A tissue culture hood now often referred to as a Biosafety cabinet is an essential if you are performing tissue cuture experiments. This instructable describes the way I made my own (first picture).
Just to give you an idea on how these Biosafety cabinets work - the second image describes the approach to creating a sterile environment in the cabinet. Air is recirculated through a HEPA filter that filters out particles like bacteria and most viruses. The cleaned up sterile air flows in from the top and then moves through a slot in the front of the cabinet and a slot in the back of the cabinet and up through a hollow cavity at the back to a blower than pushes the non-sterile air through the filter and the cycle repeats.
Some of the air escapes through a smaller filter into the atmosphere and some air enters the cabinet from the front opening.
These cabinets are expensive. New ones cost almost US$10,000 and used ones may go for about $2,000. A bigger issue is that these are massive and heavy. Hard to fit into small labs. So, I thought i would try making my own. The trick was to use an existing HEPA filtration unit found in large room air purifier units. The end result turned out well.
Step 1: The Overall Design and Making the Base
I decided to diverge from the standard design by a) pulling the air through the filter rather than pushing it through which would result in less turbulence on the filter and b) having a bottom with a regular grid of holes so that the air flow is more even - again less turbulence. Also decided to use light weight materials such as wood and plastics so that I would not need a forklift to move it around.
The second figure shows the overall construction of the cabinet carcass. The unit is 36 inches wide and about the same in height. Depth is 28 inches. Primed wooden strips (2.5 inches wide and 3/4th inch thick) were used to form the bottom and the back first. Glue and screws were used to make the base as shown in the 3rd image. The strips were covered with recycled 1/4th inch plywood.
I then glued small pieces of Pergo flooring to the primed wooden strips to support the vertical wooden pieces that would make th back.
Step 2: Making the Back of the Cabinet
The back was made by gluing 4 vertical strips to the Pergo flooring pieces. Once the glue had dried I glued and screwed a piece of hardboard to the back.
Step 3: Making the Sides
Strips were temporarily clamped into place to get the actual sizes for the pieces that would make the sides. The front is to be angled 10 degrees from the vertical. I then cut two identical sets of strips for the two sides and glued and screwed to the existing wooden pieces.
I then cut two panels from scrap hardboard and glued these to the outside of the strips creating two sides for the cabinets.
Step 4: Covering the Cabinet With Epoxy Acrylic Coats
The reason I was confident about using wood instead of metal or plastic for the cabinet was I knew that by coating all exposed surfaces with a polymer used for sealing flat roofs I would be able to make all the exposed surfaces impervious to water.
I had a couple of buckets of the SureCoat material left over from a roof repair DIY job that I had done a while back. I first rolled a layer of the gel like SureCoat onto all surfaces and then applied a polyester mesh over the edges of the cabinets and applied another layer of SureCoat over this mesh. SureCoat has very high adhesion and dries into a hard surface
For the design air would enter the bottom of the cabinet and move back up through the back and the sides of the cabinet. I had forgotten to provide air ingress for the cabinet sides so therefore cut some grooves for the air to enter the hollow cabinet sides. And because of the newly exposed wood I had to repaint these surfaces with SureCoat.
Step 5: Making the Back Panel With a Cutout for AC Power Socket and Switch
The cabinet was beginning to take shape. For the inside back panel I decided to go with the very stiff Pergo flooring panels that I had got from a freecycler. I cut these to size and then test fit them in the back.
To the bottom most panel I added a slot for an AC GFC socket (bought new) and a power switch for the socket. The power switch and the plastic panel that would cover the opening were also from a freecycler. This AC socket would provide power for stuff like Pipet Aids etc to be used inside the tissue culture hood.
Step 6: Inside Back Panel With Wired Socket and Side Panels for the Cabinet
I wired the socket as shown. Solid mains wiring was used. The bare copper wire went to the 'Earth' hole in the AC socket and then wrapped around the earth screw on the switch. The black Live wire went to one end of the switch. A black wire connected the output of the switch to the AC socket. The white wire (Neutral) went directly to the AC socket.
The socket was attached to the back panel with a foam insert to prevent air leaking through the whole socket assembly.
The back panels were fitted into the cabinet, followed by a test fit of the bottom perforated panel. The bottom panel is a polypropylene peg board that I bought online. Two inside side panels were cut to measure and attached to the side wooden strips.
