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Small intestine mucosal adhesivity to in vivo capsule robot materials
journal of the mechanical behavior of biomedical materials 15 (2012) 24 –32
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/jmbbm
Research Paper
Small intestine mucosal adhesivity to in vivo capsule robot materials Benjamin S. Terrya,n, Anna C. Passerniga, Morgan L. Hilla, Jonathan A. Schoenb, Mark E. Rentschlera a
Department of Mechanical Engineering, University of Colorado at Boulder, 427 UCB, 1111 Engineering Drive, Boulder, CO 80309-0427, USA Department of Surgery, University of Colorado at Denver, 12631 E 14th Avenue, Aurora, CO 80045, USA
b
ar t ic l e in f o
abs tra ct
Article history:
Multiple research groups are investigating the feasibility of miniature, swallowable, in vivo,
Received 18 April 2012
untethered robots that are capable of traversing the small intestine for the purpose of
Received in revised form
acquiring biometrics and performing simple surgical procedures. A mathematical model of
18 June 2012
the intraluminal environment will speed the development of these so-called Robotic
Accepted 28 June 2012
Capsule Endoscopes (RCEs), and to this end, the authors, in previous work, initiated a
Available online 6 July 2012
comprehensive program for characterizing both the active and passive forces exerted by
Keywords:
the small intestine on an RCE-sized solid bolus. In this work, forces due to adhesivity
Small bowel
between RCE materials and the mucosa are investigated. The experimental factors are
Mechanical characterization
adhesive modality (peel and tack), material (polycarbonate, micropatterned polydimethyl-
Mucosa adhesivity
siloxane, stainless steel, and mucosa), and bowel region (proximal, middle, and distal). The
Mucoadhesion
mucosa is excised from a fasting pig, stored in lactated ringer’s solution at 3 1C, and then
Robotic capsule endoscope
tested at room temperature within 43 h of excision. The results show the mean tack strength of the mucosa to engineering materials was 0.19870.070 mJ cm2. The mean peel strength was 0.05570.016 mJ cm2. This study marks the first time, to the authors’ knowledge, that adhesivity between small intestinal mucosa and RCE engineering materials has been measured. The adhesivity values acquired from this study will provide a valuable input into analytical and numerical models of the gastrointestinal tract, specifically models that account for the interfacial properties of the tissue. & 2012 Elsevier Ltd. All rights reserved.
1.
Introduction
Multiple research groups are investigating the feasibility of miniature, swallowable, in vivo, untethered robots that are capable of traversing the small intestine for the purpose of acquiring biometrics and performing simple surgical procedures (Glass et al., 2008; Woo et al., 2011; Sliker et al., 2011; n
Twomey, 2009; Quirini et al., 2008; Simi et al., 2010). This effort has been hindered, in part, by the lack of knowledge concerning the material properties of the intraluminal environment. An analytical model of this environment will speed the development of these so-called Robotic Capsule Endoscopes (RCEs), and to this end, the authors, in previous works, initiated a comprehensive program for characterizing both
Corresponding author. Tel.: þ1 3039488165. E-mail addresses: [email protected] (B.S. Terry), [email protected] (A.C. Passernig), [email protected] (M.L. Hill), [email protected] (J.A. Schoen), [email protected] (M.E. Rentschler). 1751-6161/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jmbbm.2012.06.018
journal of the mechanical behavior of biomedical materials 15 (2012) 24 –32
the active and passive forces exerted by the small intestine on an RCE-sized solid bolus (Terry et al., 2011, 2012a, 2012b). As an extension of that work, forces due to adhesivity between RCE engineering materials and the mucosa are investigated. Mucosa lines the inner surface of the small intestine and consists of an epithelial layer of simple columnar goblet cells that continually secrete a protective mucus coating that adheres to intraluminal contents. The secretions form a layer of firmly adhered gel-like substance covered by a loose layer that is readily removed by mechanical shear or suction (Atuma et al., 2001). The glycoprotein molecules in the mucus have hydrophilic and viscoelastic properties and an ability to adhere to solids, such as an RCE. The mechanical properties of the mucus facilitate the removal of particulate matter from the gastrointestinal (GI) tract without damaging the mucosa. The mucus layer behaves like a liquid on the nanoscale and as a non-Newtonian gel on the macro-scale due to its rate dependent response to shear stress (William, 2005). At the time of writing, there is no experimental data regarding the thickness of the mucus layer in the human small intestine; however, the thickness in a single porcine model was measured at 25.9711.8 mm at the proximal end of the duodenum and gradually thickened to 31.0715.7 mm at the distal end of the ileum (Varum et al., 2010). The thickness is a function of the secretion rate of the goblet cells, erosion by mechanical shear, and bacterial digestion (Hoskins and Boulding, 1981). The mucus layer in humans regenerates approximately every 24–48 h (Hanes and Lai, 2010). Mucosal adhesivity is its interfacial bonding ability and is quantified by the energy required to separate the two adhered surfaces. Several factors affect mucosal adhesivity, such as hydration, mucus surface tension, wettability, temperature, and dwell time (the amount of time the mucosa is in contact with the solid surface prior to separation) (King, 1998). During enteroscopy, the mucosa is in intimate contact with a mechanical device, and this contact plays a role in device performance, especially for untethered, robotically controlled devices that have limited power supply. RCEs experience forces from a variety of sources. For example, the myenteron exerts contact pressure (Terry et al., 2012; Miftahof and Fedotov, 2005), the pumping action caused by segmentation and peristalsis of the GI tract generates pressure against the robot fore and aft (Camilleri, 1997), and hydrostatic pressure created by respiration, skeletal muscle movement, and gravity all impart forces on an RCE (Samsom et al., 1998). In addition to these active forces, reactions from the biomechanical response of the small bowel tissue act on the robot (Sacks and Sun, 2003; Higa et al., 2007; Lim et al., 2009). These forces are transmitted to the RCE via the inner surface of the small bowel, the mucosa. Therefore, knowledge of the mucosal-RCE surface interaction is necessary in order to understand the effects of the active and passive forces on robotic mobility. In addition to contact pressure, surface interactions are characterized by adhesivity, dry friction, and fluid shear. The authors’ previous work and that of other groups have begun to investigate tribological interactions (Glass et al., 2008; Lyle et al., 2011; Wang and Meng, 2010; Wang and Yan, 2009), but the authors are not aware of any study regarding the adhesivity of the mucosa to
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RCE engineering materials. The purpose of this study, therefore, is to measure the adhesivity of the proximal, middle, and distal small intestine to typical engineering materials used in the construction of an RCE.
