the present invention generally relates to sensor systems for use in devices that can raise a blister ( e . g ., a suction blister ) and cut the raised blister , i . e ., a blister raising device integrated with a cutting member . such devices are useful for harvesting skin grafts . in particular the devices and systems are adapted to infuse a fluid into skin at a donor site to enhance blister formation . in certain embodiments , the devices according to the invention include a head portion that can be removably coupled to a harvester body that is positioned at the donor site of a subject &# 39 ; s skin . the head portion and the body portion define a sealed chamber therebetween so that a fluid can be instilled and removed , and so that negative pressure can be applied to skin following fluid infusion . although shown and described as part of the head portion , it should be clear that the coupler or conduit for fluid delivery and evacuation of the chamber can be part of either the head portion or the body portion and that the fluid and negative pressure can be applied separately via multiple couplings or via a single conduit as illustrated . fig1 a is a schematic view of a skin graft harvester 50 for use in accordance with various aspects of the present teachings . in this illustrative embodiment , the harvest 50 includes a detachable head portion 52 and harvester body 54 . the harvester body 54 is adapted for placement on a patient &# 39 ; s skin at a donor site where skin grafts are to be obtained , e . g ., on the inner thigh , and secured in place , for example , with strap 56 ( shown in phantom ). the head 52 can further include a heater ( not shown ) powered via a coupler 60 adapted to couple with a power source in a base unit ( not shown ). the head 52 further includes a seal 63 which permits a reduced pressure chamber to be formed when the head 52 and body 54 are joined together and the harvester 50 is coupled to a vacuum pump or other source of reduced pressure , e . g ., via coupler 60 connecting the harvester 50 to its base unit . the head 52 can further include one or more windows 58 for observation of skin blisters being formed within the chamber by application of reduced pressure , heat or both . once the blisters have been formed , the head 52 can be removed , e . g ., by deactivating the source of reduced pressure and by actuation of release levers 62 , which break the seal 63 and allow the head 52 to be lifted off the harvester body 54 . additional details on harvesters useful in connection with the present invention can be found in u . s . patent application ser . no . 13 / 839 , 518 filed mar . 15 , 2013 ; u . s . patent application ser . no . 13 / 346 , 329 filed jan . 9 , 2012 ; u . s . patent application ser . no . 13 / 436 , 318 also filed jan . 9 , 2012 ; u . s . patent application ser . no . 13 / 014 , 737 filed jan . 27 , 2011 ; u . s . patent application ser . no . 12 / 851 , 656 filed aug . 6 , 2010 ; u . s . patent application ser . no . 12 / 851 , 621 filed aug . 6 , 2010 ; u . s . patent application ser . no . 12 / 851 , 703 filed aug . 6 , 2010 ; and u . s . patent application ser . no . 12 / 851 , 682 filed aug . 6 , 2010 . the contents of each of the above - referenced related applications are herein incorporated by reference in their entireties . fig1 b is a schematic view of the skin graft harvester 50 of fig1 a with the head 52 removed and the cutting mechanism 74 exposed . the harvester body 54 can include a base portion 70 , a sled 72 , and actuator handle 80 . the cutting mechanism 74 can include a plurality of plates with initially aligned holes through which skin blisters are drawn by heat and / or application of suction when the head 52 is joined to the harvester body 54 and activated . once the blisters are formed , they can be cleaved by the cutting mechanism 74 . for example , below the top plate depicted in fig8 , one or more additional plates , e . g ., a cutter plate and a bottom plate can be deployed with aligned holes . by actuation ( e . g ., pulling up ) of handle 80 , the sled 72 is caused to move horizontally such that one of the plates below the top plate , e . g ., the “ cutter plate ” ( not shown ) also moves ( because of its linkage to the sled 72 ), thereby occluding the alignment of holes 78 and cleaving the raised blisters from the donor &# 39 ; s skin . as explained in more detail below , the sensor systems of the present invention can be incorporated into either the harvester head 52 or harvester body 54 — or both . fig2 a and 2b are schematic illustrations of harvester device 50 having a cutter mechanism 74 in contact with a donor site of a patient &# 39 ; s skin 2 and equipped with a non - contact blister sensor 10 to reduce burden and variability in deciding when a skin graft is ready to be harvested . the sensor is connected ( e . g ., by a wired connection 13 or wireless path via transceiver 11 ) to a controller 100 which can