Patent Publication Number: US-2022214328-A1

Title: System and methods for detection of volatile organic compounds in air

Description:
This application claims priority to U.S. Patent Application No. 63/134,380 filed Jan. 7, 2021, now pending and incorporated herein by reference. 
    
    
     The field of the invention is detection of volatile organic compounds (VOCs) in the air. 
     BACKGROUND OF THE INVENTION 
     Volatile organic compounds (VOCs) are natural or manmade compounds which readily diffuse into air, due to their volatile characteristics. Many VOCs are toxic to humans and the environment with extended exposure. VOCs are also associated with explosives. Thus, detecting VOCs is important to human safety and security, and for better preserving the environment. Although various techniques have been proposed and used for detecting VOCs, they have been met with only varying degrees of success. Accordingly, improved systems and methods for detecting VOCs are needed. 
     Overview 
     A system for detecting VOC&#39;s uses living biological cells. From an evolutionary perspective, biological cells as a system have been fine-tuned over millions of years for the purpose of sensing various molecules. Cells have evolved to be energetically efficient and sturdy. Cells can repair themselves and adapt to environmental changes. Cells can also be reprogrammed and manipulated in a variety of ways through genetic modifications. 
     In humans, the sense of smell is generally achieved by a type of neuron located in the nasal epithelium, which express olfactory or odorant receptors (OR) on their surfaces. Each odorant neuron usually expresses only one OR gene among the hundreds present in the organism&#39;s genome. When an odorant molecule, or VOC, from inhaled air binds to a matching receptor, the event triggers a chain of reactions that result in electrical signals. These signals, or spikes, propagate into the brain and are further processed to give rise to a complex sense of smell. 
     A cell may be modified to express a receptor. The receptor may be an odorant receptor. The receptor may be a wild-type receptor. The receptor may be a modified receptor, such as a genetically modified receptor. A receptor may be modified to enhance a binding specificity to a particular compound or to alter the receptor from a broadly tuned receptor to a narrowly tuned receptor or vice versus. The cell may be modified to express only one unique receptor, or more than one unique receptor. The cell may be modified to express two unique receptors. The cell may be modified to express three or more unique receptors. A receptor may be a human receptor, a mouse receptor, a canine receptor, an insect receptor, or other species type of odorant receptor. 
     OR activation eventually results in an increase in cytosolic calcium concentration, which can be measured using a calcium sensitive fluorescent reporter. These may include FIP-CBSM, Pericams, GCaMPs TN-L15, TNhumTnC, TN-XL, TN-XXL, Twitch&#39;s, RCaMP1, jRGECO1a, or any other suitable genetically encoded calcium indicator. The binding of an odorant molecule to its receptor induces an increase in the fluorescence emitted by the cells. An optical detector can therefore be used to measure cellular response in a contactless manner. The present system and methods detect VOC&#39;s using an optical detector that detects fluorescence. 
     A biochip used in the present system has one or more wells containing genetically modified living cells expressing an odorant receptor capable of binding to a volatile organic compound, and a fluorescent reporter that fluoreses in response to binding of the volatile organic compound to the odorant receptor. A capillary connects each well to a liquid source. An air flow channel is separated from each well by a membrane. Living cells are bound to a first side of the membrane, and a wall of the air flow channel is formed by a second side of the membrane. At least a portion of the biochip may be transparent. 
     Other objects, features and advantages will become apparent from the following detailed description and drawings, which are provided as examples for explanation, and are not intended to be limits on the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, the same element number indicates the same element in each of the views. 
         FIG. 1  is a schematic diagram of a VOC detection system. 
         FIG. 2  is a schematic diagram of the optical system of the VOC detection system of  FIG. 1 . 
         FIG. 3  is a bottom perspective view of a microfluidic biochip. 
         FIG. 4  is a top perspective view of the microfluidic biochip shown in  FIG. 3 . 
         FIG. 5A  is a bottom perspective view of the microfluidic biochip of  FIGS. 3 and 4  with the top foil or seal layer shown in  FIG. 4  removed for purpose of illustration. 
