General Considerations

The main premise behind the design of this unit was to ensure that both cost and maintenance were kept to a minimum, even if this is at the expense of operational efficiency. Due to the small scale of the unit and its application in both residential and industrial settings, means that it must be economically feasible to both purchase and then run. People are inherently lazy by nature and as such the maintenance of the unit must be a small as possible, otherwise few would be willing to invest in the technology.

Based on this information as many moving parts with the potential to break have been removed wherever possible including; mechanical agitators (seals and motor), ring Sparger, pumping for nutrient feed, water inlet and across filter membrane, a heating jacket and as many control systems as possible have also been removed.

Following this a slow release nutrient feed has been developed so that nutrient only needs to be input at the beginning of each batch. These batches have also been designed to last and extended period of time, thus reducing the required cleaning frequency of the unit and filters within.

The unit must remain small enough so that it can be of an acceptable size to fit under sink units in both residential and industrial settings. Such a challenge introduced the proposal of having a solid nutrient feed to reduce its size, as well as combining both the bioreactor filter and holding tank into a single unit to reduce piping requirements and system control in-between. Such an approach of combining units does reduce their individual and hence overall efficiency as well as their flexibility in terms of being able to isolate certain units during operation or commissioning. Since the unit will be operating at set rates and within set operational parameters the need for flexibility in operation is reduced.

The connections between the unit and the existing pipework have been selected to ensure that the initial fitting of the unit and its subsequent removal and re-fitting can be done as quickly and efficiently as possible without compromising the sterility of the unit, allowing bacteria to flow back into the existing water systems whilst keeping costs low.

Materials of Construction

Due to the biological nature of the unit and therefore its subsequent ties to pharmaceutical and medical products and devices, the materials looked into were those already approved for such applications. A key area of research was looking into the materials used in disposable Bioprocessing units, as these would give the best cost without affecting the credentials of the unit to operate effectively and in sterility.

The materials were chosen to withstand operational conditions that would traditionally be outside of its standard use, thus ensuring that when used correctly the unit shall withstand what is required of it with ease. The material cannot be allowed to be degraded during standard operation or during the more abrasive and aggressive cleaning procedures. Nor can they leach chemicals into the water supply as this may affect both the growth of the bacteria but also the composition of the downstream water, potentially creating a new unforeseen issue.

Materials must have the following properties;
● Non-porous (where applicable)
● Retards bacterial growth on surface
● Low protein affinity
● Abrasive
● Non-shedding
● Maintenance free
● Fully sealed – joint free preferably
● Sterile – silicone gaskets (if required)
● Non-cracking
● Cleanable
● Able to withstand a suitable load
By looking at materials already used in similar fields means suppliers and backup suppliers can be more easily found, allowing for a more stable and continuous production approach after development.

Bioreactor Walls

Based on the above information there were three clear front runners for the material to be used for the bioreactor walls; 316L Stainless steel, UHWPE, and HDPE.

316L SS is traditionally used in large scale bioreactors and in most large scale sterile unit operations due to its corrosion and heat resistance, ability to be used to fashion large units from a number of panels and is easily cleaned and maintained. However for such a small unit, with a limited cost outlay 316L would be a very expensive option, and would also limit the ability to visually inspect the tank, removing the need for float switches and level alarm systems, thus reducing the cost of the unit further.

UHWPE is very abrasion resistant and structurally strong, absorbs very little moisture, has a low coefficient of friction and is corrosion resistant. Thus lending itself to being put under various torsional loads during installation and being able to withstand contact with bleach during harsh cleaning cycles. Absorbing little moisture means the material shouldn’t lose any structural integrity nor will it negatively affect the process within the reactor to a great extent. Further testing will be required to determine just how effective this material would be in a submerged sterile environment; therefore it is difficult to trust a material that is relatively unproven. UHWPE is also an opaque material lending the same issue of visibility noted for the 316L SS.

HDPE is much like UHMWPE however it is slightly less resistant to corrosion, is transparent and is cheaper. HDPE is also a better documented material due to being around for a longer timeframe. As such the HDPE will be used as a compromise material to the UHMWPE due to being better documented, cheaper and allows for ease of internal visual inspections.

Future testing would be carried out to directly compare the products performance when fashioned from both HDPE and UHMWPE in conjunction with a full cost analysis including product lifetime, labour costs, energy costs and material and processing yields. A suitable supplier that could cope with supplying for the required unit forecast and maintains strict product quality and full traceability throughout manufacture.

