Introduction

The gastrointestinal (GI) tract has historically been a region of intense interest for the assessment of health. The diagnosis of gut disorders initially relied on external observation via x-ray, endoscopy and surgery until 1957 when two papers1 2 from the same research group simultaneously reported the development of a swallowable capsule, termed an Endoradiosonde. The ingestible device could sense the gut physiological parameters internally and transmit the data through radio communication.3 4 One capsule reported the measurement of pH, while the other reported the measurement of pressure and temperature with a capsule size of 28 mm in length and 9 mm in diameter.1 2 Another group of researchers in the same year reported the development of a pressure measuring capsule with a capsule size of 30 mm in length and 10 mm in diameter.5

These advancements allowed the development of similar sized capsules to sense different parameters within the gut, but the detection of bleeding through ingestible sensors remained a challenge. In the 2000s a capsule endoscope was invented that used a small camera to record a video to detect gut lesions.6 7 This technology was approved by the US Food and Drug Administration in 2001 and was followed by the launch of a series of other commercial endoscopic capsules.8–10 However, these capsule endoscopes neither controlled their motion (eg, by moving quickly through sites of little/no interest or slowing their movement at target-sites) nor accurately determined their position hence still lacked efficient diagnosis potential. These limitations were addressed by various locomotion, anchoring and localisation techniques, but they are out of the scope of this review, and are discussed in detail in other papers.8 9 11–13

Furthermore, the commercial capsule endoscopes that capture images of the gut lining have laid the foundation for similar-sized smart capsules that can perform monitoring, therapeutic and diagnostic functions such as sensing10 14 and drug delivery10 15 along the GI tract.16–18 There are various review papers on the scope and development of medical devices for the gut,8–12 14 15 however, latest progress on sensing and sampling capsule development have not been covered in literature. In this review, recent advances in the field of gut sensing and sampling devices are described with a focus on understanding their benefits while addressing the limitations of these devices.

The review starts with fundamentals of gut structure and physiology and details of gut microbiota. Then it continues with smart capsules for sensing different gut parameters and robotic capsules for sampling gut. Finally, the review discusses potential future avenues and implications of these capsules for personalised medicine, personal diet and early diagnosis of various gut diseases.

The GI tract and microbiota

The human GI tract is a 7–9 m long passage and in an average human lifetime, around 60 tonnes of food passes through it.19 The food is digested and absorbed by the gut using a range of physical and chemical processes. The gut mainly comprises four distinct segments namely oesophagus, stomach, small intestine and large intestine. The segments have different pH levels, transit profiles, contain diverse chemical compounds and follow unique behaviour to move food through them, as outlined in figure 1. Any chemical or biological changes from normal within each segment can be used as a biomarker for a range of health conditions. The unique features of each of these segments is discussed in detail below.

Figure 1

(A) Human GI (digestive) tract with each distinct segment of the gut identified.15 (B) Temperature and gas profiles of the gut, measured internally, which shows the daily response in each segment.16

Oesophagus

The oesophagus is a tube-like structure that connects the mouth or oral cavity with the stomach. The food is chewed in the mouth and broken down into smaller particles and mixed with saliva that prepares the food for absorption. The food transits through the oesophagus in a few seconds to the stomach where it stays for a few hours depending on the type of food, activity level and fluid intake. The oesophagus and stomach are separated by a sphincter that allows passage of the food from the oesophagus to the stomach while stopping the backflow of stomach fluid into the oesophagus.

Gastro-oesophageal reflux disease (GORD) is a health condition that produces irritation, heart burn and inflammation in the oesophagus, if the oesophageal sphincter is not closed properly.20 This allows stomach acid to enter the oesophagus creating acid reflux episodes that may damage oesophageal tissue. Another health condition affecting the oesophagus is achalasia in which the lower oesophageal muscles fail to relax which restricts the passage of food to the stomach.21 Furthermore, dysmotility is a dysfunction in which contraction from the oesophageal muscles are impaired resulting in imbalanced and uncoordinated peristaltic movement, leading to gut-related diseases.22 The peristaltic movement of the oesophagus is an important physiological process transporting food into the stomach and any disorder can be harmful. Therefore, this region sometimes requires monitoring to observe potential health problems related to the upper gut.