2.
Methods
Presently, there is no standard for measuring the adhesivity of biological tissue to engineering materials. Mucosa, however, can be thought of as a pressure sensitive adhesive (PSA). Similar to the mucosa, PSAs, such as self-stick tapes, form bonds between surfaces simply by pressure without the aid of activating agents such as water, solvents, or heat. Therefore, the rigorous, well-developed ASTM test protocol and apparatuses for characterizing PSAs were used in this work to measure mucosal adhesivity. Adhesion between the mucosa and the RCE primarily follows two modalities: tack and peel. Tack is when two adhered surfaces separate without undergoing shear while maintaining parallel and flat orientation. Peel is the separation of two adhered surfaces by applying a force to the leading edge of one of the surfaces so that it is no longer flat relative to its mating surface. Both tack and peel were tested using an Insight II tensile testing machine (MTS systems), 2 or 5 N loadcell (MTS Systems, PN 569326-01), and custom fixtures.
2.1.
Validation of test apparatus
The purpose of the validation protocol is to measure the repeatability of the adhesive tack and peel strength test apparatuses, and to provide an intuitive feel for the adhesive strength of a commercial tape for anecdotal comparison with the biological adhesive strength of the mucosa. A commercial adhesive (3M, PN 9471LE) was used for this validation due to its excellent geometric uniformity and its environmental stability. Cast polypropylene was used as the adherend due to its moderate surface energy characteristics. Since testing lasts 1 h (a fraction of the tape’s 2-year shelf life), it is assumed variation in tack and peel results is due only to the apparatuses and methods and not a result of deviation in tape material properties. The adhesivity of mucosa, on the other hand, is expected to be highly variable, similar to the other mechanical properties of biological tissue. The test procedure used to measure the tack adhesivity of the mucosa was modeled after the ASTM standard test method for tack (designation: D2979) (ASTM Subcommittee D14.50, 2009). The standard specifies a protocol and apparatus (Fig. 1, left) to determine the pressure sensitive tack strength of a commercial adhesive to a test material. Tack is defined as the force required to separate an adhesive and the adherend shortly after they have touched. As mentioned, the tack test apparatus was validated using a commercial adhesive in place of the mucosa, and polypropylene as the test material (the adherend). To validate, the commercial adhesive was transferred to a substrate and mounted on the lower grip of the test apparatus. A 1 cm2 square piece of cast polypropylene was brought into contact with the adhesive and held for 10 s with a mean pressure of 5 kPa. After the 10 s dwell time, the polypropylene was pulled
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journal of the mechanical behavior of biomedical materials 15 (2012) 24 –32
Fig. 1 – The tack test apparatus consists of a square piece of test material attached to a loadcell (left, A), which is brought into contact with the test adhesive (left, B). The peel test apparatus consists of a linear slide that translates horizontally and a grip attached to a loadcell that peels the adhesive from the test material at 901. The vertical movement of the grip is matched by the horizontal movement of the linear slide to maintain the 901 peel angle (right). from the tape at 10 mm s1. The pull force was recorded during pull-away for ten samples and the mean tack strength was calculated by Z dt F ds ð1Þ Wtack ¼ 0
where F is the force required to separate the two surfaces and dt is the displacement of the upper grip during the tack test. Note that zero displacement corresponds to the no-load position prior to separation. Tack strength per unit area is found by dividing (1) by the contact area in cm2. The test protocol to measure the peel adhesivity of the mucosa was modeled after the ASTM standard test method for 901 peel adhesion (designation: D3330/D3330M-04) (ASTM Subcommittee D10.14, 2010). The standard specifies a protocol and apparatus (Fig. 1, right) to determine the peel strength of an adhesive. Peel strength is defined as the average force required to cleanly pull tape at 901 from a substrate. Similar to the tack test, the peel test is validated by peeling the commercial adhesive from a polypropylene substrate. A 5.170.25 mm 147 mm length of transfer tape was rolled onto polypropylene. Following 30715 s of dwell time, one end of the tape was pulled 901 at 10 mm s1 while the pull force was recorded. As described in the standard, the pull force is maintained at 901 by a linear slide that moves in the horizontal direction at the same rate as the vertical pull rate. The total force experienced by the loadcell during the peel is FT ¼ Fa þ g
ð2Þ
where Fa is the adhesive force and g is the weight of the adhesive suspended by the grip. During validation, the weight (g) is negligible due to the small mass of the tape and its relatively strong adhesivity (i.e. Fa 4g). However, during biological testing, the weight of the tissue is significant because it is large relative to its adhesive force (Faog). Assuming uniform adhesive samples, g is a function of instantaneous peel length s: gðsÞ ¼ sðgT =dp Þ
ð3Þ
where gT and dp are the total weight and length of the peeled tissue sample, respectively.