Partially Permeable Membrane

The partially permeable membrane will be used to help provide O2 to the unit to allow the cells to respire aerobically, thus reducing the formation of by products, affecting yields and pH. The membrane needs to be sterile to ensure that the integrity of unit is maintained otherwise an open top reactor would have already been selected. The purpose of selecting a partially permeable membrane over using a pump is that it would also require a filter to maintain air sterility, unless compressed air is used, all of which increase the size of the unit, the relative maintenance required and its overall complexity and cost; hence have been rejected at this stage.

After searching materials that would be capable of achieving these parameters, a solution was found from an initially surprising source. Transparent dressing used in hospitals for various catheters (including I.V.) and to cover surgical incisions post operation. These materials have the following properties and characteristics making them ideal for this application;

● Colourless allowing ease of visual inspection inside unit
● Waterproof and breathable, allowing O2 to diffuse in, whilst trapping liquid inside the unit and preventing any contaminated water entering the unit.
● Used in medical devices – high grade, completely sterile and extensively safety tested – ISO 9001
● Prevents airborne and waterborne bacteria and viruses entering the bioreactor
● Shear resistant due to application in joint/ high movement areas, so should be sturdy enough to cope with transport and initial installation comfortably The potential risks to using a material such as this are as follows;
● The material does come provided with an adhesive back as standard, therefore production may become more expensive to omit the adhesive backing, as opposed to the standard material already produced on mass.
● The material is thin and therefore at risk being pierced. The unit is designed to be kept out of the way, so ensuring due care is taken with the unit this can be avoided. Another point to note is that the potential to make the material thicker to help prevent piercing would be unwise as this would drastically reduce the membranes ability to effectively diffuse O2 into the unit.

Filter membrane

“To be effective for separation, membranes should exhibit appropriate characteristics, such as good chemical resistance (to both feed and cleaning fluids), mechanical stability, thermal stability, high permeability, high selectivity and general stability in operation” (Sutherland, 2008).

As such the following properties were taken into consideration for the selection of the membrane material.

Membrane Hydrophilicity

With an aqueous feed stream the ideal membrane should be hydrophilic. If the material is hydrophobic it will adsorb components that are hydrophobic or amphoteric, resulting in fouling. For example many proteins have hydrophobic regions in their structure. Thus to reduce the fouling of Lipase and protein on the membrane surface it is advised to operate with a hydrophobic membrane. Many polymeric membranes are hydrophobic and thus lead to protein and oil attraction, resulting in increased membrane fouling. A means of measuring the Hydrophilicity of a membrane is to examine the contact angle of water, when in contact with the membrane. The smaller the angle the hydrophilic the membrane is, the more suitable it is to this process.

Hydrophilizing the membrane can be used to minimise oil fouling. However the application of cleaning solutions to hydrophilized membranes has not been widely published, and thus the respective reduction in flux and membrane life cannot be accurately considered.

Amicon’s regenerated cellulose membranes have the smallest contact angles, however as previously alluded to, these membranes would struggle to undergo harsh cleaning methods, thus resulting in a reduced flux over time, and a reduced membrane life (Sutherland, 2008). Hence a PES membrane has been selected (for the budget unit) for its combined attributes of moderate robustness and a relatively low contact angle. Effectively this choice is a compromise between the membranes required properties and cost, a ceramic membrane would be the ideal selection for this application due to an even lower contact angle and improved abrasion and chemical resistance, but for its cost.

Membrane porosity

Big holes lead to low initial membrane resistance, however depth pore blocking could result in a significant reduction in flux over time. If the particles to be separated are of the same order of magnitude as the membrane pores, smaller particles become lodged in pores in the membrane. Significant drop in flux occurs due to larger pores becoming plugged as these provide the greatest flux, however they are also the most likely to become plugged. The effects mentioned above are compounded if the membrane is operating at a high pressure, as the adsorbed cake can become compressed, forcing partially lodged particles to become entirely lodged in the pores.

Smaller pore size however corresponds to a large membrane resistance which can mask the contributions of the fouling layer and polarised layer. A smaller average pore size ensures that no molecules become lodged in the pores, but roll off due to turbulence at the surface, thus causing less of a drop in flux. However, the benefits of having a greater rate of initial flux are outweighed in the long term due the increased levels of fouling on the membrane surface.

