BactoBot: Bacteria-Inspired Soft Underwater Robot

• BactoBot is a bacteria-inspired soft underwater robot that uses 12 flexible silicone arms on a 3D-printed dodecahedral frame for gentle marine exploration.
• It passively adapts arm shape via hydrodynamic forces to mimic bacterial flagellar propulsion, ensuring safe interaction with delicate underwater habitats.
• Built with accessible DIY methods and open-source electronics, its freshwater tank tests validate its cost-effective and eco-friendly motion capabilities.

BactoBot is a low-cost, bacteria-inspired soft underwater robot developed for safe and gentle marine exploration. Rather than using rigid propellers or thrusters, it abstracts bacterial flagellar propulsion at the macroscale through 12 flexible, silicone-based arms mounted on a 3D-printed dodecahedral frame. The prototype was fabricated with accessible DIY methods, including food-grade silicone molding, 3D printing, and off-the-shelf microcontrollers, and was validated in a controlled freshwater tank, where it demonstrated forward motion and turning. In the literature, it is positioned as an environmentally conscious and accessible platform for marine science, especially in resource-constrained settings (Chowdhury et al., 25 Sep 2025).

1. Motivation and design rationale

BactoBot was conceived in response to limitations of conventional underwater vehicles such as ROVs and AUVs, which are described as rigid, heavy structures driven by high-speed propellers or thrusters and as expensive and equipment-intensive. Those properties make them effective for inspection, construction, and related industrial tasks, but problematic in coral reefs, seagrass beds, shallow cluttered environments, and other fragile marine habitats. The central design objective is therefore twofold: to reduce the risk of physical damage to delicate ecosystems and to lower the cost and fabrication barrier for underwater robotics.

The platform addresses these issues by using soft, flexible silicone arms instead of rigid propellers, by relying on inherently compliant contact behavior, and by being manufacturable from low-cost, widely available materials and electronics. The project also emphasizes open-source designs for replication and extension. This combination gives BactoBot an explicitly ecological and practical orientation: it is intended not only as a soft robot, but as a soft robot that can be built in university laboratories or hobbyist settings using typical 3D printers, Arduinos, and readily available silicone.

A common misconception is to treat BactoBot as a generalized replacement for industrial underwater vehicles. The reported prototype instead targets safe interaction and affordability. Its significance lies less in raw performance than in demonstrating that a biologically inspired propulsion principle can be realized as an accessible, compliant underwater platform.

2. Bacterial inspiration and hydrodynamic abstraction

The biological template for BactoBot is bacterial flagellar propulsion. Many motile bacteria, including E. coli, swim with one or more flagella: long flexible filaments driven by rotary motors embedded in the cell wall. The rotating filament adopts a helical, corkscrew-like form and generates thrust against the surrounding fluid. In bacterial locomotion, the relevant regime is characterized by low Reynolds number, conventionally written as

$\mathrm{Re} = \frac{\rho U L}{\mu},$

where $\rho$ is fluid density, $U$ is characteristic velocity, $L$ is characteristic length scale, and $\mu$ is dynamic viscosity. For microscale organisms, $\mathrm{Re} \ll 1$, so viscous forces dominate and locomotion depends on non-reciprocal shape change rather than inertial coasting.

BactoBot does not replicate that regime directly. It operates at macroscale in water, so its Reynolds number is not comparable to that of bacteria. The paper instead describes an abstraction of bacterial locomotion: flexible arms are attached to rotary DC motors; the arms begin straight; during underwater rotation, hydrodynamic drag bends and twists them until they stabilize into a helical configuration; that configuration then acts as a screw propeller and generates thrust approximately along the arm axis (Chowdhury et al., 25 Sep 2025).

The important technical point is that the helix is not machined into the arm. It is produced passively through fluid-structure interaction. This suggests a design strategy in which compliance and hydrodynamic loading replace geometric precision in the propulsor itself. The biological analogy is therefore principled but selective: BactoBot preserves the basic mechanism of a rotating flexible filament while scaling it to robot size and common materials.

3. Mechanical architecture and materials

The main body is a 3D-printed dodecahedron, with 12 regular pentagonal faces and one flexible arm mounted per face. The frame is printed from PETG using standard FDM printers. The dodecahedral geometry is used for symmetric placement of the 12 arms, isotropic arm coverage in three-dimensional space, and structural compactness with relatively short edge lengths. The paper further associates this arrangement with the potential for omnidirectional movement, since independently controlled arms could in principle generate arbitrary thrust vectors.

Each arm consists of a food-grade silicone rubber body and an embedded rigid PETG hook at the base, which couples the arm mechanically to the DC motor shaft. The arm is molded straight rather than as a fixed helix. When rotated underwater, distributed fluid drag induces bending and twisting, and the arm settles into a stable helical shape. The paper does not specify exact dimensions or measured mechanical properties for the arms, but it notes that typical food-grade silicones used in molding have Shore A hardness between 20 and 40 and are highly flexible.

The compliance of the arms is central to the robot’s safety profile. On contact, the silicone structures bend rather than imposing concentrated impact loads, which reduces peak stresses on both the environment and the robot itself. Arm placement is roughly radial from the dodecahedron center, and opposing faces define opposing arm pairs. That symmetry supports balanced thrust for translation and differential thrust for turning.

Another common misunderstanding is to equate the dodecahedral layout with demonstrated omnidirectional mobility. The paper is more limited: omnidirectional mobility is presented as a potential property of the geometry, whereas the reported prototype, because of paired actuation, primarily demonstrates forward or reverse translation and yaw rotation.

