micromotors and DNA-origami-based nano-robots with the exciting possibility of programming them to eventually travel through the human body and perform medical tasks - much like they do in the classic films (minus a crew of miniature humans).
Microbots Developed in Science
Tiny machines engineered thus far can be powered by chemical reactions, physical magnetic fields or biohybrid methods (biological cells combined with synthetic structures) to complete intended medical tasks. Three main challenges that micro-machines face while traveling through the human body are; 1. Precise, active transport in the third dimension (3D), 2. Biocompatibility and 3. The capacity to track precise location while within the human body.
Existing medical imaging techniques are not sensitive enough to track micromachines operating deep inside the human body, while biocompatibility of micromachines require they be non-toxic while also displaying active biological interactions when completing programmed tasks within humans. This led to collaborations among microrobotics researchers, materials scientists, bioimaging and medical specialists to solve specific problems and introduce better imaging methods, with some exciting results seen in the lab.
Sperm-bots – For Assisted Fertilization
Male infertility is largely due to immotile sperms, so scientists engineered helical micromotors functionalized by a magnetic field for assisted motion, to fertilize an egg (oocyte) in a petri dish at the lab. The experiment was inspired via biomimicry of nature-approved propulsion strategies coupled to an external source like a magnetic field to direct the sperm carrying microhelices. While experimental conditions were optimized to ensure favorable capture, transport, and release of live sperm cells without harming their constitution in the lab, free control of their motion inside the human body requires further improvement.
Imaging is key for controlled navigation through the uterine cavity and oviduct for assisted fertilization in the clinic. This strategy broadly applies to the varying roles of micromotors not just as sperm-bots, but also as drug carriers or biosensors to navigate through blood vessels and confined elastic surroundings.
Nano-Jets and Micro-jets - Catalytically Powered Tools
Researchers also looked at mimicking mechanisms of natural biological nanomotors that convert surrounding chemical energy into mechanical energy – as seen with cargo transport along microtubules in cells and with flagella-based bacteria propulsion. This led to the creation of the world’s smallest man-made jet engine that shrunk macroscale jet propulsion to the nanoscale subsequently awarded the Guinness World Record in September 2010. Fabricated with a magnetic, semiconductor/metal layer tube, the nano-jets catalyze surrounding medium of hydrogen peroxide to form oxygen bubbles for self-propulsion in a trajectory that can also be controlled via an external magnetic field. While these “smart and powerful” machines are expected to perform several useful tasks including intelligent surgery, drug delivery, and lab-on-a-chip biosensing, as long as their function depends on toxic fuels such as hydrogen peroxide they will be limited for applications outside biological organisms until a favorable fuel alternative is developed.
To ensure effective activity of miniature machines within humans a mechanism should also be implemented to stop or remove them. The advantage of biodegradability is that micromachines don't need removal as they can effectively dissipate once the task is complete, avoiding an immune reaction from prolonged retention within humans.
Nano-Robots - Created by DNA Origami
In a different experimental approach, scientists used a technique known as DNA origami to fold DNA and make programmable structures that can seek out and destroy cancer cells. Researchers named these structures nano-robots due to their size and capacity to perform programmed robotic tasks. Barrel-shaped devices designed and built using DNA origami, have 12 sites on the inside to carry the cargo and two positions on the surface to recognize molecules on the target cell. Once the lock (DNA origami device) finds the key (cancer cell), the device opens to release the cargo within (antibody) and kill the cancer cells.
Since nano-robot devices are biological in composition, they can be easily cleared after the task is complete. This work takes one more step along the path of smart drugs and micro/nano machines to realize the kind of medical microsurgeons of the 20th-century dream.
If the existing technical challenges of microbots and nano-robots as briefly detailed here can be overcome, an envisioned era of non-invasive therapies are predicted to go forward within a decade.
This article is primarily based on a report by Mariana Medina-Sánchez & Oliver G. Schmidt published in Nature|Comment May 2017, available via | doi: 10.1038/545406a