Stomach

The stomach is a J-shaped, hollow and dilated part of the upper gut that holds the food once it passes through the oesophagus. The gastric glands of the stomach generate hydrochloric (HCl) acid which maintains the pH at low levels (1–3.5 pH) that has two main functions. First, it kills any unwanted bacteria ingested with the food, and second, it starts the digestion of the food by activating the digestive juices (enzymes), and breaking down the mixture into a paste, known as chyme. The stomach wall secretes the digestive enzyme in an inactive form (eg, pepsinogen), which is activated by the acid into a functional enzyme (eg, pepsin). The muscular contractions (peristaltic movements) from the stomach wall blends the food and digestive enzymes creating chyme, which is released in the small intestine, with the gastric emptying time dependent on the type of food consumed.

Achlorhydria is a health condition that results from low gastric secretions by the stomach glands.23 This leads to insufficient amounts of HCl and digestive enzymes being produced which impairs food digestion. Furthermore, gastric motility disorder is an impairment that leads to gastroparesis in case of delayed gastric emptying or dumping syndrome in case of rapid gastric emptying.24 25 Therefore, several parameters in the stomach require monitoring. First, the amount and time of HCl acid secretion from the stomach glands to maintain the low pH, and the impacts of this on the digestion process are very important components. Second, the gastric emptying time is also extremely important for the digestion process and sometimes requires monitoring for nutritional studies, and lastly, pH sensing at various segments of the gut is critical to the diagnosis of gut-related problems.

Small intestine

This is the largest section of the gut measuring about 6–7 m in length and has a diameter of around 25–30 mm. The small intestine is also referred to as the small bowel and is further divided into three main segments, that is, duodenum, jejunum and ileum. The duodenum is the section nearest to the stomach and is the shortest of the segments. This segment initially secretes sodium bicarbonate to neutralise the acid contained in chyme from the stomach which increases the pH level to around 7 (neutral level). This smooth section soon gives way to more folded tissue covered with finger-like projections called villi, which are further covered in microvilli that increases the surface area. Duodenal tissue also secretes hormones to signal the release of digestive enzymes to prepare the small intestine for nutrient absorption and generates peristaltic movements to push the chyme towards the distal end of the gut, so the remaining food is moved to the next segment, the jejunum. The jejunum is around 2.5 m long, and is also covered with villi/microvilli that helps in absorption. Its main function is to absorb amino acids while moving the content further in the ileum segment through peristaltic movements. The ileum is about 3 m in length and also contain villi/microvilli similar to duodenum/jejunum. This segment helps bile acid to be absorbed by the host along with fatty acids, glucose, fructose and glycerol.

The small intestinal region can be affected by a number of health conditions that require diagnosis and treatment. Small intestinal bacterial overgrowth (SIBO) can result is malabsorption, diarrhoea, osteoporosis and nutritional deficiencies.26 Furthermore, dysmotility is a condition in which contraction from the intestinal muscles are impaired resulting in imbalanced and uncoordinated peristaltic movement, leading to gut-related diseases, for example, pseudo-obstruction.27 In addition, the release of gases due to fibre fermentation, bacterial overgrowth in the small intestine, reduced digestion or absorption of food nutrients (malabsorption) and abnormal (slow or fast) movement of food through small intestine can indicate various health conditions. Lastly, IBS, IBD and many other diseases are linked to this region that require monitoring for diagnosis and treatment purposes.

Large intestine

The large intestine or colon is the last segment of the gut and comprises the caecum, ascending colon, transverse colon, descending colon, sigmoid colon and rectum which is followed by the anus which is the last connection of the gut in the body. The overall length is about 1.5 m, and it contains densely packed bacterial populations in order of 1010–1012 organisms. The colon absorbs water and any remaining nutrients from the digesta and short chain fatty acids produced by microbial fermentation and excretes the residual undigestible food (faeces) from the body. This faecal matter is used worldwide for the diagnosis of various gut disorders, for example, detecting infections, identifying bleeding disorders, determining poor nutrient absorption, diarrhoea, constipation and in colon cancer diagnosis.28 29

Gut microbiota

A huge population of microorganisms (bacteria, archaea and fungi), collectively known as the microbiota, live inside the gut and play a major role in fibre fermentation and the synthesis of short chain fatty acids, and some nutrients (eg, vitamins).30 This population of microorganisms have been studied for a long time, and numerous studies on the relationship between microbiota and human health reveal that the microbiota can act as a biomarker for human health.31 The human meta-organism comprises bacterial colonies which include approximately 1013 prokaryotic organisms with a biomass of around 0.2 kg.32 Although the human microbiota is still not fully explored, it is pertinent that it colonises the mucosa layer, which covers the columnar epithelium of the GI tract and the digesta within the intestinal lumen,33 as depicted in figure 2.