         FIG. 5B  is a bottom perspective view of an alternative microfluidic biochip with the top foil or seal layer removed for purpose of illustration. 
         FIG. 5C  is a bottom perspective view of another alternative microfluidic biochip with the top foil or seal layer removed for purpose of illustration. 
         FIG. 6  is an exploded top perspective view of the microfluidic biochip shown in  FIGS. 3 and 4 . 
         FIG. 7  is a schematic representation of an osmolarity control system. 
         FIG. 8  is a front perspective view of a detection system with the top cover removed for purpose of illustration. 
         FIG. 9A  is a side perspective view of the detection system of  FIG. 8  with the top cover in place. 
         FIG. 9B  is a side perspective view of the detection system of  FIG. 8  having an alternative water collection container on the outside of the cover. 
         FIG. 10  is an enlarged front view of components of the detection system shown in  FIGS. 8 and 9 . 
         FIG. 11  is a front view of components of the detection system shown removed from the housing. 
         FIG. 12  is a top view of the optical system having four optical channels, for use in the detection system shown in  FIG. 1 . 
         FIG. 13  is a front view of a biochip loader. 
         FIG. 14  is a side view of the biochip loader shown in  FIG. 13 . 
         FIG. 15  is a top view of the biochip loader shown in  FIGS. 13 and 14 . 
         FIG. 16  is a top view of the biochip loader of  FIGS. 13-15  positioned for loading and unloading biochips from the detection system shown in  FIGS. 8-12 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIGS. 1 and 2 , in a basic form, a VOC detection system  20  includes a cell carrier or substrate, such as a microfluidic biochip  22 , an optical system  24  and an electronic system  26 . The microfluidic biochip  22  contains cells  30 , medium or water  32 , and a membrane  36  which provides a barrier for the cells against contaminants such as viruses, bacteria and dust. The cells bind to the membrane  36 , allowing the cells to more effectively interact with airborne odorants such as VOC&#39;s. Each channel or optical pathway of the optical system  24  includes one or more: light emitter, such as a blue LED  46 , lenses  40 A,  40 B  40 C and  40 D, optical filters  42 A and  42 B, dichroic mirror  44 , and a photodetector such as a photodiode  48 . 
       FIG. 1  shows an embodiment having two optical pathways each having the above-listed elements, although the system may be designed with a single optical pathway or multiple optical pathways, depending on the intended application. The electronic system  26  in  FIG. 1  is electrically connected to the blue LEDs  46  and to the photodiodes  48  and may include a digital lock-in amplifier  52  in the form of a field programmable gate array (FPGA). The electronic system  26  has an output device, such as a thin film transistor (TFT) display. Alternatively, the output or reporting from the detection system  20  may be provided via a WIFI, cellular, RF or wired connection. The electronic system  26  may include a GPS unit for detecting and reporting the location of the detection system  20 . The electronic system  26  may also include control software or circuitry, and memory for recording detection events and other data. The detection system  20  may be powered by a battery  28 , to allow flexibility in placement and use. 
     Turning now to  FIGS. 3-6 , in the example shown, specifically in  FIG. 6 , the microfluidic biochip  22  has a bottom or first layer  68 , intermediate layers including a second layer  66 , a third layer  64 , and a fourth layer  62 , and a fifth or top layer  60 . The layers may be laser cut from PET plastic sheets (polyethylene terephthalate) or other materials, such as silicon, fused-silica, glass, any of a variety of polymers, e.g., polydimethylsiloxane (PDMS; elastomer), polymethylmethacrylate (PMMA), polycarbonate (PC), polypropylene (PP), polyethylene (PE), high density polyethylene (HDPE), polyimide, cyclic olefin polymers (COP), cyclic olefin copolymers (COC), epoxy resins, metals (e.g., aluminum, stainless steel, copper, nickel, chromium, and titanium), or any combination of these materials. 
     The layers may be attached and sealed together via an adhesive, solvent welding, clamping or by using bio-compatible double sided tape and a hot press. The layers may optionally be made of glass and/or PDMS (silicon-based organic polymer) assembled using plasma bonding. The layers below the cells are translucent or transparent, so that the cells may be exposed to a light source such as the blue LED  46 , and so that fluorescence emitted by the cells may be detected by the photodiodes  48 . The layers above the cells  30  may optionally be transparent so the cells may be viewed from above. If not, the layers above the cells may be an opaque material such as plastic or metal. 