A good rule of thumb to follow is to use a pore size of one tenth of the particle size, to ensure a suitable compromise between initial flux, fouling rates and membrane resistance.

It has also been noted that a common rule of thumb used in industry, is to use a pore size value of between one third and one fifth of the particle size. These rules will be used to determine the ideal pore size for the filter found in the unit.

Surface Topography

Using a smoother more uniform surfaced membrane, such as cellulosic membranes can correspond to less fouling in comparison with rougher membranes as the protuberances can act as hooks containing molecules at the surface. Similarly, the largest protein deposits occur when operating with more heterogeneous membranes (Suki et al., 1984).

Charge on Membrane

It is to be assumed that the charge and polarity of the Lipase and other proteins in the tank will not have a significant bearing on the level of fouling on the membrane surface.

By using a filter medium that repels the charged particles, flux can be dramatically improved in a similar sense to the membrane Hydrophilicity. However due to the assumed and expected near neutral charge of the water (the greatest component when considering flux) the membrane charge should be kept as near neutral as possible.

It should be noted that no universally applicable means of theoretically establishing the extent of membrane fouling exists, and as such the only means of truly selecting the most suitable membrane will be during lab and pilot scale studies.

It is important to note that membrane material properties, solute properties and operating parameters can all interact with each other and give rise to quite different results, than if these factors were individually studied.

An industry standard 0.22 µm filter will be used to ensure absolutely no E.coli is able to escape from the holding tank.

Additional Points

It should be noted that the PES filter will be fitted into the unit via a vertical leaf filter arrangement, perpendicular to the flow in the unit. The frame holding the filter membrane will be constructed from HDPE to match the reactor walls and will be put into position via a vertical channel sealed with double silicon mechanical seals to ensure absolute integrity of the unit during operation and to limit the risk of contamination when removing and cleaning the filter manually (not included in standard 30 day cleaning cycle – which just incorporates back flush and chemical cleaning, not physical abrasion).

A risk to be considered is that the use of stronger cleaning solutions such as a 0.5M NaOH (suggested for use on ceramic membranes by Pall Corp.) would cause corrosion to the membrane and hence limit its capability to filter the bacteria. However the use of such ‘industrial’ chemicals should nnot be an issue in a ‘household’ environment.

Connections

In terms of the connections between the unit and the surrounding plumbing the key factors to consider were cost, ease of integration into the existing system and sterility of the connections.

In traditional BioPharma applications sanitary flanges constructed of 316L SS, with silicone 3-A Gaskets would be used. However the difficulty of fitting such connectors, their cost and incompatibility with plastic piping meant they were not suitable in this case.

Quick fit, sterile, disposable plastic connectors were another option; commonly used in bench top experiments in larger Pharma companies due to their ease of use, high level of sterility and low cost. However they would be difficult to use to connect existing pipework and due to their disposable nature it would be unwise to expect them to perform for extended periods, such as the products lifetime.

A sterile ISO 9001 approved smooth inner bore hose and barb style connection arrangement will be used for this unit. The connectors will be constructed from PVDF due to their improved ability to withstand degradation when in contact with chlorinated solutions such as bleach. The connectors themselves were selected since they are cheap and can be easily retrofitted into the existing system, as well as be moulded directly into the walls of the unit as a single uniform piece as a future development of the product.

Valves

The three main types of valves reviewed for this unit, were initially selected based on them using a linear feed and exit stream, therefore making the piping more streamlined, reducing turbulence and hence pressure loss, thus reducing the need for pumping to achieve sufficient head across the unit.

Plug; The plug valve offers a good level of shut-off and is easy to operate with a quarter turn to open and close the valve. Its main issue however is that they can be large comparatively to the piping, when considering the relatively fragile nature of the piping surrounding this unit was unwise.

Globe; This valve gives a good level of shut-off, but is again rather large comparatively to the piping

Single Disk Butterfly; The butterfly valve doesn’t tend to achieve as high a degree of shut-off as the previous, however due to the low operating pressures this should not present an issue. The butterfly style valve provides relatively little resistance to flow when fully open, can be opened with a quarter turn and is light enough to work in the current system. A single disk butterfly valve was chosen over a double disk arrangement due to cost and its reduced resistance to flow when only partially open.

Piping

More traditional style 316L SS piping was rejected due to cost and its lack of flexibility, making it more difficult to fit into different sized spaces under customers sinks.