Figure 2

Representation of microbial population across the lumen and mucosa layer in both the small intestine and colon. The villi structure is only present in the small intestine along with a mucus layer whereas the colon contains two mucus layers. Reproduced from Fellows and Varga-Weisz159 licensed under CC BY 4.0.

The knowledge of the relationship between the host and its microbiota has progressed significantly and suggests that the microbiota is a crucial component of human health. Researchers are exploring obesity, IBD, biochemical processes and diabetes with the help of microbiota.34–38 It is considered that microbiota can be informative of the health status over the life of the host and can even assist in early diagnosis of diseases like cancer, obesity and diabetes.34 35 39 Furthermore, the analysis of microbiota can lead to improved treatment of diseases such as ulceration, coeliac disease, Crohn’s disease and IBS.40 Microbiota can also help to study the relationship or interaction between nutrition and human health.40 41 Comprehensively, it can be inferred that the human gut microbiota is greatly influential to human health.

Recent research studies suggest that the microbiota can potentially determine the mood, behaviour and several other characteristics of the host.42 The gut microbiota has been shown to influence brain function and behaviour, and play a role in stress and anxiety, depression and mental disorders.43–45 Studies have looked at the impact of microbiota on mood, and revealed that the microbiota regulate emotions and cognition through the gut-brain axis which is a bidirectional communication link between the GI tract and the central nervous system.46

Impact of microbiota on host health and the limitations of sampling methods

Recent studies suggest the assessment of the human intestinal microbiome can reveal negative impacts on host health from some bacterial interactions with food. A study conducted on 40 human volunteers revealed that microbiota in the intestine produce trimethylamine-N-oxide from phosphatidylcholine (a chemical present in some foods), which has been shown to contribute towards the physiopathology of heart disease.47 The gut microbiota is an important tool to observe the effects on physiopathological parameters and on controlling inflammation. Rodent studies have observed the impact of an increase in bifidobacteria within the gut microbiota following consumption of a high fat diet which induced diabetes, suggested that the gut microbiota can be indicative of inflammation during the occurrence of diabetes and obesity.48 A study also determined that artificial sweeteners induced glucose intolerance by altering the gut microbiota.37 Furthermore, the same group of researchers revealed that dietary effects on individual animals were different due to the response of their gut microbiota in a related study.38

In general, obesity is co-related with the consumption of food. Studies using rodent models have revealed that weight regain is highly dependent on the gut microbiota,34 suggesting that the weight regain phenomenon may be understood and resolved by targeting the microbiota.

The changes in diversity of microbiota is prominent in early life but gradually decreases with age,49 and is also dependent on various other factors like diet50–53 and living conditions.54 The human gut microbiome may contribute towards the development of several diseases including type 2 diabetes,55 metabolic diseases,56 57 arterial dysfunction,58 IBS59 and cancer.60 A study using an unsupervised machine learning algorithm without age information on available 16S rRNA sequencing data from newborns to centenarians showed the ageing progression of microbial communities.61 The compiled data identified 35 genera related to age progression and when compared with the literature, they showed the loss of some beneficial genera and indicated an increase in inflammation and cancer-related genera in elderly subjects. Another study looked at the linkage between ageing and gut microbiome, and identified distinct groups of taxa that changed with ageing and discovered that the alterations were different in healthy and unhealthy ageing.62

The most common samples used as a proxy for intestinal microbiota are faecal samples, as they are easy to collect non-invasively and can be collected repeatedly from the same individual. However, faecal samples are collected at the end of the 9 m long gut that restricts extraction of spatial and temporal information from these samples as they are not collected from the actual site of digestion.63 64 This is a major limitation to data interpretation since associating diseases to certain location of the gut and site-specific treatment is not possible. This has been demonstrated in animal studies that show major changes in the intestinal microbiota are not always reflected in the faecal microbiota.65 Furthermore, the faecal sample is contaminated downstream from the site of disease by subsequent microbial populations and gut secretions during transit to the end of the GI tract.66 67 Another method used to study gut microbiota is by using flexible endoscopy with biopsy tools. However, biopsy tools are designed to collect tissue samples, and they cannot reliably capture microbial content. Hence, the current conventional tools available to collect microbial samples from the intestine without contamination have limitations. These methods are also tethered which limits their reach into the small intestine and the section of small intestine closest to colon (ileum) is a home to a different population of microbiota to the hind gut.68 This method also involves a risk of gut perforation and bleeding, and the procedure is invasive and unpleasant for a patient.69–71

In next sections, the latest methods used for sensing gut parameters and the use of untethered tools for sampling the gut microbiota are covered.