Table 1 – Factors and levels that define the 96 adhesivity tests. Factor
Level
Adhesivity test Region of bowel Pig material
Tack, peel Proximal, middle, distal One, two, three, four Mucosa, stainless steel, polycarbonate, micropatterned polydimethylsiloxane (PDMS)
Total adhesive force is found by substituting (3) into (2) and solving for Fa: Fa ðsÞ ¼ FT sðgT =dp Þ
ð4Þ
Peel strength is calculated by integrating the adhesive force, Fa(s), over the peel length, dp, and is Z dp Wpeel ¼ FT sðgT =LÞ ds ð5Þ 0
Peel strength per unit area is found by dividing (5) by the contact area in cm2.
2.2.
Mucosal tack and peel adhesivity tests
Small bowel tissue was acquired from the University of Colorado Hospital according to Institutional Animal Care and Use Committee standards (protocol number 87909-051D). The tissue was packaged in plastic Ziplocs-style bags filled with Lactated Ringer’s Solution and transported on ice to the testing facility. Care was taken not to freeze the tissue due to a study by Samuel et al. (2008) that finds cryogenically preserved and then thawed bowel tissue exhibits different adhesive characteristics than fresh tissue. Most samples were tested within 12 h of euthanization, and all samples were tested within 43 h. Every permutation of the factors and levels shown in Table 1 were tested, which resulted in 24 tests per pig intestine and 96 total tests for the four porcine models. The test order was randomized. Stainless steel, polycarbonate, and micropatterned PDMS are candidate materials for use in present or future robotic capsule
journal of the mechanical behavior of biomedical materials 15 (2012) 24 –32
endoscope designs and are therefore of interest to the authors. The micro-patterned PDMS is manufactured by the authors and is a drive component of a robotic capsule endoscope (Sliker et al., 2011) discussed in previous work (Sliker et al., 2010). The surface of the PDMS is covered with 70 mm tall, 140 mm diameter cylindrical pillars that are equally spaced at 245 mm center-tocenter distance. In addition to testing the adhesivity of mucosa to these engineering materials, the adhesivity of mucosa to itself is also investigated. In Table 2 the randomized adhesivity test matrix for Pig 1 is shown. Similar matrices exist for Pigs 2–4. Tack tests were performed on each permutation of bowel region, material of interest, and porcine model. For example, for the test permutation ‘‘Distal, PC, Tack’’ shown in Table 2, a 6.45 cm2 piece of polycarbonate is gripped by the upper grip of the tensile tester. A segment of small bowel is adhered with cyanoacrylate to the lower platform with the mesentery facing upward. The bowel is cut longitudinally along the mesentery and splayed open, exposing the mucosa. The polycarbonate is brought downward at 1 mm s1 until the material of interest is pressed against the tissue with a force of 0.2 N for 10 s of dwell time. The upper grip is then raised at Table 2 – Randomized adhesivity test matrix for pig 1. ‘‘Proximal’’, ‘‘Middle’’, and ‘‘Distal’’ indicates the test was performed on tissue from that region of the small bowel. ‘‘SS’’, ‘‘PC’’, ‘‘MT’’, or ‘‘Mucosa’’ indicates that stainless steel, polycarbonate, micropatterned tread, or mucosa were in contact with the mucosa during the test. ‘‘Tack’’ and ‘‘Peel’’ are the test protocols. Note that tests are shown randomized. Middle,PC,Peel Middle,SS,Tack Distal,MT,Peel Distal,Mucosa,Peel Proximal,PC,Peel Proximal,SS,Tack Middle,MT,Tack Middle,Mucosa,Tack
Distal,PC,Peel Middle,MT,Peel Proximal,PC,Tack Distal,Mucosa,Tack Middle,PC,Tack Distal,SS,Peel Proximal,MT,Tack Distal,MT,Tack
Proximal,MT,Peel Proximal,Mucosa,Tack Distal,SS,Tack Proximal,SS,Peel Distal,PC,Tack Middle,SS,Peel Proximal,Mucosa,Peel Middle,Mucosa,Peel
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10 mm s1 until the polycarbonate is fully separated from the tissue. Force and displacement of the upper grip are measured at 500 samples s1. The test is repeated twice (on contiguous, fresh samples) to yield a total of three contiguous measurements per tack test permutation. Similarly, the peel tests are performed on each permutation of bowel region, material of interest, and porcine model. For example, for the test ‘‘Proximal, MT, Peel’’, an 8 cm 2 cm rectangular section of small bowel is excised and placed on micro-patterned PDMS tread with the mucosa facing downward, so that it is in contact with the PDMS without entrapping air bubbles between the two surfaces. One end of the rectangular section of tissue is clamped in the upper grip. The upper grip is then raised at 10 mm s1, which peels the tissue from the PDMS at 901. The section of adhered tissue travels horizontally at the same rate the upper grip travels vertically, thus maintaining the 901 peel angle. The force on the loadcell is recorded at 500 samples s1 throughout approximately 6 cm of travel during peel.
2.3.
Statistical analysis
Four-way analysis of variance (ANOVA) was performed to examine the effects of the factors listed in Table 1 on the mean adhesivity. P-values less than 0.05 were considered statistically significant. The validity of the ANOVA was verified by testing the data for normality using the Shapiro–Wilk test.
3.
Results
3.1.
Validation of test apparatus
In Fig. 2 raw tack and peel data for ten test samples each are shown. The mean tack strength of the 3M 9471LE commercial adhesive to polypropylene per unit area was 87736 mJ cm2. The mean peel strength of the tape to polypropylene per unit area was 4572 mJ cm2. Errors are one standard deviation of
Fig. 2 – Raw tack test data from ten samples of commercial adhesive (3M 9471LE) adhered to polypropylene (left). Notice the high variability beginning around 1 mm of travel. This is due to the variable nature of the location of the abrupt release of the adhesive from the polypropylene. Raw peel data from ten samples of commercial adhesive (3M 9471LE) adhered to polypropylene (right). Notice the tension ramping as the slack is removed from the adhesive tape. Adhesion calculations do not include the ramping region. For both figures, error bars are one standard deviation of the mean.