be part of the harvester or situated remotely ( e . g ., as part of the console that provides a source of negative pressure and / or current to heater elements ( not shown ) within the harvester . the controller 100 can be a dedicated device or a software application on general purpose computer , laptop , tablet or smart phone type device . the wireless connection can operate via a bluetooth or other communication protocol . as shown in the cross - sectional schematic view of fig2 b , the sensor 10 can be constructed of a matrix of non - contact proximity sensor elements ( e . g ., utilizing technology such as infrared or laser optics ). the sensor elements can include emitters 12 and receivers 14 . the sensor elements are disposed above or integrated into the cutting mechanism 74 . when initially deployed the cutter mechanism has a plurality of align holes , through which skin blisters will be drawn by suction , heating or other manner . as shown in fig3 and 4 , each emitter 12 produces a signal 16 that can be received by a receiver 14 . if emitter 12 emits multidirectional signal , it can be paired with multiple sensors , e . g ., receivers 14 a and 14 b . ( possible emitter and receiver combinations arranged around holes 78 on the top plate 82 of a cutter mechanism can be seen in fig5 - 7 with the receivers omitted from fig6 & amp ; 7 for simplicity ). when receiver 14 a does not receive an expected signal ( fig4 ) it can be assumed that the direct line of travel between the emitter 12 and receiver 14 a has been broken and that therefore the blister 4 has achieved a sufficient height to be ready for harvesting . by using multidirectional or omnidirectional emitters paired with multiple sensors , as shown in fig5 - 7 , the complexity and cost is lessened by reducing the number of components in the system . the signal can be a continuous beam or a series of timed pulses to reduce power consumption ( and also limit irradiation of skin as well as the graft to be harvested ). due to the dynamic nature of some intended donor sites , it may be necessary to take repeated readings for each emitter and receiver arrangement before an alert is given that the site is ready for harvesting . in certain embodiments , the system can be configured to ensure that a pre - assigned number of paired sensor arrangements ( emitter and receiver ) are in a ‘ ready ’ state before giving an alert to the user that the site is ready for harvesting . when the conditions have been met and the system is sure that the skin is ready for harvesting , the device can instruct the user of this status . depending on the complexity of the device , the system may even indicate the next steps in the process . in an alternative embodiment , the sensors can be contact sensors . for example , load sensors can be placed above each harvest orifice ( fig8 ); or at predetermined sites to minimize cost . when the blister contacts the load sensor 92 the device can alert the user that the skin is ready for harvesting . the system may need to periodically raise and lower the load sensors ( fig9 ) to avoid interference with blister formation during the application of negative pressure within the chamber . moreover , conductivity sensors can be used in lieu of ( or in conjunction with ) load sensors . as skin is conductive , the sensor can be a conduction touch panel that can be placed within the harvester ( or at predetermined positions ). once in contact with the skin in an acceptable number of locations or total area , the device can alert the user that the skin is ready for harvesting . as with the load sensor method , the capacitive sensor may need to be periodically raised and lowered . in another alternative embodiment , the sensors can be distance sensing sensors . utilizing laser , sonar or ultrasound techniques , a sensor ( e . g ., an emitter and receiver pair ) can sit above the openings through which the blisters are pulled in the system . by recording the time it takes for the signal to return to the distance sensor 94 ( fig1 ), the distance from the emitter and the top of the blister 4 can be calculated . when this hits a predetermined time , the skin is ready for harvesting ( fig1 ). in some embodiments , a single emitter and multiple receivers can be employed . alternatively , multiple emitters and a single receiver can be used . the signals from the receiver ( s ) can be multiplexed or averaged to determine when a sufficient number of holes have been filled with blisters ready for harvesting . in yet another embodiment , the sensor can employ “ volume sensing ” techniques . for example , ultrasound can be used to produce a volume representation of the cavity in which the blisters are being pulled within the system . this can then be used to calculate the volume of the blisters by subtracting the measured volume from the start volume . when the ultrasound signal reaches a predetermined threshold the device can alert the user that