We will be using clear ACDF piping such as that supplied by c-flex. This is due to its superior performance when in contact with chlorinated solutions compared to PVC and silicone tubing. Due to the flexible nature of the piping it is easy to fit into under sink spaces and makes it easier to connect up with the existing pipework, without the need to custom pipes or modifications.

Risk – more likely to allow biofilm formation due to high Hydrophobicity contributing to large contact angle of water, hence easier for new layer to form on liquid-solid interface, much like how FOG’s develop in sewer systems.

Processing, Machining and Installing of Materials to Create finished Product

When designing a product it is all too easy to not consider how it would actually be manufactured. An understanding of how each of the components are built and them assembled can give to rise to a critical path, this allows the production engineers to better assess any bottle necks in the current process and strive to reduce the production issues associated with them. For instance it may be that a bottle neck occurs at one of the suppliers. This would be considered as the critical process as the speed at which the rest of production can be carried is limited by this (Much like the rate determining step of a chemical reaction), a potential solution would be to look for a supplier better equipped to process/supply higher volumes of parts, or it would be to look into moving that process in house.

Another consideration outside of the critical path is how the each production step dovetails with the next. For instance, depending on tolerances a certain part may need to be cut using a zwicky saw and then surface ground to achieve the desired level of flatness and parallelism. But it may be easier to upgrade the blade on the saw, thus allowing such tolerances to be achieved without the need for the second process at all. Another example for this unit would be that the cover over the splitter wall would not be put in place during assembly until the filter had been fitted. The same would go for fitting the permeable membrane prior to fitting the impeller in the base of the tank.

Process economics; this will include the type of equipment required, the number of units, any modifications, labour, training, utilities etc. any conceivable site cost needs to be taken into account to ensure that the most economical solution is found to maximise gross profit for the production facility as this will ensure that stable growth and development of the company can be maintained over an extended period. Choosing a very fast process may save on labour costs, but could incur additional capital cost w.r.t. units. Especially if they are higher spec. as these will be difficult to find second hand. Many units for cutting, grinding and polishing operations as seen for this product can be picked up second hand for a significantly reduced fee, but will still machine the parts to the desired tolerances with relative ease.

Having a defined production process prior to beginning production is imperative to ensuring adequate traceability can be achieved.

Bioreactor walls

Raw material will be brought in, in the form of beads. This will be injection moulded using custom moulds, prior to being surface ground and surface polished prior to QC testing to ensure maximum performance.

Partially Permeable Membrane

The partially membranes will be bought in from a reliable supplier such as 3M, since they already produce a part similar to the one desired for the final product.

The membrane will be fitted into the unit by being bonded to a frame using adhesive. The frame will have the surface to be adhered grit blasted to increase surface roughness, to improve adhesion between the membrane and the frame.

The frame and membrane will be fitted to the rest of the unit during final assembly, prior to testing.

Filter Membrane

The filter membranes will be bought in from a reliable supplier such as Pall, since they already produce the desired part in question, beyond their standard lead time for orders to shipping, there should not be any other delays.

The filter will be fitted into the separator wall during assembly, prior to despatch, flux tests and leak tests (including for seal around filter) will be carried out by trained operatives.

Connections

To be purchased from reliable external supplier – product already mass produced, therefore reduced risk of failed deliveries, hence limiting our own capability to produce units.

The connectors will be supplied separately to the rest of the unit to allow for self-assembly of piping at home that best fits the space available.

Valves

To be purchased from reliable external supplier – product already mass produced, therefore reduced risk of failed deliveries, hence limiting our own capability to produce units.

All quality control will be carried out on the valves prior to receiving from the supplier. These will be assembled with lengths of piping to be cut to size by the customer during home assembly of the unit.

Piping

To be purchased from reliable external supplier – product already mass produced, therefore reduced risk of failed deliveries, hence limiting our own capability to produce units.

Piping will be assembled around valves, with separate ‘spare’ length including in the packing received by the customer.

Vents

To be purchased from reliable external supplier – product already mass produced, therefore reduced risk of failed deliveries, hence limiting our own capability to produce units.

The vents will fixed to the unit and tested for air flow rates prior to despatch.

Impeller magnetic stirrer

The impeller will be fashioned from magnetised 316L SS

The shape will be fashioned from a single block using CNC machining, before being electro-polished to achieve the desired level of surface finish.

Dimensions logged during cutting, tolerances of ± 0.02 mm from dimensions on drawing will be accepted.