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journal of the mechanical behavior of biomedical materials 15 (2012) 24 –32
Fig. 3 – Mucosal tack (left) and peel (right) tests.
Fig. 4 – Mucosal tack separation force versus separation position for mucosa adhered to polycarbonate (left). Mucosal peel separation force versus separation position for mucosa adhered to polycarbonate (right).
the mean. Notice the large standard deviation of the tack modality as compared to peel. This is due to the randomness of the location of the abrupt separation of the adhesive from the polypropylene. Also note that the first 1.5 mm of peel data is highly transient, which is due to tensioning of the tape, prior to actual separation, therefore peel adhesivity measurements do not include this region. Based on this data, separation of the commercial adhesive from polypropylene via the tack modality requires approximately 190% more energy per unit area than by peel. The larger energy required for separation via tack is probably explained by the contribution bulk deformation of the adhesive as explained by Josse et al., 2004 in their work measuring interfacial adhesion between a soft viscoelastic layer and a rigid surface.
3.2.
Mucosal tack and peel adhesivity tests
In Fig. 3 the mucosal tack and peel test setups are shown. In Fig. 4 representative raw tack and peel data for the test permutations ‘‘Middle,PC,Tack’’ and ‘‘Middle,PC,Peel’’ from Table 2 are shown. As described in Section 2.2, the tack data
consists of the three contiguous samples (indicated by Trials 1–3 in the figure), which are averaged, and peel data is derived from a single sample. As shown in Fig. 4 (right), the raw peel force (FT) increases due to the increasing weight of the tissue hanging from the upper grip. The weight is subtracted from the raw data resulting in Fa from (4). Note that the transient regions of the peel data are not used to calculate Fa, hence they are truncated as shown in the figure. In Figs. 5 and 6 the summaries of the full factorial tests for the mucosal tack and peel strength, respectively are shown. Each of these figures contain 11 box plots. The plots show the median, the 25th and 75th percentiles of the data (the box), and the extents of the data (the whiskers). Outliers are denoted by plusses. The bars are grouped first by pig, then bowel region, and finally material. For example, the four leftmost boxes indicated by the labels ‘‘Pig 1’’ through ‘‘Pig 4’’ compare the median adhesivity of all materials and bowel regions of each pig. The next three boxes labeled ‘‘Proximal’’, ‘‘Middle’’, and ‘‘Distal’’ compare the median adhesivity of all pigs and materials for each bowel region. The last four boxes labeled ‘‘SS’’, ‘‘PC’’, ‘‘MT’’, and ‘‘Mucosa’’ compare the median adhesivity of all pigs and bowel
journal of the mechanical behavior of biomedical materials 15 (2012) 24 –32
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Fig. 5 – Summary of the adhesivity tack strength. From left to right, boxes 1–4 show the strength per porcine model for all regions and materials. Boxes 5–7 show strength per region of the small intestine for all pigs and materials. Boxes 8–11 show strength per material tested against the mucosa for all pigs and regions: SS is stainless steel, PC is polycarbonate, MT is micropatterned PDMS tread, and Mucosa indicates the tack strength of mucosa adhered to itself.
Fig. 6 – Summary of the adhesive peel strength. From left to right, boxes 1–4 show the strength per porcine model for all regions and materials. Boxes 5–7 show strength per region of the small intestine for all pigs and materials. Boxes 8–11 show strength per material tested against the mucosa for all pigs and regions: SS is stainless steel, PC is polycarbonate, MT is micropatterned PDMS tread, and Mucosa indicates the peel strength of mucosa adhered to itself.
regions for each material. Note that Figs. 5 and 6 are both interpreted in this way. Overall, the mean tack strength of the mucosa per unit area was 0.19870.070 mJ cm2. The mean peel strength of the mucosa per unit area was 0.05570.016 mJ cm2. This result follows the same trend as the commercial adhesive where separation via tack requires more energy per unit area than by peel.
3.3.
Statistical analysis
The Shapiro–Wilk test confirmed normal distributions within the tack and peel tests for both the commercial adhesive and mucosal testing but rejected the null hypothesis for lumped tack and peel means. Therefore, the two adhesive modalities were analyzed separately. For both tack and peel tests,
30
journal of the mechanical behavior of biomedical materials 15 (2012) 24 –32
Fig. 7 – Live porcine small intestine of a fasting animal (left). Notice the intestine has collapsed on itself and is flat. Schematic of an RCE traveling through the collapsed small intestine (right). Examples of adhesivity are indicated by the dotted regions. The RCE will expend energy overcoming adhesivity on its leading edge as the mucosa peels away from itself, and also on the trailing edge as the mucosa is peeled from the tread material.
ANOVA identified significant differences between porcine models (p ¼0.05 and 0.00 for tack and peel, respectively). The differences in adhesivity between bowel regions were not significant (considering all pigs and materials), although there may be a weakening trend from proximal end to the distal end. The adhesive strength of the mucosa to itself is significantly stronger than that of mucosa to the engineering materials (p¼ 0.02 and 0.01 for tack and peel, respectively). Although not significant, the mean adhesive strength of the engineering materials to mucosa varied from least to greatest as follows: stainless steel, polycarbonate, and PDMS micropatterned tread. This trend is manifested in both the tack and peel tests.
4.
