a safe and efficient hemostasis is one of the most important factors in minimally invasive surgery . hemostasis can be achieved by sutures , clips , application of hemostyptica or thermic coagulation .   for thermic coagulation , high - frequency ( hf ) coagulation in monopolar or bipolar mode , cold plasma , and ultrasonic coagulation klingele compared the ultrasonic technique with the bipolar technique in dissected vessels and examined the tissue damage . underwood et al . measured the differences in operative time , blood loss and complications when using bipolar or ultrasonic technique in colectomies and fundoplications [ 7 , 8 ] . sutton   et al . examined the lateral thermal spread using monopolar and bipolar diathermy , the harmonic scalpel and ligasure by measuring the maximum temperature . in our present study the tests were performed on perfused , isolated pig liver in a minimally invasive model with a pulsatile organ perfusion ( p.o.p . ) trainer . for analysis a complete laparoscopic equipment setup for minimally invasive surgery was used together with a p.o.p . the experimental procedure began with heating the isolated , frozen liver in a water - cooled vacuum in order to warm it up to about 30c35c before the actual start of the experiment . the pump of the trainer was activated to check the correct function ( figure 2 ) . small open vessels on the liver surface were closed with sutures . after a final check of the correct connection of blood vessels , after 30 minutes , the tissue temperature was detected by the laboratory thermometer at 5-minute intervals until it reached 37c .   to start the experiment , a 2 - 3  cm thick slice of the liver was resected to create a deep tissue bleeding . after closure of the cover , the pump was activated again ; the bleeding area was photodocumented ( figure 3 ) . the cut surface was divided horizontally in two halves . only the upper half was coagulated to see the difference to untreated tissue and calculate the tissue loss ( figure 4 ) . the coagulation was started using one of the four techniques until the bleeding could be stopped .   using ultrasonic coagulation , it was not possible to create hemostasis in preliminary experiments for hemostasis   of large bleeding areas in perfused pig liver . after 5 minutes , the cell walls were highlighted and a first approach of coagulated tissue became visible . the bleeding did not stop ; the extraction of tissue by the mechanical vibrations of the ultrasonic applicator could be observed . after 10  minutes , there was an increase of foam ; after 15  minutes , we saw extracted tissue without sufficient hemostasis ( figures 5 and 6 ) . based on these preliminary tests , the ultrasonic technique was not considered for   further experiments and pathological examination .   using the other devices , after stopping the pump and opening the cover , the tissue was fixed on a cork plate and placed in formalin solution after photodocumentation .   from the resected tissue , 10  cuts in 510  mm distance were made , each cut was examined at 10 points microscopically ( figure 7 ) . furthermore , the maximum tissue damage was determined to compare the tissue loss caused by the different techniques . the high - frequency coagulation devices were used with the settings shown in table 1 . the gas flow had little effect ( cooling ) on the coagulation performance . with 1.5  l / min gas flow , forced coag  was chosen because the required output voltage of the monopolar technique for a similar coagulation is significantly higher than in bipolar technology . the tissue structure changes on the necrotic side the color is darker in the thermally damaged area . microscopically the necrotic cells are not clearly defined and shrank because of the evaporated water . in addition , the color is very dark compared with the nondamaged tissue . the 10 measuring points are lined up linearly , so that the measurements cover a distance of 1  mm of the tissue sample . the tissue loss is measured in relation to the lower half of the untreated sample at the point of greatest loss . the measurement can be easily carried out macroscopically ( figure 4 ) .   for statistical evaluation of tissue damage and tissue loss , we measured the coagulation time of 3  different techniques ( monopolar , bipolar , and cold plasma ) on 4 experimental days . ultrasonic coagulation was excluded , because hemostasis of large bleeding areas could not be reached .   as shown in figure 9 , the shortest time there was almost no difference between monopolar ( 11  min ) and cold plasma ( 11.25  min ) . to determine the tissue loss , the height difference between the coagulated area and the untreated area was macroscopically analyzed and measured in mm ( figure 10 ) . the minimal tissue loss was 4  mm ( bipolar coagulation ) ; the maximal tissue loss of 8  mm occurred using cold plasma coagulation . on average , the tissue loss was the least in bipolar technique ( 4.5  mm ) , more in monopolar technique ( 6.0  mm ) , and most in cold plasma coagulation ( 7.5  mm ) . the depth of tissue necrosis was analyzed microscopically and measured in mm ( figure 11 ) . the lowest value was found in cold plasma coagulation ( 0.157  mm ) , the highest in monopolar coagulation ( 0.598  mm ) . the slightest average damage to the