Smart capsules for sensing the gut

The wireless capsule endoscope (WCE), M2A, was developed as an alternative to tethered endoscopes by Given Imaging in the beginning of 21st century.6 This WCE was a pill-sized capsule (26 mm×Ø 11 mm) that was swallowed and used intrinsic peristaltic forces to move along the GI tract until it could be recovered from human faecal waste. Through its passage, it captured images that were transferred wirelessly to a recorder, which were examined after the trial.9 Although this device assisted with diagnostic procedures, provided ease and comfort to the patient as compared with tethered endoscope, the visual analysis provided incomplete information about gut diseases. Therefore, subsequent similar-sized capsules with a wider range of sensors as shown in figure 3 used to measure the physiological, biological and chemical parameters are detailed below and shown in table 1.

Figure 3

Gut sensing devices for measuring various gut parameters. (A) alphaOne pH monitoring capsule by the Jinshan group75 and (B) Bravo reflex capsule by Medtronic72 are used to record acid reflux episodes to diagnose gastro-oesophageal reflux disease by attachment to the oesophageal wall. (C) Heidelberg pH capsule for upper GI tract use to determine achlorhydria by holding it inside the stomach.78 (D) SmartPill by Medtronic to measure pH, temperature and pressure throughout the gut that can also generate a transit profile.81 (E) Laboratory prototype to measure pH.87 (F) Atmo Gas Capsule by Atmo Biosciences to measure gases from inside the gut that simultaneously generates the gas profile (results) on a phone application.112 (G) VitalSense by Philips Respironics97 and (H) eCelsius by BodyCap96 used to measure core body temperature. (I) Capsule prototype to measure peristaltic pressure.106 (J) Capsule to measure gases in the gut.16

Table 1

Sensing technologies using smart capsules

pH sensing

GORD producing irritation, heart burn and inflammation in the oesophagus has been traditionally monitored though catheter-based pH measurement tools which are uncomfortable and restrict the daily activities of patients. A wireless Bravo reflex capsule (Medtronic, USA)72 overcame these limitations by attaching the capsule to the oesophageal wall using an endoscope and continuous pH monitoring became possible for several days.73 74 Recently, Jinshan group (China) launched the alphaONE pH monitoring capsule (26.5 mm×6 mm×5.5 mm)75 as shown in figure 3A that was formerly known as JSPH and had proven its efficacy for patients with GORD in clinical trials.76 77 The wireless capsule technology showed similar results to the traditional catheter while improving the feasibility and safety for performing GORD tests.

An insufficient amount of HCl in the stomach due to low gastric secretions results in achlorhydria that can be detected by a Heidelberg pH capsule (Heidelberg Medical, Germany),78 a method invented in 1960s.79 The Heidelberg pH capsule is used in the upper GI tract and is held in the stomach using tethered methods, that is, a thread is attached to the capsule that is tied outside mouth, to continuously measure the pH level while sodium bicarbonate is introduced in the stomach to neutralise the gastric juice.80 Afterwards, the time taken by the gastric secretions to convert the gastric juice from a base to acid nature is used to determine the health of the patient. This capsule is a unique tool for detecting achlorhydria by measuring the acidic content in the stomach which can impair the food digestion process.

The normal pH level of distinct segments of the GI tract changes with the development of some GI-related diseases (eg, ulcer, UC, Crohn’s disease, GORD), that require pH monitoring of the whole GI tract and wireless capsules are best suited to monitor such situations. A commercial capsule SmartPill (Medtronic, USA)81 measures the pH of the entire gut along with temperature and pressure measurements, and offers an alternative to gastric emptying scintigraphy (GES) for measuring gastric emptying times. It has been used to determine the complete transit profile of the GI tract including small and large bowel transit times to help treat patients with chronic constipation.82 83 Another commercial capsule IntelliCap (Medimetrics, The Netherlands),84 used for drug delivery, has also been used to determine the transit times of the whole gut by measuring the pH and temperature in humans and animals.85 86 The capsule technology offers significant benefits over traditional GES method as the capsule generates complete transit profile of the gut and the method has not reported any after-effects on gut motility (peristaltic movement).