Sizing (incl. filter size, wall thickness, connections Diameter, valve, pp membrane, magnetic stirrer and nutrient block)

See excel sheet

Venting

Suitable, sterile venting will be required to ensure that vacuums do not form within the unit, with the potential of this leading to structural collapse. The Vent must be able to provide an adequate flow rate through to achieve this with ease. The vent must also have the following properties;

● 0.03 µm pore size to ensure it is fully sterile w.r.t. both bacteria and viruses
● Capable of operating and maintaining sterility and structural shape during humid operation
● Must retard the growth of bacteria
● Can be steam cleanable
● Can be easily replaced in a sterile fashion
● Be cheap to replace
● Must have a low protein affinity to ensure flow through is as high as possible
● Must have long operational shelf life
Based on the above selection criteria the Pall Emflon series of filter would be ideal for this situation

Pumping requirements

In the ideal scenario no pumping will be required as this will reduce the number of mechanisms that can break during operation. If a pump is required a peristaltic pump will used in conjunction with a submerged outlet, therefore allowing reversible flow, ideal for back flushing, thus extending operating time between cleaning cycles.

NONE REQUIRED –for more advanced units lumen flush or pulsed back-flushing can be incorporated to ensure that a higher operational efficiency can be achieved. This has been considered and discussed in the development of the premium unit.

Unit Commissioning

Commissioning of the unit will primarily be carried out in house, prior to sale to the customer. This will ensure the safety and quality of the product to ensure it will operate as expected provided it is used correctly.

The following commissioning process will be carried out on a random sample of the units as a Quality Control Check;

● Leak test (water)
● Pressure test (water)
● Check connections
● Perform visual checks for faults, imperfections, cracking etc
● Torque tests carried out on magnetic stirrer during operation
● Electrical components tested (ASK MUSTAFA on specifics)
● OZUF test carried out on filter to ensure performance
● Test at 60 oC and 2 oC to ensure operation in more extreme environments
● Carry out punch list check of all components and their condition

Start-up of Unit

Much like the maintenance of the unit, the start-up procedure must be kept as simple as possible. It must also be noted that the initial start-up and commissioning of the unit should incorporate enough checks to ensure the operation of unit is adequate and to eliminate potential future problems from arising by being properly set up in first place.

The start-up of the unit shall have the following procedure based on Figure 1 above;

Membrane Cleaning

There are three levels of cleaning that can be applied to a membrane;

● Physically clean membranes are those which are free from visible cake, slurry or other particulate matter
● Chemically clean refers to “all” foulants and impurities being removed
● Biologically clean or Sanitised membranes are entirely free from all viable microorganisms (Dychdala, 1993)

The importance of cleaning the membrane effectively is of tremendous importance when it forms such an integral part of the unit. The cleaning method required needs to be harsh enough to remove sufficient amounts of particles fouled in the membrane pores but without impacting on the ability of the membrane to function after cleaning has taken place. Over the life of the membrane it is expected for the achievable flux across the membrane to reduce after each cleaning cycle, thus reducing the impact of each cleaning cycle, and reducing the frequency at which cleaning cycles are required will be greatly beneficial to extending the life of the membranes, with the ultimate aim to restore the previous process flux after each cycle.

Three energy inputs are required to clean a membrane;

● Chemical energy – Detergents and cleaners are used to hydrolyse, solubilise or disperse the foulant by physiochemical reactions.
● Thermal energy – the addition of heat in most cases improves the effectiveness at which chemical detergents can remove foulants by increasing reaction rates.
● Mechanical energy – physical scouring of the membranes and shear forces from fluids passing through the membrane channels at high velocities will help dislodge foulants from membrane pores.

The effect the above have on the cleaning efficiency is affected by a final overriding factor; time (Dychdala, 1993)

The unit will be designed to be cleaned with water at 20 oC however should be more effectively cleaned at a greater temperature should the owner decide to clean using hot water.

Cleaning Cycles

The frequency at which the unit should be cleaned should be kept to a minimum and the frequency at which it requires manual cleaning should be limited to zero if at all possible. By designing the unit to operate for extended periods of time without the need to be cleaned, ensures that the user has less opportunity to damage the unit during cleaning.

It should be noted that there will be a point at which lack of cleaning of the unit will cause a build-up of the cake layer formation of the filter membrane surface (thus reducing flux), to a point where the filter will become relatively ineffective.