     As shown in  FIGS. 5 and 6 , the fifth layer  60  and the fourth layer  62  have through holes providing wells  72  for holding cells  30 . A membrane  65 , such as a PTFE membrane, on the bottom surface of the third layer  64  closes off the bottom of the wells  72 . The membrane may be treated to make it transparent and to promote cell adhesion. Cell adhesion to the membrane allows for better detection of VOCs, which move from the air flow channel  84  in the biochip  22  through the membrane  65 . Although the example shown has four wells  72  in a square array, other numbers, patterns and shapes of wells may be used. Capillaries  80  in the fourth layer  62  connects a water inlet  76  in the fifth layer  60  into each of the wells  72 . The capillaries  80  may be etched into the fourth layer  62  before assembly. 
     An air inlet  74  extends through the fifth layer  60 , the fourth layer  62 , the third layer  64  and connects into the air flow channel  84  which is formed in the second layer  66 . As shown in  FIG. 6  the air flow channel  84  extends under each of the wells  72 , in an S-shaped configuration. The membrane  65  encloses the air flow channel  84  from above while the first layer  68  encloses the air flow channel  84  from below. The membrane  65  separates the cells  30  in the wells  72  from the air flow channel  84 . The air flow channel  84  may be wider at positions under the wells  72 , so that the cells  30  are better exposed to elements such as VOCs moving through the membrane  65 . Alternatively, positions may be inverted with the air inlet  73  extending through the first or bottom layer. Where the biochip has an on-chip reservoir or liquid source as in  FIGS. 5B and 5C , locating openings on the top of the biochip may be convenient for filling the reservoir(s) and/or the wells. 
       FIG. 6  shows the second layer  66  attached to the first layer  68  and to the third layer  64  using layers of double-sided tape  66 A and  66 C, as one example. The layers may alternatively be attached using adhesives, fasteners, plastics welding, or other techniques. Alignment holes  82  may be provided at the corners of each layer to precisely align the layers on a fixture during assembly of the layers into the microfluidic biochip  22 . 
       FIGS. 5B and 5C  show alternative biochips  22 B and  22 C which, unlike the biochip  22  in  FIG. 5A , has no water inlet  76 . Rather, the biochip  22   b  has an on-chip reservoir  81  filled with water or media. Water is supplied to each of the wells  72  via capillaries  83 . The water may introduced into the biochip  22 B when the cells  30  are placed into the wells. The water may contain nutrients. In  FIG. 5C  the biochip  22 C has multiple separate reservoirs  85 . Each reservoir  85  supplies water via a capillary  87  to a single well  72 . Depending on the specific biochip design and number of wells, a single reservoir may supply water to all of the wells, as in  FIG. 5B , or each well may be connected to a separate reservoir as in  FIG. 5C , or one or more reservoirs may be connected to one or more wells. Biochips  22  having varying numbers of wells may be used, for example 2, 4, 8, 16, 32, 64, 96 or 98, 100, 128 and up to 1000 or above is specialized applications. 
     After the microfluidic biochip  22  is assembled and ready for use, cells  30  are placed into the wells  72  from the top of the fifth layer, the cells are seeded on top of the membrane  65 , and the cells bind to the membrane  65 . A foil or pierceable seal layer  70  may then be adhered onto the top surface of the fifth layer  60  to cover and seal the wells  72 , as well as the water inlet  76 , the air outlet  78 , and the air inlet  74 . The foil or seal layer  70  also prevents light from entering the top of biochip  22 . This reduces evaporation and avoids stray light affecting the signal from the photodetectors. The microfluidic biochip  22  is then effectively sealed against the environment. The biochip  22  may be manufactured as a disposable unit intended for replacement e.g., every 30 days. 