the skin is ready for harvesting . alternatively , changes in the volume of the chamber can be determined by dead - space leak detection . by utilizing the vacuum source , the cavity in which the blisters are being formed can periodically be vented . if the leak within the cavity is known , then utilizing the time it takes for the cavity pressure to hit a predetermined threshold the system can calculate the volume of the blisters that are protruding through the harvester and alert the user the skin is ready for harvesting . in yet another embodiment , as shown in fig1 the sensor can be a colorimetric sensor 120 capable of detecting changes in color , e . g ., in a plate having the holes through which the blisters will be raised . the sensor can include a color photo detector 122 ( or an array of such detectors ) and an optional lens 124 ( for widening the view of view ). when a sufficient portion of the sensors field of view registers one or more colors associated with the skin or blister , a controller associate with the sensor can alert the user that the skin graft ( s ) are ready for harvesting — or automatically initiate steps to cut the blister ( s ). the colorimeter can further include filters 126 to preferentially pass colors associated with skin tones ( or skin components such as melanin ) the detector can be deployed within the device &# 39 ; s chamber . alternatively , light from the chamber can be collected via a waveguide or optical fiber ( or fiber bundle ) and transmitted to a detector outside of the device . the device / sensor can be configured to allow the user to define the blister height before initiating graft formation . this can be done mechanically by adjusting the space between the sensor and the forming hole , or electronically by adjusting the calibration of the sensor for proximity , pressure , reflection , time to receive a sound wave , etc . for example , as shown in fig1 a , the controller 100 , e . g ., a smartphone app , can communicate with the sensor to select a desired height . the device / sensor can also be configured to allow the user to define the minimal number of successfully formed grafts that need to be formed before alerting the clinician that the grafts are ready ( 50 %, 70 %, 100 %). in addition , the device can count the number of grafts formed and either display as a total , a percentage of the total , or in a light grid pattern ( one light for each graft , changing from red to green when formed ). this allows the user to decide if they have enough grafts and in the pattern / orientation suitable for them . for example , as shown in fig1 b , the controller 100 , e . g ., a smartphone app , can communicate with the sensor to select a desired fill factor to be obtained or , as shown in fig1 c , the controller 100 can monitor and display the progress of blister formation so that the user can determine when to initiate graft formation . the term “ load sensor ,” as used herein , is intended to encompass pressure transducers , touch sensors , piezometers and other piezoelectric devices , force sensors , force sensing variable resistors and / or capacitors and like devices which can determine pressure or contact based on changes in force or electrical or magnetic behavior . the term “ conductivity sensor ,” as used herein , is intended to encompass devices that can measure or determine changes in light , electrical resistance , capacitance and / or impedance , associated with the presence of skin within or near a target site . the term “ in proximity ” encompasses situations wherein objects are close to each other as well situations where objects are in contact with each other . closeness is not absolute quantity but rather denotes a distance wherein an object , e . g ., a sensor , can perform its intended function . references and citations to other documents , such as patents , patent applications , patent publications , journals , books , papers , web contents , have been made throughout this disclosure . all such documents are hereby incorporated herein by reference in their entirety for all purposes . the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof . the foregoing embodiments are therefore to be considered in all respects illustrative rather than limiting on the invention described herein . scope of the invention is thus indicated by the appended claims rather than by the foregoing description , and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein .
skin graft harvesting systems and methods are disclosed that utilize sensors to automate the harvesting of skin grafts or assist a user in deciding when the skin graft is ready to be harvested . such systems and methods can reduce the burden of visual observation and ensure greater reliability and consistency of the grafts . the invention is particularly useful with harvesters that rely upon suction and / or heating to raise a plurality of small or “ micro ” blisters simultaneously .