▼ HARDWARE ▼

Overview

This section aims to record all procedures and products made by the Hardware division of our iGEM team. The hardware we created is capable of storing the modified HotPETase to act as a bioreactor, and to cycle water through a centrifugal pump, thus allowing microplastics to enter the chamber within the model and be digested.

Magnetic Centrifugal Pump

1.1 Overview

Magnetic centrifugal pumps are specialized pumps that combine the principles of centrifugal fluid movement with magnetic coupling technology to achieve efficient fluid transfer. The benefits of these types of pump eliminates the need for mechanical seals, which can reduce the risk of leakage. In addition, with the limited moving parts (impeller and magnetic drive assembly), the pump requires relatively less maintenance compared to traditional centrifugal pumps. For our model, we have used a magnetic centrifugal pump of 220-240V, and a head of approximately 1.8 meters.

1.2 Internal Structure

The pump consists of a front casing, spindle, O ring, impeller, rear casing, drive magnet, and motor. Refer to the below diagram for further details

Figure 1. A diagram showing the Magnetic Centrifuge Pump

• Front casing: The entry point of the fluid. Its spiral-shaped diffuser slows down the fluid leaving the impeller, increasing the internal pressure. A hose connection is used for easier attachments of pipes.
• Impeller and spindle: The impeller's vanes are curved to sling fluid towards the pump discharge leading to the front casing. The spindle holds the impeller at a fixed axis, maintaining alignment and minimising wear and tear.
• O ring: Acts as a static seal between the impeller and rear casing, preventing fluid from seeping through the gaps.
• Rear casing: A thin, stationary wall which is non-magnetic. It supports the spindle and keeps the pumped fluid from being in direct contact with the motor. The magnetic force drives through the casing to spin the impeller instead of blocking the magnetism.
• Drive magnet: Created the rotating magnetic field to drive the pump. The casing is specifically designed to retain its magnetism permanently such as with the use of neodymium. The magnet spins alongside the motor.
• Motor: The power source of the pump. It converts the electrical energy into rotational force, allowing the impeller to spin.

3D Model

2.1 Chamber

We have made a 3D model with the platform Blender to show a more comprehensive view of our design. Its purpose is to store the enzymes for PET degradation and as a container to hold the circulated water.

• Upper section: Used for collected water flowing down the stream. We have adopted an inverted funnel approach to increase the collection surface area and utilized the Venturi effect to create a low pressure zone after passing through the constriction. The top hole is 15cm in diameter and the constriction is 5cm wide, allowing for a 3:1 ratio for the Venturi effect. The component is removable.
• Middle section: Used to house the enzyme. The chamber is 22.5 cm in diameter and stands at a height of approximately 20cm. This allows for a larger volume of water and enzymes to be stored. A discharge tube is also installed near to the bottom and is connected to the centrifugal pump.
• Bottom lid: A detachable lid drilled with a hole to allow the unwanted products to leave the chamber.

Original design:

Figure 3. The original designs of the hardware

Layout:

Figure 4. Animation for Hardware

2.2 Sieve

The sieve acts as the container for calcium alginate beads, and is the first layer of the components that the water passes through. To prevent the heavy metals in water from contaminating the bioreactor, the sieve, 20cm in diameter and 4cm in height, acts as the filtration component of the hardware, allowing for the bioreactor to be used for an extended duration.

Figure 5. A sieve

2.3 Bearing

Figure 6. A Bearing

The bearing is installed on the surface of a metal frame, which allows for sufficient distance between the blades of the stirrer and the frame in order not to hinder the blades' movement. In addition, a flexible plastic indentation is inserted under the bearing to hold onto the stirrer. The frame is lodged on 3 plastic supports with the purpose of setting a fixed distance between the frame and the bioreactor.

2.4 Stirrer

Figure 7. A Stirrer

The stirrer comprises 2 components: The upper part (white) and the lower part (yellow). The upper part takes the brunt of the force of water and spins under the constant flow of water. The upper part is then glued to a tough PVC tube connected to the lower part, which spins along the upper part and stirs the products post-reaction.

2.5 Bioreactor

The bioreactor stores the modified PETase used for digestion. A removable lid allows for replacement of PETase when needed, along with a fine wire gauze to prevent the viscous PBS solution with bacteria from leaking while simultaneously allowing microplastics to enter the bioreactor and be digested. The resultant products can then leave the bioreactor and be circulated by the pump repeatedly.

2.6 Piping

Figure 8. Piping

The piping consists of 3 main elements.

