Fabrication and Implantation of Miniature Dual-element Strain Gages for Measuring In Vivo Gastrointestinal Contractions in Rodents.

The overall goal of the following procedure is to demonstrate the fabrication of a dual element strain gauge. This is accomplished by first bonding to aligned individuals, single element strain gauge components. In the second step, the connecting wires are assembled and soldered to the bonded elements, taking care not to damage the backing material and the gauge element.

The dual element assembly is then embedded between layers of a silicone sheet to form a flexible, watertight and reusable strain gauge. In the final step, the gauge is combined with a stable connector for secure coupling to a suitable bridge amplifier. Ultimately, the fabricated gauge can be used to record the rate and relative magnitude of in vivo contractions along the digestive tract.

Once fabrication of the individual strain gauges has been mastered, the gauges offer a relatively low cost option for use along multiple regions of the gastrointestinal tract that are of interest to the investigator. The strain gauge technique has several advantages over alternative methods like using balloon manometers, pressure transducers or piso, electric crystal transmitters and receivers as the string gauges may be used acutely in anesthetized subjects or may be chronically implanted for long-term recordings in awake freely behaving animals. To fabricate the dual element strain gauge, begin by attaching a small, clean self-adhesive piece of paper with the adhesive side facing up on the workbench.

Then secure the elements to the work surface to limit any unwanted movement or contamination of the elements. Next, grasp each element in turn with a pair of number five Mont forceps, and then clean the back of the element films with isopropyl alcohol. After allowing the elements to air dry, use a clean 10 zero camel hair brush under a stereo microscope to apply a thin film of epoxy phenolic adhesive to the back of one of the elements.

Then immediately place the opposing back of the second element onto the adhesive and align the foil grids. Place the bonded elements in a 50 to 60 degrees Celsius oven overnight to fully cure the epoxy. The next morning use a single edged razor blade to trim the bonded dual elements to a final size of three by three millimeters, avoiding trimming the top of the elements.

Next, disassemble a 30 centimeter braided strand of a three conductor wire into its three constituent wires. Then to make a four wire cable first pair, one of the resulting single wires with a light colored wire contained within a second 30 centimeter length of three conductor wire. Next, strip approximately one millimeter of Teflon insulation from both ends of each wire and use activated rosin soldering flux and low temperature solder.

Tin the wire ends with a soldering pencil, then flux the solder pads on one side of the bonded dual element with a clean 10 zero brush and solder one single lead and one of the paired common leads to the solder pad. After repeating the process on the opposite side, ensuring that the remaining common wire lead is soldered to the pad opposite the original common lead, use a clean brush dipped in resin solvent to remove any residual flux following the cleaning procedure. Partly cure two part silicone rubber epoxy resin for 20 to 30 minutes.

Then solder gold socket connectors to the free ends of the wire leads and insulate the solder joints on the element solder pads with a thin layer of the resin. Next, cut three pieces of 0.5 millimeter thick silicone sheet to 15 millimeter by 15 millimeter squares and clean the silicone with distilled water. Then cut one of the sheets into a U-shape to accommodate the final dual element assembly without deforming the encapsulating silicone coat, the inner surfaces of the notch free silicone sheets with clear silicone adhesive, and then sandwich the dual element assembly within the notch and the aligned outer sheets.

Gently pressing any excess silicone and air bubbles from the center outward. Carefully clamp the encapsulated assembly between two blocks of metal bar stock for 24 hours to ensure a uniform thickness and the absence of deformation the next day, leaving the excess silicone along the boundaries of the assembly, reinforce the solder joint of the gold socket. Connectors on the individual terminal wire leads with three millimeters of shrink tubing, aligning the tubing within a plastic electrode pedestal prior to the first use of a new strain gauge.

Trim the excess silicone from the boundaries of the gauge to achieve the final desired dimensions. Take care not to damage the dual element or wire leads. Then secure the electrode pedestal and wires with 0.125 and 0.25 inch diameter shrink tubing to prevent disconnection during the experiment.

To implant the dual element strain gauge, begin by using a number 14 taper 0.3 eighth circle needle to thread the four corners of the strain gauge with four to five centimeter lengths of four zero or smaller sterile silk suture. Next, after performing a laparotomy section, the rectus abdominis musculature along the connecting linear elbow to prevent bleeding. Then make a very superficial midline in the parietal peritoneum to avoid lacerating the underlying abdominal viscera using saline soaked cotton tipped applicators.

Carefully placed the stomach on a saline soaked gauze padd at the coddle end of the abdominal incision. Keeping the organ in its anatomical position align the grid of the encapsulated strain gauge in parallel with the circular smooth muscle fibers. Then using the previously threaded sutures and a number 14 taper 0.3 eighth circle needle, attach the corners of the gauge to the ventral cirus surface of the gastric corpus.

Now begin the suture pattern of the gauge along the greater curvature of the stomach near the fundus corpus boundary, and proceed along this boundary toward the lesser curvature to obtain the best results. The ci rosa underlying the strain gauge should neither be slack as in this image nor overly stretched as in this image. After placing the suture, use saline soaked cotton tipped applicators to return the stomach to its anatomical position.

For an acute model exteriorize, the strain gauge leads at the coddle end of the midline incision before closing the abdominal incision. Then close the rectus adom muscles and the abdominal skin with separate three zero nylon sutures. Secure the free wires to the animal to provide strain relief during manipulation of the animal or the terminal wire connector.

Finally, place the animal in a stereotaxic frame to support the head and elevate the upper torso to reduce the respiration artifacts during recording. Maintaining the rectal temperature at 37 plus or minus one degree Celsius using a feedback controlled heating pad. Representative data from Aobut Barbital anesthetized rat are shown in the graph.

The top trace represents the gastric corpus contractions from the animal during the brainstem administration of thyrotropin releasing hormone, a known motility enhancing peptide, and illustrates the baseline contraction levels prior to the increase in the phasic gastric smooth muscle activity. The second trace demonstrates a reduction in the baseline gastric smooth muscle tone from the same animal. In response to the nitric oxide donor sodium nitropress side, this voltage signal can then be used to derive the equivalent static load in grams.

These representative data demonstrate the bidirectional capabilities of a dual element strain gauge that has been properly attached to the gastric cirr rosa. The third trace represents the basal smooth muscle contractions recorded by a sub miniature strain gauge sutured to the ciero surface of the duodenum of a fasted rat. In this representative experiment, the orientation of the strain gauge elements was also in parallel with the circular muscle of the duodenum.

While the strain gauge onto the gastrointestinal tract, it is important to remember to suture the gauge with minimal disruption of the ci and vasculature. After watching this video, you should have a good understanding of how to fabricate sub miniature strain gauges while avoiding some of the early pitfalls associated with a more technically challenging aspects of the process such as discretely soldering small, delicate components.