The use of filter aid has been considered as a means of improving the efficiency and functionality of the filter, however the added complexity of introducing this at the correct flow rates to the system has been regarded as too much of an expense to what is fundamentally a budget unit. The addition of filter aids has also not been considered in terms of the growth rates of the bacteria in the unit.

Turbulence promotion

Promoting turbulence will increase the rate of flux across the filter membrane, so far this has been considered by inserting baffles into the tank design as well as into the membrane design itself. However for ease of construction and thus reduced cost, only the baffles present on the tank walls will be incorporated into the final design. Lab scale and pilot scale studies will be carried out as to the effect the additional turbulence will have on the flux rates and the corresponding most economical selection and positioning of baffles will be chosen.

Disposal of Bacteria and Drainage

The Disposal of the bacteria in this unit is of paramount importance as transgenic material cannot be allowed to enter the public sewer system, with the potential of giving harmful bacteria antibiotic resistance. As such a thorough and effective disposal process must be considered and developed to ensure such a release does not occur. However this operation must still remain easy to carry out and with easily accessible and relatively safe chemicals to ensure the safety of the user.

Nutrient Feed Design

The main initial considerations for the nutrient feed are as follows;

● Sterility – if introduction or replacement of the nutrient feed itself is in any way unsterile then this will detrimentally affect the growth rates of the existing K12 bacteria and hence the yield and flow rates of lipase into the drainage system.
● Linear release profile – although an initial exponential feed rate might positively benefit the growth rates of the K12 w.r.t. to ensuring aerobic respiration. A linear, near zero release rate profile is ideal for maintaining the level of existing nutrients in the unit to just above the bacteria’s maintenance energy and hence allows them to maintain a near constant OD. Thus excess nutrient isn’t being used up for growth and anaerobic respiration but is being used to maximal effect for secreting the lipase enzyme over a sustained period of time. Such a release profile means it is easier to more accurately predict the rate at which the nutrient will run out and hence the time at which the bacteria will stop secreting the lipase at beneficial rates.
● Slow nutrient release – by ensuring the nutrient release is slow enough, a compact nutrient source can be used to provide enough energy for the bacteria to live off of for a sustained period of time. Therefore contributing to the ideal scenario of the unit being able to carry out extended batches for a month or more before needing to be cleaned and have the nutrient and bacteria replaced. The release of the nutrient should be a process control where possible by physical phenomena, thus increasing reliability and removing the need for using complex control systems or motorised parts, thus reducing cost and increasing reliability.
● Simplicity – by keeping the design as simple as possible, especially with regard to replacing the block sterility and performance should be easier to maintain during the lifetime of each batch.
● Compact – by ensuring the feed system is as compact as possible the overall volume and size of the unit can be kept as low as possible. In doing so, the unit can have a greater overall throughput for a set size capable of fitting under a conventional sink unit in a household environment.
The overall concept for the nutrient delivery system is as follows;

Shape: The nutrient block will be in a cylindrical shape, with a sterile sheath around the circumference of the block. Thus only the end of the block will be exposed at any given time to the external environment. This will ensure the same surface area is exposed and assuming that the rate of diffusion of nutrient into the broth is constant a near-zero linear release profile will be achieved.

Sterile Packaging: The block will be embedded into a handle, much like a lolly-pop. It will be sheathed in a sterile polymer, which will also remain with the block during operation. An injection moulded cap will protect the end of the block being exposed to external bacteria. In the handle there will be a plastic bag that covers the block, its sheath and the end cap. This will be removed by firmly pulling prior to use, ensuring that the sheath does not harbour any bacteria that would be added into the solution when inserted. Inside the protective bag will be a sterile atmosphere. The handle will also have a thread around its circumference and two depressed sections on the base to allow the base unit to be screwed into the unit and thus place the nutrient block in with ease. This will also make removal of the block simple. There will be silicon rings found before and after the thread to ensure the block is securely in place and remains sterile during operation.

Surface eroding matrix: Based on the previous design features it should be clear that a surface eroding matrix (not bulk erosion) would be the ideal supporting matrix for the nutrient to ensure the most accurate near zero linear release profile possible.

To achieve this, a hydrophobic polymer will be needed to ensure that the bonds on the surface are hydrolysed faster than the water is able to penetrate through the matrix. Hence surface erosion will be the dominant physical mechanism and control the rate of diffusion. To maximise this effect the nutrient should be evenly distributed within the matrix.

There are two potential matrix materials that present the properties above that could be effectively used in an aqueous system such as this.