Step 7: Support for the Filter and Blower
I had run out of the white wooden strips that were long enough so used 2.5 inch bare wood strips to create a frame to hold the HEPA filter assembly. Four strips were attached to each other (glue and screw butt joints) to create a rectangle that fit about 10 inches from the top of the cabinet. The rectangular frame was then attached to the sides and back of the cabinets with screws.
Step 8: Making the HEPA Filter and Blower Assembly
The HEPA filter will rest on the rectangular frame. The whole air moving assembly is made up of two main parts - the actual HEPA filter set up plus and ancillary set of fans that would increase the air flow of required. Hopefully the first diagram explains this a bit.
I had a room air purifier from RabbitAir that we had not used for the last three plus years so though I would recycle this for the tissue culture hood. The unit has a four stage air purification - a prefilter to hold back hair and lint, the HEPA filter that would hold back 99.97% of particles of 0.3um size or larger, an activated carbon filter and finally an air ionizer. More importantly replacement filters are available easily and at much lower cost (about $50) than filters for commercial tissue culture hoods which cost about $500 or more.
The 3rd figure shows the general assembly of this HEPA set up. I am planning on adding a lot more detail on taking apart the air purifier in a youtube video.
The back of the air purifier is the shroud for the blower. This back was attached by screws to a piece of hardboard. Air from the outlet would be directed by a deflector made of triangular pieces of plywood and a cover to form the duct. Images show the steps to create this duct.
Step 9: Assembling the HEPA Filter
The blower motor unit and the associated circuit boards fit into the gray base. The frame that hold the filter was trimmed of extra plastic and then attached to the base with the original screws. The filters were then placed inside the frame.
Step 10: Adding the Ancillary Helper Fans
I created a layer of fans (again from a freecyler) to help increase the airflow if required and to even out the airflow.
These 110V fan were from a freecycler that I attached to a perforated hardboard. The fans were attached with double sided foam tape after cutting out holes for the fan and covering the perfboard with mesh. The fans were wired in parallel. The wires from the connected fan was passed through a hole in the HEPA panel (second image).
The panel with fans was held attached to the bottom of the rectangular wooden frame while the HEPA panel rested on the top of the rectangular wooden frame.
Step 11: Final Panels and Front Glass
The top-front panel was made from Pergo flooring and attached with screws to the cabinet. The top panel was cut from hardboard.
I did not have a 36 inch by 18-ish inches of glass but had two 18 inch square tempered glass panels. I used these. They are supported by an aluminum L-channel that is screwed into the sides of the cabinet. I may add hinges to these panels or convert the setup into a sliding panel. Not decided yet.
I slide the bottom perforated polypropylene sheet in. This bottom panel is removable to facilitate cleaning.
Step 12: Wiring and Testing
An IEC AC In socket was added to the bottom of the cabinet on the back panel. The wiring was led through the hollow back to the top. Wires were connected through a terminal block. The wiring diagram shows the mains wiring coming from the IEC socket - earth wire is connected to the earth wire for the cabinet-AC-socket attached to the back panel. Neutral wires are directly connected from the IEC In socket to the HEPA unit, to the fans, to the cabinet-AC-socket and for a cabinet light and a UV light. The live wire from the IEC in socket goes to the fuse holder first. The other end of the fuse is connected to a power switch. The output of the power switch connects to one end of three switches. The other ends of these three switches connect to: the live end of the ancillary fans, to the cabinet light wire and the UV light wire. The live output of the power switch also connects to the HEPA filter and the live wire for the cabinet-AC-socket.
The front panel had a rectangular hole cut in to hold the LCD panel from the air purifier. The PCB was then attached to the bezel that fit in the hole. The original dust sensor from the Air Purifer was attached to the inside of the top chamber in the tissue culture cabinet.
I had kept testing air flow with a digital anemometer right through the later steps in the construction which helped optimize the placement of fans and holes to direct air.
Step 13: More to Do ...
I have shot video footage which I still have to edit into a youtube video. This shows the teardown of the RabbitAir airpurifier, also the testing of the cabinet with air flow meters, and of course the operation of the unit. Hopefully will get these up in a few days and then will attach the link to this instructable.
I also have to do particle retention tests and a sterility functional test but these are more an attribute of the HEPA filter - yes I should order a brand new set.
The lights (LED strip and UV light) still have to be added. I do have UV fluorescent lamps but will have to convert CFL electronics to run these UV lamps.