Some prototypes tested in vivo to measure pH have also been reported in the literature. A recyclable smart capsule prototype has used an iridium oxide-based sensor to precisely monitor the pH in a beagle dog study, however, the overall size was slightly bigger and battery life was shorter than commercial capsules.87 A battery-less prototype has also been used to determine acid reflux episodes from a pig’s oesophagus and performed well when compared with the Bravo capsule in similar in vivo experiments.88 Another capsule prototype tested in humans monitored pH with a high resolution of 0.001, in addition to measuring temperature and pressure.89 The improved design with better power consumption of sensors allowed the device to measure gut parameters for almost 10 days.90

Drug absorption is highly dependent on transit times which vary from animal to animal and depend in turn on feeding rate and time. Therefore, pH sensing throughout the gut using the Bravo capsule has been performed to determine individual (stomach, small intestine, large intestine) transit times, with the capsule allowed to pass through the gut instead of being attached to the oesophageal wall.91

The pH measurement of different segments of the gut can now be accurately and precisely measured with smart capsules, which can help diagnose abnormal conditions and assist in generating transit profiles that are useful tools in gut health diagnosis. The smart capsules for pH sensing can assist in better understanding of diseases like GORD,20 inflammation, achlorhydria,23 ulcer, UC, Crohn’s disease, chronic constipation, diarrhoea29 and has a potential to replace some of the traditional methods. These smart capsules sometimes embed a temperature sensor to measure the core body temperature and specifically to determine the time of excretion of the capsule from the body.

Temperature sensing

The normal body temperature of a healthy human is 37°C and when the body loses more heat than it generates, this condition leads to hypothermia when the core (internal) body temperature falls below 35°C. Conversely, if the core body temperature increases above 40°C, it leads to hyperthermia and can result in heat stroke that could be life threatening. For hypothermia or hyperthermia, temperature monitoring is extremely important and can help in determining safe and appropriate treatment.92 For accurate monitoring of the core body temperature, the gut is a promising candidate as sometimes axillary (outside) temperature readings via thermometer or contactless infrared pyrometer are inaccurate.93 94 Therefore, measurement of the core body temperature through ingestible devices like smart capsules which continuously measure the gut temperature can be more accurate when compared with other methods.94 Other applications include the monitoring of internal body temperatures of athletes continuously while they are training or performing as the core body temperature changes with exercise intensity and activity time which is useful in determining optimum performance level.95

A commercial ingestible capsule (eCelsius) measured the core body temperatures of athletes during the road cycling world championship.95 The temperature of 25% of the athletes was recorded above 40°C (with the highest individual measurement of 41.5°C). Although none of the athletes were treated for heat stroke (hyperthermia), it is likely that their performance was affected.95 Several commercial capsules are available to measure the core body temperature such as eCelsius (BodyCap, France),96 VitalSense (Philips Respironics, USA),97 CorTemp (HQ, USA)98 and myTemp (myTemp, The Netherlands).99 A study compared the accuracy and responsiveness of four commercial capsules (eCelsius, VitalSense, CorTemp and myTemp) and showed small differences across the tested capsules.100 Another study on eight participants compared the performance of eCelsius and myTemp capsules by measuring the rectal temperature simultaneously during and after the exercise. The myTemp device showed better performance while the eCelsius had an average systematic error of about 0.2°C, which could be eliminated by calibration of the capsule before the trial.101 Smart capsules that measure the core body temperature are more accurate than existing instrumentation and can perform measurement during exercise, which is not possible with traditional methods.95 101

One of the functions of the gut through which the temperature sensing smart capsule transits from one segment to the next and eventually is excreted from the body is peristaltic movement. Peristaltic movements apply a diverse range of forces, and sometimes these can be measured to diagnose potential health problems.

Pressure sensing

The food moves from mouth to the stomach through the oesophagus using peristaltic movement to push the food in a distal direction. Achalasia is a condition in which lower oesophageal muscles fail to relax which restricts the passage of food to the stomach. Oesophageal manometry is traditionally performed using tethered devices to monitor the oesophagus for movement of food towards the stomach by measuring the pressure applied by the peristaltic movements.21 A wireless capsule (24 mm×Ø 12 mm) has measured the peristaltic pressure in a beagle dog for 26-hour period, which showed that a smart capsule can be used for long-term monitoring as opposed to tethered devices.102 Another capsule prototype with better sensor power consumption allowed measurement of gut parameters including peristaltic pressure through a full gut transit, with the authors claiming that the smart capsule had a battery life of 233 hours.90 Another group tested a capsule prototype in 24 human subjects which monitored peristaltic pressure with a maximum error of 0.15 kPa and an 80% success rate in detecting poor gastric motility in the tested patients, along with measuring temperature and pH.103 Smart capsules that measure the peristaltic pressure of the oesophagus allows health practitioners to assess disorders of the lower oesophageal muscles, providing a better diagnosis.

