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This Robot Look & Swims Like Sperm To Deliver Drug Inside Your Body

Imagine a tiny robot that swims like a real sperm cell, changing its movement whenever it encounters a thicker or thinner liquid. Even more fascinating, this microscopic machine can switch between two different swimming styles to maintain its speed. What sounds like science fiction is now becoming a reality.

Scientists have developed a soft robotic sperm that can imitate the remarkable swimming abilities of biological sperm cells. The tiny robot can switch between two swimming patterns, known as planar and helical propulsion, allowing it to move more efficiently through liquids with different viscosities. This technology could open new possibilities for targeted drug delivery, medical diagnosis, and other advanced biomedical applications.

How Do Real Sperm Cells Swim?

Sperm cells are among nature's most interesting microscopic swimmers. A typical sperm cell measures approximately 40–50 micrometres in length and consists of a head and a long, flexible tail called a flagellum.

Unlike larger swimming organisms, sperm cells cannot move by simply pushing water backward with powerful movements. Instead, they use waves travelling along their flexible tails to generate forward motion.

Interestingly, their swimming patterns can change depending on the physical properties of the surrounding fluid, particularly its viscosity. Viscosity describes how thick or resistant to flow a liquid is.

For example, swimming through a thin liquid is different from moving through a thick, sticky fluid. To deal with these changing conditions, some biological sperm cells can modify their flagellar movements.

Researchers have observed that certain sperm species switch between planar and helical swimming patterns as the viscosity of their surroundings changes. These adaptations help them maintain effective movement under different conditions.

This natural ability inspired scientists to develop a robotic version that could perform similar movements.

Introducing the Soft Robotic Sperm

A research team led by Khalil developed a microscopic soft robot designed to reproduce the swimming behaviour of biological sperm cells.

The artificial swimmer consists of two main components:

  • Magnetic head: A tiny head that responds to externally applied magnetic fields.

  • Flexible flagellum: An extremely thin, soft tail that bends and moves to generate propulsion.

The robot is controlled using external magnetic fields in the millitesla range. By changing the direction and movement of these fields, researchers can control how the robot's head moves and how waves travel along its flexible tail.

Unlike earlier microrobots that could perform only one swimming pattern, this new design can switch between two different propulsion modes.

This is an important development because the ability to change swimming patterns gives the robot greater flexibility when navigating through liquids with different physical properties.

Two Swimming Styles That Make the Robot Faster

The most interesting feature of the robotic sperm is its ability to switch between planar and helical flagellar propulsion.

1. Planar Propulsion

In planar propulsion, the robot's head moves back and forth within a single plane. This movement creates a wave that travels along the flexible tail, pushing the robot forward.

The resulting movement resembles the side-to-side swimming motion commonly associated with sperm cells.

2. Helical Propulsion

In helical propulsion, the robot's head performs an out-of-plane wobbling movement. This creates a wave that travels along the flagellum in a three-dimensional, spiral-like pattern.

Instead of moving within a single flat plane, the flexible tail produces a helical wave that helps the robot move through the surrounding liquid.

By controlling the magnetic field, scientists can switch between these two movements and study how each affects swimming speed.

How Does Switching Swimming Patterns Improve Speed?

To understand the robot's performance, researchers combined theoretical modelling with laboratory experiments.

They developed a mathematical model based on a scientific approach called resistive-force theory. This method helps scientists understand how tiny flexible objects move through fluids and how the surrounding liquid resists their motion.

The team then tested the robotic sperm in silicone oils with different viscosities.

Their results revealed that helical propulsion could provide a significant speed advantage over planar propulsion at relatively low actuation frequencies.

The improvements were particularly interesting:

  • In silicone oil with a viscosity of 1 Pa·s, helical propulsion made the robotic sperm approximately twice as fast as planar propulsion.

  • In silicone oil with a viscosity of 5 Pa·s, the helical swimming pattern increased its speed by approximately 1.2 times compared with planar propulsion.

These results demonstrate that changing the shape and movement of the flagellum can significantly influence how efficiently a microscopic robot travels through a liquid.

However, helical propulsion is not always the best option.

At higher actuation frequencies, the robotic sperm needs to switch from helical to planar propulsion to avoid a significant reduction in swimming speed.

This behaviour is similar to the adaptive swimming strategies observed in biological sperm cells.

Why Is This Discovery Important?

One of the biggest challenges in developing microscopic medical robots is controlling their movement inside the human body.

Different bodily fluids have different physical properties. Their viscosity can vary depending on their location and composition. A microscopic robot designed to travel through these environments must be capable of adapting its movement to changing conditions.

Traditional microrobots often have limited flexibility because their structures are relatively rigid or their magnetic orientation is fixed. As a result, changing their swimming patterns can be difficult.

The new soft robotic sperm addresses this limitation by combining a flexible structure with controllable magnetic actuation.

Its ability to switch between two swimming modes could help researchers develop microrobots that move more effectively through complex biological environments.

In the future, similar technology could support several medical applications, including:

  • Targeted drug delivery: Microscopic robots could potentially transport medicines to specific locations inside the body.

  • Medical diagnosis: They could help explore difficult-to-reach areas and deliver diagnostic tools.

  • Assisted reproduction: Advanced sperm-inspired microrobots could contribute to technologies designed to assist sperm movement.

  • Targeted therapy: Researchers could investigate using these robots to deliver therapeutic materials directly to affected tissues.

These applications are still under development and require further research before they can become practical medical treatments.

A Step Towards Smarter Medical Microrobots

The development of this soft robotic sperm represents an important step towards creating microscopic machines that can adapt their movements to their surroundings.

By demonstrating controlled switching between planar and helical propulsion using the same robot, Khalil and his team have addressed a major challenge in microrobotics.

The research shows that swimming speed is not determined only by how quickly a robot moves its tail. The pattern of movement also plays an important role.

Inspired by the natural abilities of sperm cells, scientists are now exploring ways to build artificial swimmers that can respond more effectively to different environments.

Although much work remains before these microscopic robots can be used widely in medicine, this research brings us closer to a future in which tiny machines could navigate the human body and perform highly targeted medical tasks.

The fascinating lesson from nature is that sometimes moving differently, rather than simply moving faster, is the key to reaching a destination.

Reference: Khalil et al., "Controllable switching between planar and helical flagellar swimming of a soft robotic sperm", Plos One, November 2, 2018. https://doi.org/10.1371/journal.pone.0206456

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