     The microfluidic biochip  22  is designed for operation in the detection system  20  shown in  FIGS. 1, 2 and 8-10 , although it may also be used in other systems as well. Referring to  FIGS. 8, 9 and 10 , in the detection system  20 , the optical system  24 , the electronic system  26  and the battery  28  are contained within a housing  90 . A frame  112  is positioned on top of the base  110 . The frame  112  has a detection system slot or front opening  136  adapted to receive the microfluidic biochip  22 . The base  110  and the frame  112  may be fixed in position on guide-posts  128 . A top plate  114  is supported on one or more jack screws  120  which are rotated by one or more jack screw motors  122 . The jack screws  120  and the jack screw motors  122  form an elevator to raise and lower the top plate  114  towards and away from the frame  112 . Bushings at the corners of the top plate  114  slide on the guide-posts  128  and prevent lateral movement as the top plate  114  moves vertically. Alternatively, the top plate  114  may be fixed in position with the frame  112  and the microfluidic biochip  22  moved vertically. 
     A water or liquid medium supply container  94  is connected to a water supply tube  96  which passes through the top plate  114 , at a position in alignment over the water inlet  76  of the microfluidic biochip  22 , when the microfluidic biochip  22  is installed in the detection system slot  136 . A vacuum tube  100  extends from a water collection container  104 , through the top plate  114 , to a position aligned over the air outlet  78  of the microfluidic biochip  22  when the microfluidic biochip  22  is installed in the detection system slot  136 . An air inlet tube similarly extends through the top plate  114  to a position aligned over the air inlet  74  of the biochip  22 . With the top plate  114  in the up position, the biochip is sealed from the environment. When the top plate  114  moves down to engage the biochip  22 , the water supply tube  96 , the vacuum tube  100  and the air inlet tube pierce through the seal layer  70  to make fluid connections with the biochip  22 . 
     A pump tube  102  connects the inlet of a vacuum pump  98  to the water collection container  104 . The outlet of the vacuum pump  98  leads to an outlet  108 . In an alternate design, a positive pressure pump may be used instead of the vacuum pump  98 , with air pumped into the air inlet and through the air flow channel under positive pressure, rather than drawing air through the air flow channel via vacuum. 
     The detection system components may be in or on a housing  90  enclosed by a cover  92 . As shown in  FIG. 9A , sight windows  106  may be provided through side walls of the housing  90  aligned with the water supply container  94  and the water collection container  104 , to allow visual inspection of the water level in the containers. The detection system  20  does not actively remove water from the biochip  22 . However, humidity in the air moving through the air flow channel may condense into liquid water, which moves into and is collected in the water collection container  104 . 
     As shown in  FIG. 8 , the outlet  108  may extend through a front wall of the housing. Also as shown in  FIG. 8 , the electronic system  26  may include an on/off switch  132  on the housing  90 , and a USB port  134  for charging the battery  28  or for interfacing the electronic system  26  to another device via a USB cable. As shown in  FIGS. 10-12 , rollers  126  projecting into the detection system slot  136  are rotatable to guide the biochip  22  into the detection system slot  136 . Optionally, the rollers  126  may be rotated by one or more load motors  124  for this purpose. In this case, one or more sensors or switches  125  detects the presence of the biochip at detection system slot, causing the load motors  124  to turn on. The load motors  124  and rollers  126  provide a biochip mover for moving the biochip  22  horizontally. Alternate forms of biochip movers may be used instead of the load motors  124  and rollers  126 , such as linear actuators, rack and pinion platforms, solenoids, etc. A biochip mover may be provided with single direction actuators and/or spring elements. The battery  28 , the LEDs  46  and photodiodes  48  of the optical system, the jack screw motor  122  and the load motor  124  are electrically connected to a control board  130  of the electronic system  26 , which controls the operations described below. 
     In use, cells  30  and water or medium are provided into the wells  72  of the microfluidic biochip  22 . The foil layer  70  is then applied over the fifth layer  60  to seal the wells  72 . The microfluidic biochip  22  is then ready for use, although the microfluidic biochip  22  may optionally also be stored for days or weeks with the cells having sufficient water and nutrients to maintain life. 