Application
Understanding the adhesivity of the mucosa to RCE materials leads to a more comprehensive knowledge of the in vivo forces experienced by such a device. For example, in Fig. 7 (left) is a section of live porcine small intestine that illustrates the flat, collapsed condition of the bowel in a fasting animal. In this condition, the mucosa is adhered to itself, and the RCE will expend energy at its leading edge to separate the two adhered surfaces as it actively travels through the collapsed bowel (Fig. 7, right). In addition, the RCE will expend energy at its trailing edge as the tank-style tracks (the drive mechanism for the authors’ RCE) peel away from the mucosa. Following is an example of how the adhesive properties of the mucosa measured in this work can be used to make a simple estimation of the energy requirements for overcoming it. The authors’ RCE (Fig. 8) is used in the following analysis as an example, however the effects of mucosal adhesivity should be considered for any RCE design. An estimation of the work required to overcome adhesion at the leading edge is Wle ¼ ðC=2ÞLWpeel; mucosa
ð6Þ
where C is the circumference of the inner surface of the small intestine, L is the axial length traveled, wpeel,mucosa is the peel adhesivity of mucosa adhered to mucosa (indicated in Fig. 6,
Fig. 8 – Front and side views of the authors’ RCE. The device has a rectangular cross section with tank style treads on the four sides. It is also equipped with a video camera.
right-most box plot). An estimation of the work required to overcome adhesion at the trailing edge is Wte ¼ NdLWpeel; MT
ð7Þ
where N is the number of treads on the RCE, L is the axial length traveled, Wpeel,MT is the peel adhesivity of mucosa adhered to PDMS micropatterned tread (indicated in Fig. 6, 2nd box plot from the right), and d is the width of a tread. Consider, for example, a typical small intestine with a nominal inner diameter of 3 cm, the authors’ RCE with tread width of 6 mm, a travel distance of 1 m, and a travel speed of 10 mm s1. An estimate of the energy required to overcome mucosal adhesivity at the leading and trailing edges due to mucosa–mucosa separation and mucosa–tread separation are Wle ¼ ð3p=2Þ100 :055 ¼ 25:9 mJ
ð8Þ
and Wte ¼ 4 0:6 100 0:054 ¼ 13:0 mJ
ð9Þ
respectively. Energy requirements for other adhesive modalities (like tack) can be calculated in a similar manner by understanding the interfacial geometries and then deriving simple equations such as those presented in (6) and (7). Note that RCE speed was given in the above example (10 mm s1) because that is the speed at which all peel and tack testing were conducted.
5.
Discussion and conclusions
To the authors’ knowledge, this is the first study of porcine mucosal adhesivity to engineering materials. As such, the test
journal of the mechanical behavior of biomedical materials 15 (2012) 24 –32
apparatuses are used to first quantify the adhesivity of a commercially available, environmentally stable, geometrically uniform adhesive. Several attributes of the tack and peel test were learned by first using commercial adhesive. For example, tack adhesivity measurements are highly variable due to the abrupt and random nature of the release of the adhesive from the adherend. Also, the adhesivity of the mucosa is several orders of magnitude less than that of the commercial adhesive, which offers an intuitive feel for the adhesive strength of the mucosa. Separation via the tack modality requires much more energy, which provides an additional consideration for RCE designers who are interested in design optimization. The relative tack and peel strengths show similar trends for both the commercial adhesive and the mucosa. Estimates of energy (and power) requirements for overcoming adhesivity appear to be well within the range of the capabilities of an untethered RCE. When applying these results, however, recall that mucosal adhesivity is a function multiple variables such as separation rate, temperature, dwell time, and postprandial conditions. This work considers a simple subset of those conditions which were of interest to the authors and that should be of general use to others pursing similar work. Future work will investigate adhesivity as a function of these variables. Also note that this is a preliminary study, and although care was taken to maintain hydration of the tissue, future work will investigate in situ measurements that are more representative of the RCE environment.
Acknowledgments This work was funded in part by a Junior Faculty Pilot Award from the Colorado Clinical and Translational Sciences Institute (CCTSI). This publication was supported by NIH/NCRR Colorado CTSI Grant no. UL1 RR025780. Its contents are the authors’ sole responsibility and do not necessarily represent official NIH view.
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Atuma, C., Strugala, V., Allen, A., Holm, L., 2001. The adherent gastrointestinal mucus gel layer: thickness and physical state in vivo. American Journal of Physiology–Gastrointestinal and Liver Physiology 280 (5), G922–G929. ASTM Subcommittee D14.50, 2009, Standard Test Method for Pressure-Sensitive Tack of Adhesives Using an Inverted Probe Machine (ASTM D2979-01). ASTM Subcommittee D10.14, 2010, Standard Test Method for Peel Adhesion of Pressure-Sensitive Tape (ASTM D3330/D3330M–04). Camilleri, M., 1997. The duodenum: a conduit or a pump? GUT 41 (5), 714. Glass, P., Cheung, E., Sitti, M., 2008. A legged anchoring mechanism for capsule endoscopes using micropatterned adhesives. IEEE Transactions on Biomedical Engineering 55 (12), 2759–2767. Hoskins, L.C., Boulding, E.T., 1981. Mucin degradation in human colon ecosystems. Evidence for the existence and role of bacterial subpopulations producing glycosidases as extracellular enzymes. Journal of Clinical Investigation 67 (1), 163–172.