Dysmotility is a dysfunction in which contraction from the intestinal or oesophageal muscles are impaired resulting in imbalanced and uncoordinated peristaltic movement, leading to gut related diseases. For monitoring the manometry of the entire gut, a few commercial capsules are available, for example, the SmartPill motility capsule and the Bravo capsule.72 81 A capsule prototype tested in both healthy and constipated subjects revealed that the colonic contraction in normal patients was significantly higher than the patients with constipation which resulted in the chyme (food residue) staying longer in the colon of constipated patients and higher absorption of water leading to dryer stools and more difficult defecation.90

The pressure measured by commercial capsule, for example, SmartPill, Bravo and other laboratory prototypes mentioned earlier, is intraluminal pressure that is the cumulative pressure within each distinctive region (eg, stomach, small intestine, large intestine) and is different from peristaltic forces so provides less information on the amount and frequency of peristaltic contraction. A prototype used two Micro Electromechanical Systems (MEMS)-based sensors to nullify the impact of intraluminal pressure and removed the disturbances from breathing and heartbeat signals of live subjects, hence successfully estimated the peristaltic contraction forces.104 105 In another paper, peristaltic forces were measured using a force sensor that provided real-time information on gut motility including contraction frequency and the amount of peristaltic forces generated by the intestines for better diagnosis of the gut health.106

Another application area for pressure sensing capsules is the estimation of transit times through distinct sections of the gut, for example, oesophagus, stomach and colon. A capsule prototype used MEMS-based pressure sensor to measure the intraluminal pressure of the gut, determined that the pressure in each region represented different cumulative pressures so pressure signals could be used for estimating transit times.107

Smart capsules with pressure sensing capability provide a viable solution for the diagnosis of gut disorders (eg, achalasia and dysmotility) as they offer accurate information on gut motility, for each distinctive segment of the gut that is a crucial factor used to determine overall gut health. These capsules can also be used to determine transit profiles which are helpful in determining the digestion and absorption that provide better insight of the gut health.

Another way to determine the transit profile is by sensing the gases throughout the length of the gut as each segment generates different types and quantities of gases.

Gas sensing

Gases are present inside the gut partly because of swallowed air, but other significant sources are fibre fermentation, bacterial overgrowth in the small intestine, reduced digestion or absorption of food nutrients (malabsorption) and unnatural (slow or fast) movement of food through small intestine. These gases are key in detecting various gut-related disorders like IBS, carbohydrate malabsorption and SIBO. The current gold standard is breath testing as it is easy to perform and non-invasive, but it does not provide any information about the origin (point of gas generation) of the detected gases.108 A hydrogen sensing capsule was developed to overcome the limitations of breath tests and tested in vivo in a pig model as a proof-of-concept demonstration.109 The sensing parameters were then extended to include the measurements of methane and carbon dioxide gases.110 Finally, the gas-sensing capsule with the capability to measure oxygen, hydrogen and carbon dioxide was tested in human subjects and gathered precise information on the origin of the gas generation which was unable to be obtained by a breath test performed simultaneously in human subjects.16 111 The Atmo Gas Capsule (Atmo Biosciences, Australia)112 as shown in figure 3F was tested in humans and compared with the SmartPill Motility Capsule (Medtronic, USA)81 to generate gut transit profiles.113

An important parameter considered in gut health diagnosis is accurate digestion and absorption measurement, which is assisted by gas-sensing smart capsules that provide accurate measurement of gut performance.

Robotic capsules for sampling the gut

The study of gut microbiota is gaining massive attention due to its significant impact on human health. The gut microbiota is an important factor in human health, and it can act as a biomarker for early diagnosis of diseases like diabetes, cancer and obesity. The gut microbiota can also be used for better identification of IBD, ulcer, coeliac disease, Crohn’s disease and IBS. The current gold standard for collecting a sample of gut microbiota is faecal sampling but it is highly contaminated, and it is not possible to localise faecal samples which means it is not possible to associate a particular disease to a certain location within the 9 m long gut. The microbiota contains a depth of information that cannot be fully captured by smart capsules through sensing or imaging, so robotic capsules are being developed to collect samples from the gut to allow detailed examination after capsule retrieval. Devices that can collect a sample from the gut are divided into two broad categories. First, biopsy devices that can collect a small tissue sample from the gut wall, and second, sampling devices that can collect gut content-based (fluid) samples which contain digesta, mucus, microbiota and exfoliates.