     The detection system  20  is placed in the desired location. Since the detection system is compact and requires no external connections, the detection system may be used in wide variety of locations. The detection system  20  is turned on via the switch  132 . The microfluidic biochip  22  is loaded into the detection system slot  136 . The jack screw motor  122  is turned on, rotating the jack screws  120  which lowers the top plate  114  towards the microfluidic biochip  22 . The tips of the water supply tube  96  and the vacuum tube  100  pierce through the foil layer  70  and engage into the water inlet  76  and the air outlet  78  of the microfluidic biochip  22 , respectively. The vacuum pump  98  is turned on, drawing air through the air flow channel  84 . The optical system  24  is also turned on. An extension tube may optionally be provided on the air inlet to better sample air from a specific location rather than sampling ambient air around the detection system. In use, the air inlet or extension tube draws in an air sample or a volume of ambient air for testing for the presence of VOC&#39;s. 
     VOC&#39;s in the air drawn into the microfluidic biochip  22  pass through the membrane  65  and bind to an appropriate OR of the cells  30 , transducing a signal that ultimately produces fluorescence when illuminated by the blue LED  46  or other light source reflected into the wells  72  by the mirror  44 . When present, the fluorescence is detected by the photodiode  48 . The detection event may then be displayed, transmitted and/or recorded. 
     With the tip of the water supply tube  96  engaged into the water inlet  76 , water or other medium flows via capillary action from the water supply container  94  (if used) through the capillaries  80  and into the wells  72  to supply the cells  30 . The cells  30  are consequently supplied with water from the capillaries  80  (via the water supply container or via an on-chip reservoir), and are exposed to VOC&#39;s passing through the membrane  65 , but the cells  30  are otherwise sealed off from the environment. 
     When air sampling is completed, the microfluidic biochip  22  is removed or ejected from the detection system  20  and may be replaced with a new microfluidic biochip  22 . 
     The detection system  20  may be provided with a biochip loader  150 , together forming a combined unit  148 , shown in  FIG. 16 , which can store multiple biochips  22  and automatically load and unload biochips  22  into and out of the detection system  20 . The loader  150  allows the detection system  20  to operate unattended for an extended period of time.  FIGS. 13-15  show the loader  150  with no housing. Generally, the loader  150  is contained within a housing which may be similar to the housing  90  shown in  FIGS. 8-9 . Alternatively, the loader  150  and the detection system  20  may be provided together in a single housing. In either case, the loader  150  is secured in a fixed position relative to the detection system  20 , to allow biochips  22  to be moved between them. The loader  150  may also be electrically connected to the control board  130  or other component of the electronic system  26  of the detection system, with the control board  130  controlling both the detection system and the loader  150 . 
     As shown in  FIGS. 13-14 , the loader  150  has a frame  152  including guide-posts  128  attached to a frame base  154  and a motor plate  158 . A lift plate  166  is movable vertically on the guide-posts, driven by jack screw motors  122  rotating jack screws  120 . Bushings  168  allow the lift plate  166  to slide vertically on the guide-posts  128  while reducing sliding friction and preventing lateral movement. A guideway  160  is formed within the frame  152  by columns  164  attached to the frame base  154  and the motor plate  158 . The columns  164  pass through openings in the lift plate  166 . The guideway  160  is configured to hold a stack of biochips  22  on the lift plate  166 . A loader slot  180  is provided at the top of the guideway  160  to allow biochips  22  to be placed into the guideway  160 . 
       FIG. 13  shows a stack of 3 biochips  22  in the loader  150 , although the loader may have capacity to hold e.g., 2-10 or more biochips  22 .  FIG. 13  which is a front view of the loader  150 , shows the loader slot  180  formed by openings or cut away sections  182  through the upper ends of the front columns  164 . The rear columns may have the same design, so that the loader slot  180  extends entirely through the guideway  160  from the front to the back of the loader  150 . 
     A limit switch or sensor  174  may be located at the bottom of the guideway  160  to sense when the lift plate  166  is in the full down position. A camera  170  or other optical detector may be provided on the bottom side of the motor plate  158  to visually detect the presence and/or number of biochips  22  in the loader  150 , and/or to read an identifier on a biochip, such as a bar code on the seal layer. Referring to  FIGS. 13-15 , the loader  150  has a biochip mover, which may be provided in the forms of four load motors  124  on the motor plate  158 . Each load motor rotates a roller  126 , for moving biochips  22  into and out of the loader  150 . 