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Hanes, J., Lai, S.K., Issued 2010, Compositions and Methods for Enhancing Transport Through Mucus. Patent Application 2010/0215580. Higa, M., Luo, Y., Okuyama, T., Takagi, T., 2007. Characterization of the passive mechanical properties of large intestine. International Journal of Applied Electromagnetics & Mechanics 25 (1/4), 595–599. Josse, G., Sergot, P., Creton, C., Dorget, M., 2004. Measuring interfacial adhesion between a soft viscoelastic layer and a rigid surface using a probe method. The Journal of Adhesion 80 (1), 87. King, M., 1998. Experimental models for studying mucociliary clearance. European Respiratory Journal 11 (1), 222–228. Lim, Y.-J., Deo, D., Singh, T.P., Jones, D.B., De, S., 2009. In situ measurement and modeling of biomechanical response of human cadaveric soft tissues for physics-based surgical simulation. Surgical Endoscopy 23 (6), 1298–1307. Lyle, A., Terry, B., Schoen, J.A. Rentschler, M.E., 2011. An experimental evaluation of small bowel friction forces when eliminating edge effects, In: ASME 2011 International Mechanical Engineering Congress and Exposition, Denver. Miftahof, R., Fedotov, E., 2005. Intestinal propulsion of a solid non-deformable bolus. Journal of Theoretical Biology 235 (1), 57–70. Quirini, M., Menciassi, A., Scapellato, S., Stefanini, C., Dario, P., 2008. Design and fabrication of a motor legged capsule for the active exploration of the gastrointestinal tract. IEEE/ASME Transactions on Mechatronics 13 (2), 169–179. Sliker, L.J., Kern, M., Rentschler, M.E., 2011. Surgical evaluation of a novel robotic capsule endoscope using micro-patterned treads. Journal of Surgical Endoscopy, SEND, 11–1291. Simi, M., Valdastri, P., Quaglia, C., Menciassi, A., Dario, P., 2010. Design, fabrication, and testing of a capsule with hybrid locomotion for gastrointestinal tract exploration. IEEE/ASME Transactions on Mechatronics 15 (2), 170–180. Samsom, Smout, Hebbard, Fraser, Omari, Horowitz, Dent, 1998. A novel portable perfused manometric system for recording of small intestinal motility. Neurogastroenterology and Motility 10 (2), 139–148. Sacks, M., Sun, W., 2003. Multiaxial mechanical behavior of biological materials. Annual Review of Biomedical Engineering 5, 251–284. Samuel, N.T., Vailhe´, E., Vailhe´, C., Vetrecin, R., Liu, C., Maziarz, P.E., 2008. Comprehensive characterization of the effect of tissue storage conditions on tissue–adhesive interaction. Journal of Biomaterials Science, Polymer Edition 19 (11), 1455–1468. Sliker, L.J., Wang, X., Schoen, J.A., Rentschler, M.E., 2010. Micropatterned treads for in vivo robotic mobility. Journal of Medical Devices 4 (4), 041006–041008. Twomey, K., 2009. Swallowable capsule technology: current perspectives and future directions. Endoscopy 41 (8), 732. Terry, B.S., Lyle, A.B., Schoen, J.A., Rentschler, M.E., 2011. Preliminary mechanical characterization of the small bowel for in vivo robotic mobility. Journal of Biomechanical Engineering 133 (9), 091010–091017. Terry, B.S., Schoen, J.A., Rentschler, M.E., 2012a. Measurements of the contact force from myenteric contractions on a solid bolus. Journal of Robotic Surgery, 1–5. Terry, B.S., Schoen, J.A., Rentschler, M.E., 2012b. Characterization and experimental results of a novel sensor for measuring the contact force from myenteric contractions. IEEE Transactions on Biomedical Engineering 99, 1. Varum, F.J.O., Veiga, F., Sousa, J.S., Basit, A.W., 2010. An investigation into the role of mucus thickness on mucoadhesion in the gastrointestinal tract of pig. European Journal of Pharmaceutical Sciences 40 (4), 335–341. Woo, S.H., Kim, T.W., Mohy-Ud-Din, Z., Park, I.Y., Cho, J.-H., 2011. Small intestinal model for electrically propelled capsule endoscopy. BioMedical Engineering Online 10 (1), 108.
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William, K.J., 2005. Krause’s Essential Human Histology for Medical Students. Universal-Publishers. Wang, X., Meng, M.Q.-H., 2010. An experimental study of resistant properties of the small intestine for an active capsule endoscope. Proceedings of the Institution of Mechanical
Engineers, Part H: Journal of Engineering in Medicine 224 (1), 107–118. Wang, K.D., Yan, G.Z., 2009. Research on measurement and modeling of the gastro intestine’s frictional characteristics. Measurement Science and Technology 20 (1), 015803.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/jmbbm
Research Paper
Small intestine mucosal adhesivity to in vivo capsule robot materials Benjamin S. Terrya,n, Anna C. Passerniga, Morgan L. Hilla, Jonathan A. Schoenb, Mark E. Rentschlera a
Department of Mechanical Engineering, University of Colorado at Boulder, 427 UCB, 1111 Engineering Drive, Boulder, CO 80309-0427, USA Department of Surgery, University of Colorado at Denver, 12631 E 14th Avenue, Aurora, CO 80045, USA
b
ar t ic l e in f o
abs tra ct
Article history:
Multiple research groups are investigating the feasibility of miniature, swallowable, in vivo,
Received 18 April 2012
untethered robots that are capable of traversing the small intestine for the purpose of
Received in revised form
acquiring biometrics and performing simple surgical procedures. A mathematical model of
18 June 2012
the intraluminal environment will speed the development of these so-called Robotic
Accepted 28 June 2012
Capsule Endoscopes (RCEs), and to this end, the authors, in previous work, initiated a
Available online 6 July 2012
comprehensive program for characterizing both the active and passive forces exerted by
Keywords:
the small intestine on an RCE-sized solid bolus. In this work, forces due to adhesivity
Small bowel
between RCE materials and the mucosa are investigated. The experimental factors are
Mechanical characterization
adhesive modality (peel and tack), material (polycarbonate, micropatterned polydimethyl-
Mucosa adhesivity
siloxane, stainless steel, and mucosa), and bowel region (proximal, middle, and distal). The
Mucoadhesion
mucosa is excised from a fasting pig, stored in lactated ringer’s solution at 3 1C, and then
Robotic capsule endoscope
tested at room temperature within 43 h of excision. The results show the mean tack strength of the mucosa to engineering materials was 0.19870.070 mJ cm2. The mean peel strength was 0.05570.016 mJ cm2. This study marks the first time, to the authors’ knowledge, that adhesivity between small intestinal mucosa and RCE engineering materials has been measured. The adhesivity values acquired from this study will provide a valuable input into analytical and numerical models of the gastrointestinal tract, specifically models that account for the interfacial properties of the tissue. & 2012 Elsevier Ltd. All rights reserved.
