The mechanical and neurological mechanisms contributing to intractable chronic constipation and fecal incontinence are not yet fully understood, in part because of challenges in measuring the slow actions of the bowel. Currently however, the study of neural mechanics underlying bowel function during natural behavior (e.g. eating, movement, and defecation) is challenging. Without versatile wireless systems, gut manometry measurement inevitably relies on either restraint of conscious animals or the use of anesthesia. The few available telemetric systems involve major surgery and have only unimodal sensor probes [1]. To enable multimodal data collection with a route to endoscopic implantation, we developed the Colonic Monitor of Conscious Activity (ColoMOCA). The ColoMOCA was designed for use with electrophysiology tools and to be affixed to the colon wall. After implant, the ColoMOCA wirelessly transmits colonic pressures and electrical properties of stool over multiple days. In this study, we evaluated the accuracy of the ColoMOCA compared to micro-tip catheters acutely in conscious swine, and the capability for this system to characterize bowel activity in awake, untethered swine over several days.

The ColoMOCA included battery-powered pressure-sensing electronics, stainless-steel electrodes for measuring salinity of bowel contents [2]. The ColoMOCA supported wireless battery recharge at 10-20 cm, with a battery lifespan of about 40 hours per charge, and a data transmission range of 30 cm. The ColoMOCA measured pressure from two locations 63 mm apart (Figure 1), and electrically measured stool conductivity in 3 overlapping regions (approximately 36, 28, and 20 cm long). The ColoMOCA transmitted data 10 times per second to a pager-like wearable radio for ambulatory data recording. ColoMOCAs were assembled using flexible, polyimide circuit boards, and encapsulated using biocompatible, silicone rubber. This produced a soft, cylindrical implant measuring 85 mm long and 8 mm in diameter. Benchtop and in vitro experiments validated wireless sensor performance and anticipated lifespan. Sensors were sterilized by ethylene oxide gas. 

Experiments were performed at two sites. At one site, a ColoMOCA was implanted for chronic studies in each of 3 adult Yucatan minipigs (weight 20-25 kg). At the other site, a ColoMOCA was inserted via colon stoma alongside reference micro-tip catheters for in vivo acute survival validation in 1 adult Yucatan minipig (weight approximately 50-60 kg).

For acute ColoMOCA insertions, non-anesthetized animals were transferred to a restraint cart. The ColoMOCA and micro-tip catheters were taped together such that their respective sensors were aligned (Figure 1). The ColoMOCA-micro-tip assembly was then inserted into the proximal colon via the cecal stoma. Reference pressures from the micro-tip catheters were collected by a data acquisition system. Data were recorded for approximately 30-40 minutes per session, and then the assembly was removed. Data from 3 sessions were included in this analysis.

For chronic survival implantations, under isoflurane anesthesia, a laparotomy was performed. ColoMOCAs were fed through a 1-cm incision in the colon and sutured to the colon wall by passing a suture through one or both solid silicone endcaps of the device. Incisions were closed in layers using sutures and dermal staples. In vivo daily recording sessions of up to 2 hours of colonic activity began the day after implantation. Colonic pressures and electrode readings were made with animals fully conscious and untethered. Data were received by a wireless radio worn by the animal. The radio recorded data to an onboard memory card, and forwarded data over Bluetooth to a computer that plotted data in real time. During recordings the animal ate food, walked, urinated, and defecated freely. Animals carried the ColoMOCA implant for up to seven days. ColoMOCAs were explanted in a terminal surgery under isoflurane anesthesia, and colonic tissue was harvested for histologic assessment.

Recorded pressure data were analyzed in MATLAB. The amplitude, duration, and periods between rhythmic peaks in colonic pressure waveforms were calculated. Statistical analysis between extracted contraction information from all sensors was performed in JASP. Data from implanted animals were quantitatively summarized, and qualitatively compared to animal behavior.

A total of 100 minutes of ColoMOCA data with concomitant micro-tip catheter measurements were obtained in the acute experiments. Four sensors were measured simultaneously: Micro-tip catheter sensors and ColoMOCA sensors. Sensors were located proximally in the colon relative to sensors located approximately 6 cm distally. A similar number of contractions were identified by all sensors (range 214-276). Median contraction periods were similar, as were median contraction durations for all sensors (Table 1). Median contraction amplitude differed by location, i.e., distally-measured pressures overall were lower, regardless of sensor type (Table 1). Differences in outcomes measured by all sensors were compared using ANOVA with Kruskal-Wallis nonparametric test, and Tukey post-hoc group comparison.

In the chronic study, all animals showed rapid recovery from surgery, returning to normal movement within 24 hours. No animals showed signs of distress with the ColoMOCA implanted or inserted. Implanted ColoMOCAs remained patent and did not obstruct stool for 6-7 days of implantation. The two devices secured with sutures at each end both remained in the implanted location for the study duration. Two implanted devices allowed for data collection over three days (excluding weekends), while one device stopped transmitting approximately one day after implant for unknown reasons. One animal developed an abdominal hernia due to breakage of abdominal sutures. Gross histology of explanted colon tissue did not show inflammatory response.

In the chronic study, approximately 435 minutes of ambulatory, catheter-free colon pressure data from both ColoMOCA sensors were analyzed, from which 1,129-1,192 colonic contractions were identifiable. Some activity was not analyzed due to occasional data loss during wireless recording. Freely-moving animals demonstrated regular colonic activity with similar median contraction period and duration (Table 1). As in the acute case, implanted sensors showed a significant difference in contraction amplitudes between proximal and distal sensors (Table 1).

Our initial results suggest that small wireless sensors can be surgically implanted in the colon for ambulatory monitoring of bowel function. Animals tolerated the wearable radio harness well, suggesting that continuous monitoring may be feasible. Recorded electrode data, although qualitatively analyzed, showed periodicity possibly caused by stool movement. 

While chronic recordings did not have a reference for comparison, measured pressure waveforms were similar to those recorded acutely. Extracted contraction summary data from ColoMOCA and micro-tip catheters were in agreement (Table 1). However, all pressure sensors showed time-domain differences, with low-to-moderate overall correlation (r = [0.04 0.57] and r = [-0.02 0.55] for proximal/distal sensor pairs, and r = [0.01 0.25] for micro-tip catheters and r = [0.02 0.26] for ColoMOCA sensors). This was unexpected, given that all sensors were within a single short section of colon, and was possibly caused by the localized forces exerted by stool.

Small wireless sensors can remain in the bowel for at least 7 days, permitting catheter-free untethered recordings of colonic pressures and conductivity of bowel contents. Comparable colonic luminal pressure features recorded by the ColoMOCA and the standard manometric tools in an awake pig support the use of ColoMOCA for in vivo monitoring of intestinal motility. As a research tool, these sensors can improve neurophysiological research of the bowel. Clinical translation of this technology could enable rectally-inserted sensors for chronic diagnostic testing in humans without catheters.

J. Mcrorie, B. Meerveld, C. Rudolph, “Characterization of Propagating Contractions in Proximal Colon of Ambulatory Mini Pigs.” Digestive Diseases and Sciences, vol. 43, 1998, pp. 957–963.
A. Smiley, S. J. A. Majerus, I. S. McAdams, B. Hanzlicek, D. Bourbeau, and M. S. Damaser, “Sensors Selection for Continuous Monitoring of Bowel State and Activity,” in 2018 40th Annual International Conference of the IEEE Engineering in Medicine and Biology Society (EMBC), 2018, pp. 2997–3000.