     In use, the lift plate  166  of the loader  150  is lowered to or near the bottom of the guideway via the jack screw motors  122  rotating the jack screws  120 . Multiple new or unused biochips  22  are inserted (by hand) through the loader slot  180  onto the lift plate  166  in the guideway  160 . The biochips  22  may be keyed with the loader slot  180  so that the biochips can only be loaded in a single correct orientation. Alternatively, the biochips  22  may have a projection or other feature that allows loading in only the single correct orientation. In the combined unit  148 , the detection system  20  and the loader  150  are fixed in position (e.g., bolted into place in a housing or a mounting plate), with the front of the loader  150  facing the front of the loader  150 , and with the loader slot  180  of the loader adjacent to, and vertically and horizontally aligned with the detection system slot  136 . In this design, the biochips  22  may be loaded into the loader  150  through the loader slot  180  at the back of the loader  150 . 
     With the combined unit  148  placed or located in the desired room or space, the electrical system is turned on using the switch  132 . The control board  130  confirms the presence of one or more biochips  22  in the loader  150 , and optionally performs other functions, such as system checks, recording, reporting, etc. The control board activates the jack screw motors  122  to raise the lift plate  166  to vertically align the top-most biochip  22  with the loader slot  180 . The load motors  124  of the loader  150  and the detection system  20  are turned on in the forward direction causing the rollers  126  to move the top-most biochip out of the loader  150  and into the detection system  20 . The detection system  20  operates to detect VOC&#39;s as described above. 
     The cells in an operating biochip  22  can effectively operate for several days, for example from 3 to 10 days. The duration of biochip operation is a function of the ability of the receptors (OR) to last, and not of cell viability. Cells with improved ORs may be able to operate longer than 10 days. The ORs in cells in a sealed biochip may be stored in the loader  150  for up to six weeks. Regardless of the OR effective duration, after a prescribed time interval, or after other factors determine the ORs are no longer operating sufficiently, the control board  130  initiates replacement of the used biochip  22 . The load motors  124  are turned in the reverse direction, with the load motors of the detection system  20  causing the used biochip  22  to move out of the detection system  20  and back into the empty loader slot  180  in the loader  150 . The load motors  124  of the detection system  20 , also rotating in the reverse direction, move the used biochip  22  through the empty loader slot  180  and the used biochip is ejected out from the back of the loader  150  into a collection location. The control board operates the jack screw motors  122  to lift the lift plate  166  to vertically align the next biochip in the guideway  160  with the loader slot  180 . The load motors  124  are again switched on in the forward direction moving the next biochip from the loader  150  into the detection system  20 . This sequence is continued until all of the biochips  22  in the loader  150  have been used. The control board may wirelessly communicate with a technician to provide detection results, and/or diagnostic and status data, or to allow the technician to remotely control operation of the combined unit  148 . 
     When used with a biochip having water reservoir(s) as shown in  FIGS. 5B and 5C , the water supply container  94  may be omitted. Referring to  FIG. 9B , the water collection container  104  may be replaced by an external collection container  97  supported in a holder  101  on an outside surface of the cover  92 . In this case the pump tube  102  is connected to the entry tube  93  of the external collection container  97 , which may be removably secured into the external collection container  97  via a fitting  95 . Water removed from the system is collected in the external collection container  97 , which may contain a gel or other water absorbing material. The external collection container  97  may be removed and replaced with a new external collection container  97 , without opening the cover, when the biochip is replaced, or after a selected number of biochips have cycled through the system. 
     The OR&#39;s (olfactory receptors) may be sequences extracted from the human (600 ORs) and mouse (1300 ORs) genomes, or from other animals such as dogs, elephants, insects, etc. Synthetic ORs with sequences that are not found in nature may be used. Such synthetic constructs are still considered ORs based on their sequence and functional similarities to natural ORs. 