1.
Introduction
Multiple research groups are investigating the feasibility of miniature, swallowable, in vivo, untethered robots that are capable of traversing the small intestine for the purpose of acquiring biometrics and performing simple surgical procedures (Glass et al., 2008; Woo et al., 2011; Sliker et al., 2011; n
Twomey, 2009; Quirini et al., 2008; Simi et al., 2010). This effort has been hindered, in part, by the lack of knowledge concerning the material properties of the intraluminal environment. An analytical model of this environment will speed the development of these so-called Robotic Capsule Endoscopes (RCEs), and to this end, the authors, in previous works, initiated a comprehensive program for characterizing both
Corresponding author. Tel.: þ1 3039488165. E-mail addresses: [email protected] (B.S. Terry), [email protected] (A.C. Passernig), [email protected] (M.L. Hill), [email protected] (J.A. Schoen), [email protected] (M.E. Rentschler). 1751-6161/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jmbbm.2012.06.018
journal of the mechanical behavior of biomedical materials 15 (2012) 24 –32
the active and passive forces exerted by the small intestine on an RCE-sized solid bolus (Terry et al., 2011, 2012a, 2012b). As an extension of that work, forces due to adhesivity between RCE engineering materials and the mucosa are investigated. Mucosa lines the inner surface of the small intestine and consists of an epithelial layer of simple columnar goblet cells that continually secrete a protective mucus coating that adheres to intraluminal contents. The secretions form a layer of firmly adhered gel-like substance covered by a loose layer that is readily removed by mechanical shear or suction (Atuma et al., 2001). The glycoprotein molecules in the mucus have hydrophilic and viscoelastic properties and an ability to adhere to solids, such as an RCE. The mechanical properties of the mucus facilitate the removal of particulate matter from the gastrointestinal (GI) tract without damaging the mucosa. The mucus layer behaves like a liquid on the nanoscale and as a non-Newtonian gel on the macro-scale due to its rate dependent response to shear stress (William, 2005). At the time of writing, there is no experimental data regarding the thickness of the mucus layer in the human small intestine; however, the thickness in a single porcine model was measured at 25.9711.8 mm at the proximal end of the duodenum and gradually thickened to 31.0715.7 mm at the distal end of the ileum (Varum et al., 2010). The thickness is a function of the secretion rate of the goblet cells, erosion by mechanical shear, and bacterial digestion (Hoskins and Boulding, 1981). The mucus layer in humans regenerates approximately every 24–48 h (Hanes and Lai, 2010). Mucosal adhesivity is its interfacial bonding ability and is quantified by the energy required to separate the two adhered surfaces. Several factors affect mucosal adhesivity, such as hydration, mucus surface tension, wettability, temperature, and dwell time (the amount of time the mucosa is in contact with the solid surface prior to separation) (King, 1998). During enteroscopy, the mucosa is in intimate contact with a mechanical device, and this contact plays a role in device performance, especially for untethered, robotically controlled devices that have limited power supply. RCEs experience forces from a variety of sources. For example, the myenteron exerts contact pressure (Terry et al., 2012; Miftahof and Fedotov, 2005), the pumping action caused by segmentation and peristalsis of the GI tract generates pressure against the robot fore and aft (Camilleri, 1997), and hydrostatic pressure created by respiration, skeletal muscle movement, and gravity all impart forces on an RCE (Samsom et al., 1998). In addition to these active forces, reactions from the biomechanical response of the small bowel tissue act on the robot (Sacks and Sun, 2003; Higa et al., 2007; Lim et al., 2009). These forces are transmitted to the RCE via the inner surface of the small bowel, the mucosa. Therefore, knowledge of the mucosal-RCE surface interaction is necessary in order to understand the effects of the active and passive forces on robotic mobility. In addition to contact pressure, surface interactions are characterized by adhesivity, dry friction, and fluid shear. The authors’ previous work and that of other groups have begun to investigate tribological interactions (Glass et al., 2008; Lyle et al., 2011; Wang and Meng, 2010; Wang and Yan, 2009), but the authors are not aware of any study regarding the adhesivity of the mucosa to
25
RCE engineering materials. The purpose of this study, therefore, is to measure the adhesivity of the proximal, middle, and distal small intestine to typical engineering materials used in the construction of an RCE.
2.
Methods
Presently, there is no standard for measuring the adhesivity of biological tissue to engineering materials. Mucosa, however, can be thought of as a pressure sensitive adhesive (PSA). Similar to the mucosa, PSAs, such as self-stick tapes, form bonds between surfaces simply by pressure without the aid of activating agents such as water, solvents, or heat. Therefore, the rigorous, well-developed ASTM test protocol and apparatuses for characterizing PSAs were used in this work to measure mucosal adhesivity. Adhesion between the mucosa and the RCE primarily follows two modalities: tack and peel. Tack is when two adhered surfaces separate without undergoing shear while maintaining parallel and flat orientation. Peel is the separation of two adhered surfaces by applying a force to the leading edge of one of the surfaces so that it is no longer flat relative to its mating surface. Both tack and peel were tested using an Insight II tensile testing machine (MTS systems), 2 or 5 N loadcell (MTS Systems, PN 569326-01), and custom fixtures.