4. Actuation, control, and DIY fabrication

BactoBot uses 12 high-torque DC geared motors, one per arm, mounted inside the dodecahedral frame. Because of space constraints, the motors are wired in opposing pairs. Each pair is driven by a single BTS7960 high-current H-bridge driver, so the system uses six BTS7960 modules in total. Power is supplied by an 11.1 V 3S LiPo battery, while a buck converter provides a regulated 5 V rail for logic and control electronics. The central controller is an Arduino Mega 2560, selected for its I/O capacity and programming simplicity (Chowdhury et al., 25 Sep 2025).

This architecture is explicitly a trade-off. Pairing motors reduces driver count, wiring complexity, and cost, but it removes individual motor control and therefore limits gait richness. Control is open-loop: there are no feedback sensors, no onboard state estimation, and no closed-loop thrust regulation. Forward motion is generated by commanding opposing arm pairs so that their thrust components align while net body torque is approximately canceled. Turning is generated by asymmetric actuation of subsets of arms to create yaw torque.

The fabrication workflow is similarly pragmatic. The frame was designed in SolidWorks and printed in PETG. Mold tools for the silicone arms were also CAD-designed and 3D-printed. Silicone preparation required accurate two-part mixing by weight, dry molds, correct hook placement, and full curing under the manufacturer’s specified conditions. The paper reports early failures caused by moisture in molds and incorrect mixing ratios, after which a strict molding protocol was adopted. Material selection also evolved during prototyping: ABS hooks were abandoned because of brittleness and frequent failure, and PETG was retained for improved durability and water resistance.

The reported system is therefore not only a robot design but also a fabrication method. Its technical contribution includes the claim that a soft, biologically inspired underwater robot can be assembled from relatively ordinary digital-fabrication tools and commercially available electronics.

5. Waterproofing, buoyancy calibration, and experimental validation

Waterproofing was identified as a central engineering challenge, especially around rotating shafts. The solution is a multi-layer sealing strategy. Static joints and seams are sealed with marine-grade sealant. Critical electrical entry points, including wire feedthroughs and motor housings, are sealed with Araldite epoxy and hot glue. Most importantly, each rotating shaft uses a multi-O-ring system housed in a grease-filled chamber, where the grease both lubricates the O-rings and fills microgaps. The paper states that this protocol prevented water ingress during extended tank tests (Chowdhury et al., 25 Sep 2025).

Buoyancy calibration was based on Archimedes’ principle. The buoyant force was written as

$F_b = \rho_{\text{water}} g V_{\text{displaced}},$

and neutral buoyancy was enforced through the condition

$m = \rho_{\text{water}} V.$

The robot volume was estimated from SolidWorks, then ballast was added iteratively in a freshwater tank. The procedure began with six 500 g weights, followed by two 1 kg weights, and continued until the robot neither rose nor sank significantly. The final tuned mass was approximately 11.25 kg. Ballast placement was then adjusted so that the center of mass aligned appropriately with the center of buoyancy, suppressing unintended roll and pitch.

Experimental evaluation was conducted in a freshwater tank under open-loop Arduino control. The reported outcomes are qualitative rather than fully quantified: the paper does not provide numerical speed, turning radius, or energy-consumption measurements. It does report three principal results. First, no water ingress occurred in the electronics during extended operation. Second, the robot demonstrated clear forward motion and turning maneuvers. Third, the final ballast configuration yielded stable attitude without undesired roll or pitch. These results validate the feasibility of bacteria-like flagellar propulsion realized through low-cost DIY methods, the viability of the multi-layer sealing system, and the basic effectiveness of the paired-actuation strategy.

6. Ecological role, limitations, and development trajectory

BactoBot is explicitly framed as a platform for ecologically sensitive marine work. Its soft arms, absence of sharp blades, and avoidance of high-speed jets make it attractive for coral reefs, seagrass beds, underwater caves, and other cluttered or delicate habitats. The paper further identifies marine science and conservation, shallow-water survey, education, NGOs, and universities in developing regions as natural use cases. The open-source orientation reinforces this accessibility claim (Chowdhury et al., 25 Sep 2025).

Within soft underwater robotics, the project occupies a specific niche: macroscale flagellate locomotion implemented as a low-cost and replicable system. The paper notes that bio-inspired underwater robots have often focused on fish-like, jellyfish-like, turtle-like, or octopus-like locomotion strategies, whereas BactoBot is centered on flexible rotating filaments. It also notes that related flagella-inspired designs have tended to be more specialized and expensive, while this platform prioritizes DIY accessibility.

Its present limitations are substantial and are acknowledged as such. The robot is telecommand open-loop, with no IMU or exteroceptive sensing, so navigation precision is limited and station-keeping is not available. The 12 motors are wired as six parallel pairs, which under-actuates the system and prevents truly independent omnidirectional gaits. The prototype has been demonstrated only in calm tank conditions, and its performance in real marine environments with currents, waves, and debris remains unknown. Dynamic shaft sealing, long-term material fatigue, and electronics reliability remain open engineering questions. The paper also notes that an earlier driver choice, TB6612FNG, failed, and that the BTS7960 solution, while more robust, adds bulk.

The development roadmap is correspondingly clear. The authors propose integrating an IMU for attitude estimation and heading stabilization, along with a PID controller of the form

$$u(t) = K_p e(t) + K_i \int e(t)\,dt + K_d \frac{de(t)}{dt}.$$

They also propose adding cameras, sonar, depth sensors or pressure transducers, and chemical sensors; revising the actuation architecture to permit per-motor control; optimizing arm geometry, stiffness, and fluid-structure interaction; improving shaft seals and pressure resistance; and conducting field trials with quantitative measurement of speed versus PWM, turning radius, and mission energy consumption. The central implication is not that BactoBot is already an autonomous marine robot, but that it establishes a physically validated foundation for one.