Force-Sensing

Laparoscopic Grasper

Client: Charles P. Heise, MD, FAS, CRS Advisor: William L. Murphy, Ph.D Team Leader: Adam Dahlen Communication: Darshan Patel BWIG: Andrew Eley BSAC: Clara Zhang

Problem Statement Goal: design and build a prototype laparoscopic grasping tool –Provides feedback auditory or tactile Tissue-damaging force –Redesigned jaw mechanism Eliminate pinching at the pivot point of the jaws –Educational tool for surgical teaching

Background Laproscopy: minimally invasive surgery viewed by video screen (see figure on right) –Minimal incisions (5-10mm) Current Problems –Unknown force on tissue –Grasper pinching at pivot point –Causes damage to tissue

Wikipedia [Online]. March 3, A surgeon's view using a laparoscope. The top photo is of a gallbladder. The bottom photo is a cystic duct.

Design Constraints Foremost, re-design grasper to reduce pinching to a floating-point jaw mechanism Increase awareness by providing feedback Dimensions: –Fit to 5mm port –About 30cm long Varying closure setting Autoclavable

Design Specifications

“floatable” jaw to reduce pinching of bowel Provide feedback of pinching pressure must fit through 5mm port must be about 30 cm long optional ratcheting for locking autoclavable or disposable grasp a large portion of bowel without causing damage design must not be hazardous to surgeons or the patient No separate or loose parts that could possibly get lost in the patient must be precise enough to ensure that no damage is done to the tissue grasping mechanism must be solid with little slack durable material such as stainless steel The circuit should need little maintenance This device should last several years in a hospital environment. The handle should be easy to use and grasp by a surgeon. One large model will be prototyped for show; if successful, further manufacturing of the recommended design can be done and utilized for future testing or redesign. Cost should be under a few hundred dollars FDA approval is not required at this stage

Effect of jaw design on grip security, measured as peak load. Conducted independently of squeeze pressure. Damian D. Marucci, John A. Cartmill, William R. Walsh, Christopher J. Martin. Patterns of Failure at the instrument-Tissue Interface. Journal of Surgical Research, 2000 Jaw Design Research

Grey boxes = tissue samples that tore with respective jaw patterns No tearing occurred at any squeeze pressure with plane and wave pattern jaws. Damian D. Marucci, John A. Cartmill, William R. Walsh, Christopher J. Martin. Patterns of Failure at the instrument-Tissue Interface. Journal of Surgical Research, 2000

With increasing the jaw size and contact area (from 32mm 2 to 64mm 2 to 128mm 2 ), the allowable force before tissue damage was increased from 15 to 37 N. the maximum pressure applicable to the tissue without damage was computed to be 4.7 x10 5 Pa. E. A. M. Heijnsdijk, H. deVisser, J. Dankelman, D. J. Gouma. Slip and damage properties of jaws of laparoscopic graspers. Surg Endosc, 2004.

Research Conclusions 2 mm wave pattern is the optimal design option in terms of both grip security and minimal tissue damage. 4.7x105 Pa is the maximal pressure applicable to the tissue. With a safety factor of 1.5, the threshold pressure is: (1/1.5)x (4.7x105 Pa) =3.12x105Pa

Design Alternatives Strain Gages: –Widely accepted method of measuring strain on bendable materials –Small (reduces obstruction) –Inexpensive (under 10 dollars) –High-temperature resistant models (immovable after fixation and reduces limitations on placement location) –Functions by producing a change in resistance when strain is applied

Strain Gages on the Shaft Measure the compression and tension stresses on the shaft

Less clutter on the instrument, which will create a minimal nuisance to the user. Custom molds necessary for each gage during fixation procedure Manufacturing difficulty and practicality of putting the strain gages in the 5mm diameter of the shaft Increased cost for high-temperature resistant materials Less accurate, as compression and tension forces are small in comparison to those in the handle Signal conditioner necessary to supply voltage to and monitor output from the sensors

Strain Gages on the Handle Measure the compression and tension stresses on the handle

Larger fixation area on handle than on the shaft Larger compression and tension forces in the handle than on the shaft Extraneous deformation due to variations in handling Disadvantages of first design

Final Design Jaw Structure Design

The parallel jaw mechanism extends when the actuator is pulled.

Circuit Design

Buzzer circuit powered by a 6.0V battery which directly drives the supply voltage of an operational amplifier and a 3.3 V voltage regulator Potentiometer used to divide the 3.3 V and inputs to the inverting side of an opamp Opamp used as a voltage comparator which drives a buzzer if the non-inverting input voltage is higher then the inverting input, or the reference voltage. The variable voltage is dependent on the force sensor. As the force sensor is compressed the voltage goes up. If the sensor is pressed too much, the voltage will rise above the reference voltage causing the comparator to output 6.0V and turning on the buzzer.

Prototype Design: –Made to show feasibility of design idea and provide a physical reference –Not constructed to proposed scale of design due manufacturing constraints of available machining equipment –Tests were conducted to demonstrate that an equal force of N was experienced at various locations across the jaws when a tensile force of N was applied to the actuator Circuit: –Can be calibrated to specific value chosen by user to demonstrate a threshold force applied to the handle, which is proportional to the force experienced at the jaws

An example run was conducted with a compressive force of 10 N between the jaws of the Stryker laparoscopic grasper model. –Voltage was increased until 3.8 V, sounding the buzzer. –The threshold voltage would vary when connected to the proposed scale model design, as the ratio of input to output force would differ from the prototype model.

Future Work Construct an actual model to the design specifications and dimensions provided using stainless steel Conduct test of ratio of input to output force of scaled design Conduct testing of the circuit connected to the scaled design to set the threshold voltage to sound the buzzer when an output force of 10 N was observed.