     Cell types used include the Hana3A cell line, derived from the commonly used HEK293 (human embryonic kidney) cells. This cell line contains accessory proteins that help the expression of ORs, such as receptor transporter proteins RTP1, receptor expression enhancing proteins REEP1 and REEP2, as well as the protein Gaolf(s) necessary to transduce the signal. The second cell type which may be used is primary astrocytes, extracted from rat embryonic brains and expanded in vitro. Both cell types have been shown to function equally well in detecting VOCs. ORs as disclosed in U.S. Patent Application No. 63/189,015, incorporated by reference, may be used. 
     The number of cells needed to generate a measurable response depends on the brightness of the cells and the sensitivity of the fluorescence detector. In the portable system  20  described, about 10,000 cells are used for each well. In the design shown in  FIG. 12 , the optical system has four optical paths, one for each well, with each optical path including the components as shown in  FIG. 2 . 
     In the example shown in  FIG. 1 , band-pass filters  42  and a dichroic mirror  44  are used to separate excitation light from emitted light. The excitation source for each cell population may be a blue LED  46  (Nichia NSPB500AS) with a viewing angle of 15 degrees, coupled to a collimating lens (Thorlabs LB1157) and a blue excitation filter (Semrock FF01 469-35). The dichroic mirror (Semrock FF506) reflects the excitation light towards the cells in the biochip  22 . A doublet of lenses focuses the excitation light onto the cells, and in turn collimates the emitted light back. Emitted light crosses the dichroic mirror and is filtered from scattered excitation light by a green emission filter (Semrock FF01 525-39). The filtered emitted light is focused by a lens on a silicon Photodiode  48  (Vishay VEMD5510C). 
     As shown in  FIG. 2 , the fluorescence reporter may be excited by blue light and emit green light. The conversion rate greatly increases (over 30 times) when the reporter is in the presence of calcium, which leads to an increase in emitted green light when the cells detect an odorant. 
     In order for the cells to generate a quick response, they are advantageously directly seeded on the membrane  65  that separates them from the outside environment. As it is difficult to embed electrodes on such thin membranes, the system monitors calcium flux in a contactless, optical way. 
     Fluorescence collected from one population depends on the number of cells and expression level of the calcium reporter protein. Cell number does not change where the system uses cells that do not divide such as neuronal cells. Cell number with cells that divide such as HANA3A cells, grow to a single layer confluence by dividing based on available space, and stop dividing when they touch each other. The number of functional fluorescent reporters in each cell can decrease over time due to natural protein turnover and photobleaching (light induced damage to fluorescent molecules). However, the cells may continuously produce new fluorescent proteins that compensate for this loss. 
     The fluorescence level is converted into a voltage by the photo-diode  48 , and can easily be monitored or digitized for further processing. The change in fluorescence occurs at a timescale of a few seconds. At those low frequencies, the ambient electrical and optical noises affect the photo-diode voltage significantly more than the true fluorescence signal. This can be circumvented by providing the fluorescence signal a high frequency signature and filtering out the other frequencies. For example, the following steps may be used. 
     1. flashing the excitation LED  46  at 6 kHz, which causes in turn the fluorescence emission to have the same frequency.
 
2. multiplying the raw fluorescence signal with a reference signal of same frequency and same phase. Since the product of two periodic signals tends to zero when their frequencies are different, most of the noise (which isn&#39;t 6 kHz) is significantly attenuated.
 
3. smoothing the product with a low pass filter to remove the high frequency oscillations and only keep its DC component.
 
     As shown in the example of  FIG. 1 , the initial analog to digital converter (ADC) step is performed by a low noise electrophysiology chip (Intan RHD2132) originally designed to record action potentials. The digital lock-in amplifier is designed in Verilog and implemented on a SPARTAN6 FPGA board. The lock-in output can be displayed on a TFT screen connected to the FPGA board, or sent to an on-board computer (Raspberry Pi Zero) through a custom parallel communication protocol. 
     An on-board computer can perform live analysis in order to translate the raw fluorescence intensity into detection events. This processing may consist first in computing the mean and standard deviation of the derivative of the signal over the previous 30 seconds. Detection occurs if the instantaneous derivative is greater than the average derivative+C times the RMS (dF&gt;dF+C×((dF−dF) 2 )) 1/2  for at least n seconds, with and being chosen to favor either accuracy or speed of detection. 