tissue was observed in cold plasma coagulation with a mean of 0.264  mm . bipolar coagulation led to an average depth of 0.346  mm , monopolar coagulation of 0.459  mm . bipolar coagulation of large bleeding areas of the perfused pig liver led to a rapid   hemostasis causing moderate tissue damage and depth loss . the worst results occurred with monopolar coagulation compared to bipolar technique ( coagulation time + 33.3% , tissue loss + 33.3% , tissue damage + 32.7% ) . ultrasonic coagulation was excluded because large bleeding areas could not be treated sufficiently and hemostasis was not reached . the aim of this study was to examine whether there are differences between the common coagulation techniques in a minimally invasive model concerning the efficiency of hemostasis and local tissue damage [ 1 , 3 ] . the experiments were done with isolated , perfused pig livers . the use of ultrasonic coagulation did not lead to a complete hemostasis even after 15-minute coagulation time . the termination resulted from the application form of energy causing foam formation and extraction of tissue components . the benefits of ultrasonic technology in minimally invasive surgery are clear in the preparation and dissection of tissue , the use close to other organs , and the closure of isolated vessels of up to 57  mm . in our model , it was not useful for coagulation of large bleeding areas . the first parameter in this study was the coagulation time until a sufficient hemostasis was reached . a good instrument should be safe and quick to reduce damage to lateral tissue and to minimize the energy flow to the patient . we know from clinical use that with longer monopolar coagulation time more and more necrotic tissue sticks to the instrument and is pulled out . the average tissue loss was 4.5  mm in bipolar technique and 6.0  mm in monopolar technique ( cold plasma coagulation 7.5  mm ) . the depth of tissue damage by coagulation was the third parameter in this study . because of the high efficiency of bipolar coagulation , less energy was needed to stop the bleeding . this resulted in an average depth of tissue necrosis of 0.346  mm in bipolar coagulation and 0.459  mm in monopolar coagulation . it showed the longest coagulation time and the highest tissue loss , while the depth of tissue damage was the lowest in this study . one possible explanation could be the gas , which is pressed into the tissue during the contactless coagulation . it leads to a deep dissection of necrotic tissue and a rapid shrinkage of the particles ( figure 13 ) . this indicates a low degree of necrosis and corresponds with the previously mentioned results ( figure 14 ) . after monopolar coagulation , we regularly see black carbonized tissue ( figure 15 ) , after cold plasma coagulation grey - brown to black tissue ( figure 16 ) . in summary , the collateral tissue damage consisted of the tissue loss and the tissue necrosis in the depth . when both factors are added , the total damage can be calculated ( table 2 ) . the results of this work came from experiments on isolated pig livers in an in vitro test environment and can not be easily transferred to human application . perfusion of the isolated liver was achieved with colored saline solution and not with blood , what restricts hemostasis and prolongs the coagulation time . this is one reason for the relatively long coagulation time that we measured in the perfused liver . further studies have to show the presumed superiority of bipolar technique . in our surgical department the experimental study showed differences between several coagulation techniques in a minimally invasive model for hemostasis in deep liver bleeding . ultrasonic coagulation , which is very useful in dissection of tissue and coagulation of isolated vessels , did not lead to a sufficient hemostasis in large bleeding areas . the bipolar technique was the form of coagulation with the highest efficiency and the least tissue damage .
hemostasis in minimally invasive surgery causes tissue damage . regardless of the method of production of thermal energy , 
 a quick and safe coagulation is essential for its clinical use . in this study 
 we examined the tissue damage in the isolated perfused pig liver using monopolar , bipolar , cold plasma , and ultrasonic coagulation . in a 
 minimally invasive in vitro setup , a 2 - 3  cm slice of the edge of the perfused pig liver was resected . 
 after hemostasis was achieved , liver tissue of the coagulated area was given to histopathological examination . 
 the depth of tissue necrosis , the height of tissue loss , and the time until sufficient hemostasis was reached were analyzed . 
 the lowest risk for extensive tissue damage could be shown for the bipolar technique , combined with the highest efficiency in hemostasis . 
 using cold plasma , coagulation time was longer with a deeper tissue damage . 
 monopolar technique showed the worst results with the highest tissue damage and a long coagulation time . 
 ultrasonic coagulation was not useful for coagulation of large bleeding areas . 
 in summary , bipolar technique led to less tissue damage and best coagulation results in our minimally invasive model . 
 these results could be important to recommend bipolar coagulation for clinical use in minimally invasive surgery .