Biopsy devices for tissue sampling

Due to the limitations of tethered endoscopy with biopsy tools in terms of their reach into the gut and risk of gut perforation and bleeding, robotic capsules were developed to perform tissue biopsy. In 2003, a patent application proposed a wireless capsule system to collect a biopsy specimen in a biological body using microspectroscopy and/or biosensors.114 This capsule contained two motorised blades to capture solid tissue or a liquid specimen that could be stored in two dedicated compartments to avoid contamination until its recovery. Another design consisted of a razor connected to a torsional spring was proposed that triggered the biopsy process by melting a paraffin block which allowed the razor to rotate at high speed with the help of torsion spring to capture a tissue sample.115 Both approaches required the capsule to be in close contact (almost rubbing) with the intestinal wall to capture the tissue. To allow this to occur, a magnetic biopsy capsule was used and an external magnet held the capsule’s lateral hole against the intestinal wall while a cylindrical razor blade cut the tissue with magnetic actuation.116 However, this design lacked a triggering mechanism to effectively locate the target-site. Therefore, a design with a micro reed switch that triggered the biopsy process based on external magnetic field was presented that included an elliptical hole to affix the target tissue and a spiral spring to produce the rotational force.117 Once the capsule reached the target-site, the reed switch was triggered by an external magnetic field that heated a shape memory alloy (SMA) spring. This resulted in the cutting of a special polymer string which allowed the torsional spring to rotate the biopsy cutting tool, hence collecting the affixed tissue from the elliptical hole.

The biopsy tools that remained inside the capsule shell could not guarantee tissue collection despite the external magnetic holding mechanism, therefore some designs were presented that actively moved the tools outside the capsule shell to ensure tissue collection as shown in figure 4. A motor with a rack and pinion gear system moved a biopsy tool (forceps with barbs) from inside of a capsule shell to biopsy, but this compromised the limited power available to the capsule endoscope to perform usual imaging tasks.118 Another device used two ring-shaped permanent magnets to move the biopsy forceps outside the shell to actively cut intestinal tissue.119 An SMA actuator was used to project a biopsy razor out of the capsule shell once the capsule was at the target-site, then the rotating magnetic field rotated the capsule prototype helping the razor to cut the tissue from the gut wall and finally two restoration magnets were used to bring the cutting tool back inside the capsule shell to secure the sample.120 However, the two magnets used for restoration of the biopsy module to bring the ejected blade back inside the capsule, were too large and did not allow a telemetry system for triggering the biopsy process to fit inside traditional endoscopes. Another design used a gear-assembly to move forceps-style blades in and out of the capsule shell as shown in figure 4C that allowed the biopsy system to fit inside a traditional endoscope.121

Figure 4

Tools for tissue biopsy. (A) Cylindrical razors to cut a targeted tissue sample.126 (B) A barb-based design to penetrate the gut wall to extract a tissue sample.124 (C) A scissor-shaped razor to clamp tissue inside the jaws.121 The razor used magnetic actuation to eject from the capsule shell and cut the targeted tissue with scissor motion that could be visualised with the camera module. (D) The elaboration of tissue extraction method using the scissor-shaped razor.121

All these designs presented a biopsy tool, and some also included a triggering method but most of them lacked locomotion to help the capsule reach the target-site. A biopsy capsule with an active locomotion mechanism that could move flexibly within the gut and extract a tissue sample with rotating blade mechanism from the target location by magnetic actuation was presented.122 Later, this research group modified the blade design with a retractable biopsy punch to fit inside a traditional endoscope so the biopsy procedure could be effectively performed with visual aid from the camera module.123

Some designs considered obtaining a biopsy sample from the mucosa layer, as superficial collection from the epithelial layer was not sufficient for in-depth microbiota analysis. A unique design of a micro-spike with barbs was proposed as shown in figure 4B that was triggered by heating an SMA wire that moved the micro-spike outside the capsule shell by a slider crank mechanism with a torsion spring used to penetrate into the mucosa layer.124 The barbs helped to tear the biopsy samples which was then stored inside the capsule to avoid contamination. Another capsule captured a fine-needle aspiration biopsy, and was triggered with a magnetic actuation that squeezed the capsule allowing a sharp hollow needle to penetrate into the mucosa layer to obtain a sample, which was not explored previously by the earlier biopsy capsules.18 125

One of the challenges for most of the biopsy capsules is the lack of control of their motion as they rely on peristaltic forces to reach the target-site and stopping the capsule at the target-site is not possible. Some of the designs used an external magnetic field to anchor the capsule at the target-site but this increased the complexity and cost of the overall system while the capsule anchoring could not be precisely controlled. A complete solution with a tissue monitoring module using a camera, anchoring module using SMA springs to stop the capsule at the target-site and a biopsy module using two cylindrical razors and a spiral spring to extract the tissue did show clinical promise.126 However, the capsule required further miniaturisation as it was oversized for in vivo testing and supplying the power to all modules was challenging as the size of the battery was too big to fit inside a swallowable capsule. Another capsule used a single magnetic actuator to drive both the anchoring mechanism and biopsy spike tool using a ratchet mechanism to overcome the power limitations, but intestinal trials are yet to be realised.127

In order to collect biopsy samples from the stomach, a magnetically actuated capsule was developed that released a large number of temperature-sensitive microgrippers that self-fold themselves due to a change in temperature.128 The capsule then collected the microgrippers with an adhesive patch using its camera module.