• L-pipe bends: Needed due to the difference in the elevation of the discharge tube and the mouth of the pump. Allows the system to curve and maintain a constant inner diameter. The white bends can be rotated while the gray bends are more suitable for fitting the tough PVC tubes. However, the gray bends require silicone sealants to prevent leakage of water while the white ones do not.
• Soft PVC tubes: The translucent tubing connects the tough PVC tubes with the mouth of the pump. The flexibility of the tubes offer easier insertion of the piping system and can be tightened by using hose clamps.
• Tough PVC tubes: These tubes extend the length of the system and act as the pathways connecting the top and bottom of the chamber with the pump.

2.7 Valve

Figure 9. Valve of the Hardware

The valve allows the contents within the chamber to be collected and released, allowing the products to be obtained more conveniently.

2.8 Calcium alginate beads

Calcium alginate beads have been prepared to remove heavy metals that are present in the collected water, which could contaminate the modified PETases present in the bioreactor. Therefore, the layer of beads serves as the 1st layer of filtration of the model to minimize the effect of heavy metals towards the bacteria.

2.9 Abandoned components

The components were originally planned to be used for various functions, but were later deemed unnecessary and were not included in the final design.

• Frustum debris separator: A durable protective case installed on the top of the wire gauze. The inner diameter of the top hole is 10cm while the inner diameter of the bottom hole is 15cm. The frustum is made with stainless steel to prevent rusting and make it withstand the brunt impact of large debris. The frustum is supported by wooden beams, providing gaps for debris to fall off the layer of wire gauze if the debris enters through the top hole of the frustum.
• Wire gauze: A circular network of metal wires for filtering placed in the mouth of the chamber. 3 stacks are used to minimize the amount of solid contaminants to enter the chamber.

Contributions to Society

When tackling microplastic pollution in the ocean, two main issues usually rise as a result: Firstly, PET degradation produces unwanted products such as ethylene glycol, which biodegrades rapidly in the aquatic environment and thus may deplete the oxygen content of the nearby areas. In addition, ethylene glycol can also cause toxicity and organ damage when ingested by animals[1], invalidating the purpose of the modified HotPETase. Secondly, passing the water through the bioreactor once may not completely digest the microplastics, resulting in an inefficient deployment of the fusion constructs. Our hardware solves these problems in 2 ways: We have made a valve at the bottom of the chamber, which allows the reacted produced to be kept and stored within the chamber according to the users' need. Needless to say, the piping system and the gaps between components have been sealed shut using silicone sealants and transparent waterproof coating, preventing any harmful byproducts from seeping out into the ocean. To tackle the problem of incomplete digestion, the centrifugal pump mechanism allows the water stored within the chamber to be cycled repeatedly, resulting in a greater degree of digestion over time.

The components for the hardware have been listed above. The shapes and materials of the components are not tailor-made nor require a long duration of time to be obtained. The chamber of the hardware is also empty, so the interior components are removable and can be altered depending on the intended purpose. For example, the fusion constructs in the bioreactor can be replaced with a fresh batch periodically. We aim to make the hardware to achieve our objective in the simplest possible manner not only for reproducibility, but also to incorporate flexibility into our design. In the future, we intend to install much more features into the hardware, including a semi-permeable membrane to solely trap microplastics and sensor-controlled valves to automatically seal the entry point of water. Therefore, the simplicity of the design can achieve its degradation purpose with greater efficiency.

Cost

The cost of the hardware is listed as followed:

Part	Price (In HKD)
All sections of the chamber	1000
Magnetic Centrifugal Pump	600
Bioreactor	320
Flexible plastic indentation	200
Valve	13.70
4* Grey L-pipe bends	16 (4 per piece)
2* White L-pipe bends	3 (1.5)
2m of soft PVC tubing	40 (20/m)
2m of tough PVC tubing	34 (17/m)
Stirrer	28.5
Metal frame	13.5
Sieve	12
Bearing	10
Bioreactor	19
6* Plastic supports	8 (1.33 per piece)
4* Hose clamps	20 (5 per piece)
Total	2337.7

Our hardware significantly reduces the financial barrier for reproduction and offers an affordable option for people to use in their daily lives. This solution addresses the high costs required in order to recycle or treat plastic waste in the ocean.

Proof of Concept

In order to test for the functionality of the hardware, 2 tests have been carried out: The rotation of the stirrer and the emergence of water from the return. Together, these 2 tests will prove that the components have achieved their desired function.

Results

Figure 10. Animation for the Working of Hardware

Analysis:

Through these 2 tests, we have concluded that the stirrer can indeed spin under a water current and that the centrifugal pump is able to direct water to return to the chamber to achieve circulation, proving our hardware's functionality and designated purpose.

To improve the speed at which the stirrer spins, a L-pipe bend has been added to the return pipe and angled towards the plane of the stirrer's blades. This prevents most of the water current from sliding off the interior wall and leaving the blades untouched.