     The membrane  65  on which the cells are living provides the interface that separates the controlled cellular environment from the outside air. The membrane may advantageously allow VOCs to diffuse across the membrane in seconds; prevent bio-contaminants from entering the cell medium and damaging the cells; be optically clear in order to visualize the cells; be chemically compatible with cell adhesion and growth; and be mechanically, chemically and heat resistant. 
     The membrane may be a thin (15 microns) PTFE (Teflon®) membrane with high porosity (75%) and a maximum pore size of 30 nm, which is smaller than bacteria and most relevant viruses. The membrane may be opaque when dry, however after wetting of the membrane with a low surface tension fluid like isopropyl alcohol, the membrane becomes transparent and can be kept transparent as long as one side is kept in contact with IPA, water, or cell medium. In spite of its thinness, the membrane is sturdy and can be heated to more than 200 degrees Celsius, which allows coating it with an anti-stiction material on the external side for some applications while improving cell adhesion by treating it with plasma, and incubating it with poly-D-lysine on the inward-facing side. A silicon oxide (SiO 2 ) membrane may also be used. 
     A pre-concentrator may be used to adsorb VOCs and desorb them upon heating. 
     Evaporation of water or medium through the membrane into the air flow channel  84  is inherently tied to air sampling. Medium evaporation is one of the main causes of failure in cell culture. As water evaporates, the concentration of dissolved substances, such as salts, increase up to the point that the cells cannot function properly. Counter-acting this phenomenon helps to keep the cells alive. Referring to  FIG. 7 , measured rates of evaporation in the biochip of  FIGS. 3-5  is on the order of 60 microliters per hour (40 mL/month for the biochip of  FIGS. 3-5 ). This value is significant in comparison to the volume of medium that is sufficient for the cells to survive for one month. Indeed, based on the rate at which cells consume nutrients, they only require a few hundred microliters of medium per month. Perfusing fresh medium can compensate for this evaporation, but is wasteful since cells need water rather than fresh medium. However, perfusing pure water would flush away the vital solutes contained in the medium. 
     Thus the biochip is designed to use evaporation and capillary action inside the chip to aspirate water from a water supply container  94  or from the reservoir. If the water supply container is connected to the wells by thin enough capillaries  80 , the speed of incoming water prevents solutes in the wells  72  from diffusing back into the water supply container, which insures that osmolarity remains constant inside the wells. The system also has the advantage to be self-regulated in a passive way. If the evaporation rate increases, the depression in the well will increase and draw water faster. 
     The vacuum pump  98  is driven by an electric motor which may use less than 0.5 W when on. There is no pump for water, as the transpirative osmolarity control system is passive. The vacuum pump  98  may run continuously, or intermittently, depending on the condition of the ORs and the status of the detection system. 
     As the example of  FIGS. 3-5  uses mammalian cells, the optimal temperature is 37° C. Temperature control may be achieved by a single peltier module  140 , attached to small aluminum overlay that distributes the heat over the four wells. The peltier element acts as a heat pump, transferring heat from one side of the unit to the other based on the direction of current flow through the device. An H-bridge circuit (DRV8838) may be used to control the current direction to either heat or cool the wells based on the temperature measured with an internal thermocouple (MAX31855). The temperature measurements and the control of the H-bridge are both performed by the on board computer. 
     A method of detecting an airborne substance may include moving an air sample through an air flow channel of a biochip. The airborne substance in the air sample diffuses through a membrane in the biochip separating the air flow channel from a plurality of wells holding living cells. The living cells react to exposure to the airborne substance, such as a VOC and emit fluorescent light. The fluorescent light is detected indicating the presence of the airborne substance in the air sample. The cells may be illuminated by a light source. 
     Thus systems and methods have been shown and described. Various changes and substitutions may of course be made without departing from the spirit and scope of the invention. The invention, therefore, should not be limited, except by the following claims and their equivalents. Application Nos. 63/189,015; Ser. Nos. 17/153,747; 16/486,132; 16/344,791; and 17/251,557 are incorporated herein by reference.