The first is to use POE’s or poly Ortho Esters, however these can hydrolyse to form carboxylic acid. Not only will the affect the pH of the unit but they also catalyse the hydrolysis of POE, hence increasing the rate of reaction as time passes – no longer near zero.

The second would be to choose polyanhydrides. They can be used to make up an aliphatic-aromatic copolymer to effectively control the release rate. Bulk erosion of such copolymers is inhibited by the presence of carboxylic acids produced during their hydrolysis. This would be the best selection.

Micro Spheres: Another potential solution to the matrix dilemma comes in the form of microspheres. Such microspheres have very predictable and uniform release kinetics, due to their development for controlled release of drugs, and thus should strongly reflect whatever can be modelled in theory.

The premise behind this idea is that some medicine containing microspheres can evenly release their active ingredient over a number of days even up to as many as 100 days. As such a fluidised packed bed of these spheres could be housed in cylinder formed of a structural mesh or cage, overlaid with a membrane to ensure the spheres are not released, as they could potentially be destroyed by the impeller (causing a ‘burst’ of nutrients), sediment or cause blocking in the filter membrane.

Research has shown that using double-walled microspheres reduces the frequency and risk of rupturing or bursting. This rupturing would result in an initial spike of released nutrients followed by a significantly reduced level of release for a sustained period.

Based on research carried out by REFERENCE a PLL:PLG double shell system with an volume ratio of 40:60 respectively should result in a near-zero linear release profile over a 100 day period, given a maximum outer diameter of 60 µm for the microspheres. Based on this information an inner diameter (for the PLG sphere) of 44.2 µm. This would lead to an outer shell thickness of 15.8 µm formed of PLL. Both the PLL and the PLG are bulk eroding matrices, where the outer PLL shell becomes more porous with extended exposure to water. The inner PLG core breaks down releasing the drug progressively (in the case of this research, piroxicam). From this information the microspheres would be be manufactured using a WHAT IS THE PROCES CALLED; at an oscillatory frequency of approximately 3.5 Hz and a Jet Velocity of 0.001 m/s.

It should be noted that this is just an illustration of how the microspheres would be manufactured given extensive test data showing the release profile of the nutrient mix over time, within set outer diameter and ratio tolerances.

Support of unit

The unit will need to be supported to ensure that there is sufficient access to the drain at the underside of the unit and that the unit does not create such stress on the existing pipework that it could crack or shear, causing a leak. The support will also need to make sure that the unit is high enough up to allow the drip feed to be gravity operated, otherwise a pump will be required to achieve sufficient transmembrane pressure and for the enzyme solution to be released into the u-bend.

Safety Considerations

Due to the units nature, it will be operated and maintained by untrained and unskilled individuals (w.r.t. bioprocessing), thus presenting issues of misuse, intentional or not. The cannot where possible have the following hazards associated with it;

● Operate at high pressure or contain highly pressurised liquids or gases at any point
● Require the use of strongly acidic, basic, toxic, corrosive or harmful materials
● Require the use of any flammable or explosive materials
● Require the use of any high resistance electrical components prone to ignition or sparking
● Require any previous or technical knowledge to operate effectively and safely
● Having a complex maintenance or cleaning regime that is easy to carry out incorrectly
The most reasonable approach to developing this unit safely would be to use the Layers Of Protection Analysis.

Traceability

The unit will have full traceability. All raw materials will have full traceability, all purchased components will be fully traceable, all operations and processes carried out on site will be recorded (both manually and electronically on a system such as IFS), including test results, date and operator’s initials to ensure the quality of the product can be assured.’

Future Product Development

Further to the Oxo cube and jelly cube experiments …. The product does have the potential to be developed, so that it can operate on cheap, common food stuffs to reduce the cost and hopefully increase the ease at which the unit can be operated with. This does however impact the long term revenue gained from those wishing to continue purchasing the existing nutrient feed system. Nevertheless this is somewhat an exciting proposition that could induce a greater number of individuals to purchase the product, which is where the majority of revenue should be generated from.

As mentioned previously a more premium unit could be designed to ensure improved operational efficiency, covering issues such as filter caking, loss of nutrients and a lack of control of the secretion the enzyme. This could be achieved by introducing pumping systems, a full control system with level indicators and pH sensors, control valves linked to the control system to ensure more appropriate flows are achieved during peak and less demanding times. A less detailed design of which can be found on the Product design report – Premium file.