The biopsy devices are promising tools for collecting tissue samples from the gut wall and they can overcome the limitations of tethered devices by accessing the entire gut. The biopsy tools are used with locomotion and localisation mechanisms to efficiently capture the target tissue. However, these devices cannot be used for sampling the gut microbiota as they cannot capture content-based samples.

Sampling devices for content-based samplingPatent applications

The development of tools for sampling the gut contents are gaining attention based on the impact microbiota have on human health and the amount of information microbiota can reveal. The promising benefits of sampling devices for the collection of gut microbiota has resulted in many patents with an intent to produce a commercial device. A patent filed in 1957 which was later published in 1962, intended to track a capsule through X-ray to determine the target-site and to open an inlet through radiant energy from outside of the subject, to collect a sample.129 However, this design did not specify how to protect the sample from downstream contamination, that is, if sealing the sample storage compartment completely was not possible then samples from other locations further down the gut may continue to enter the storage. Another patent suggested the use of a low melting point spring that could be heated from outside the host using an electromagnetic field, hence detaching the spring which then opened the chamber for sample collection.130 The spring was connected to a piston inside a slider that allowed the piston to move to the other end once the fluid chamber was filled, which closed the inlet of the chamber and secured the sample from contamination. Another patent proposed to use an ether-filled bellows inside a capsule that could expand on heating from an external electromagnetic field resulting in collection of the surrounding fluid, with the capsule returned to its original position once the magnetic field was removed which secured the collected sample.131

Most of the designs used separate opening and closing mechanisms, hence making them complicated. Second, a lot of the capsules were designed for one-off use, which increased the overall cost and reduced sustainability. Therefore, SMA materials were introduced that allowed the reuse of capsules multiple times. An SMA spring was latched inside the capsule chamber which was compressed when the temperature was changed by passing electric current through the spring that allowed the chamber to sample content from an orifice.132 133 Another design with a rotatable mechanism was based on twisted concentric cylinders that used an SMA polymer which were heated to open apertures on the circumferential wall, allowed the surrounding fluid to enter the capsule.134 The device moved back to its original position when heating was stopped which secured the collected sample from contamination. Another patented design used an SMA polymer for the inlet (door) of the capsule.135 The SMA polymer shape was designed to block the outside fluid from entering the chamber. When the SMA polymer was heated using induction heating, it allowed the outside fluid to enter the capsule, once heating was stopped the polymer returned to its original shape which secured the sample inside the chamber and avoided cross-contamination.

However, most of these designs required a strong external (magnetic or electromagnetic) field to trigger the sampling process which required an expensive external setup. Therefore, some internal triggering mechanisms have been proposed that rely on internal resources from within the capsule device. One capsule used wireless communication to trigger a set of spring-loaded concentric cylinders that were joined with a meltable thread.136 The wireless receiver was used to ignite the heater that melted the thread and allowed the spring to open the concentric cylinders which in turn collected the sample via suction from a small inlet. This capsule was designed for one time use, while another patent proposed a wireless triggering mechanism that used a motor to open the sampling chambers. The capsule consisted of two motorised blades to capture solid tissue or a liquid specimen that could be stored in two dedicated compartments to avoid contamination until recovery.114

The embedded designs that incorporated both sampling and triggering mechanisms inside the capsule, left less capacity for the sample storage itself. Therefore, a simplified design was proposed that collected the content from the intestine with both active and passive mechanisms.137 The capsule had a vacuum compartment with a seal which could be dissolved by a chemical reaction when the target location was reached. For the active mechanism, the compartment opening was covered by a magnet which was displaced using an external magnetic field. However, neither the active nor passive mechanisms proposed, defined any method to close the compartment to stop downstream contamination so targeted sampling was not possible. Another patent proposed using a fluid-sensitive membrane to cover the inlet of the chamber which was dissolved by interacting with the stomach or intestinal fluid and allowed the accumulation of a sample inside the chamber. The inlet then got closed by a spring-operated valve that blocked the orifice once the chamber was filled with fluid.138 Wrigglesworth et al proposed an extendable mechanism to collect digesta from the ileum (small