2.1.
Validation of test apparatus
The purpose of the validation protocol is to measure the repeatability of the adhesive tack and peel strength test apparatuses, and to provide an intuitive feel for the adhesive strength of a commercial tape for anecdotal comparison with the biological adhesive strength of the mucosa. A commercial adhesive (3M, PN 9471LE) was used for this validation due to its excellent geometric uniformity and its environmental stability. Cast polypropylene was used as the adherend due to its moderate surface energy characteristics. Since testing lasts 1 h (a fraction of the tape’s 2-year shelf life), it is assumed variation in tack and peel results is due only to the apparatuses and methods and not a result of deviation in tape material properties. The adhesivity of mucosa, on the other hand, is expected to be highly variable, similar to the other mechanical properties of biological tissue. The test procedure used to measure the tack adhesivity of the mucosa was modeled after the ASTM standard test method for tack (designation: D2979) (ASTM Subcommittee D14.50, 2009). The standard specifies a protocol and apparatus (Fig. 1, left) to determine the pressure sensitive tack strength of a commercial adhesive to a test material. Tack is defined as the force required to separate an adhesive and the adherend shortly after they have touched. As mentioned, the tack test apparatus was validated using a commercial adhesive in place of the mucosa, and polypropylene as the test material (the adherend). To validate, the commercial adhesive was transferred to a substrate and mounted on the lower grip of the test apparatus. A 1 cm2 square piece of cast polypropylene was brought into contact with the adhesive and held for 10 s with a mean pressure of 5 kPa. After the 10 s dwell time, the polypropylene was pulled
26
journal of the mechanical behavior of biomedical materials 15 (2012) 24 –32
Fig. 1 – The tack test apparatus consists of a square piece of test material attached to a loadcell (left, A), which is brought into contact with the test adhesive (left, B). The peel test apparatus consists of a linear slide that translates horizontally and a grip attached to a loadcell that peels the adhesive from the test material at 901. The vertical movement of the grip is matched by the horizontal movement of the linear slide to maintain the 901 peel angle (right). from the tape at 10 mm s1. The pull force was recorded during pull-away for ten samples and the mean tack strength was calculated by Z dt F ds ð1Þ Wtack ¼ 0
where F is the force required to separate the two surfaces and dt is the displacement of the upper grip during the tack test. Note that zero displacement corresponds to the no-load position prior to separation. Tack strength per unit area is found by dividing (1) by the contact area in cm2. The test protocol to measure the peel adhesivity of the mucosa was modeled after the ASTM standard test method for 901 peel adhesion (designation: D3330/D3330M-04) (ASTM Subcommittee D10.14, 2010). The standard specifies a protocol and apparatus (Fig. 1, right) to determine the peel strength of an adhesive. Peel strength is defined as the average force required to cleanly pull tape at 901 from a substrate. Similar to the tack test, the peel test is validated by peeling the commercial adhesive from a polypropylene substrate. A 5.170.25 mm 147 mm length of transfer tape was rolled onto polypropylene. Following 30715 s of dwell time, one end of the tape was pulled 901 at 10 mm s1 while the pull force was recorded. As described in the standard, the pull force is maintained at 901 by a linear slide that moves in the horizontal direction at the same rate as the vertical pull rate. The total force experienced by the loadcell during the peel is FT ¼ Fa þ g
ð2Þ
where Fa is the adhesive force and g is the weight of the adhesive suspended by the grip. During validation, the weight (g) is negligible due to the small mass of the tape and its relatively strong adhesivity (i.e. Fa 4g). However, during biological testing, the weight of the tissue is significant because it is large relative to its adhesive force (Faog). Assuming uniform adhesive samples, g is a function of instantaneous peel length s: gðsÞ ¼ sðgT =dp Þ
ð3Þ
where gT and dp are the total weight and length of the peeled tissue sample, respectively.
Table 1 – Factors and levels that define the 96 adhesivity tests. Factor
Level
Adhesivity test Region of bowel Pig material
Tack, peel Proximal, middle, distal One, two, three, four Mucosa, stainless steel, polycarbonate, micropatterned polydimethylsiloxane (PDMS)
Total adhesive force is found by substituting (3) into (2) and solving for Fa: Fa ðsÞ ¼ FT sðgT =dp Þ
ð4Þ
Peel strength is calculated by integrating the adhesive force, Fa(s), over the peel length, dp, and is Z dp Wpeel ¼ FT sðgT =LÞ ds ð5Þ 0
Peel strength per unit area is found by dividing (5) by the contact area in cm2.
2.2.
Mucosal tack and peel adhesivity tests
Small bowel tissue was acquired from the University of Colorado Hospital according to Institutional Animal Care and Use Committee standards (protocol number 87909-051D). The tissue was packaged in plastic Ziplocs-style bags filled with Lactated Ringer’s Solution and transported on ice to the testing facility. Care was taken not to freeze the tissue due to a study by Samuel et al. (2008) that finds cryogenically preserved and then thawed bowel tissue exhibits different adhesive characteristics than fresh tissue. Most samples were tested within 12 h of euthanization, and all samples were tested within 43 h. Every permutation of the factors and levels shown in Table 1 were tested, which resulted in 24 tests per pig intestine and 96 total tests for the four porcine models. The test order was randomized. Stainless steel, polycarbonate, and micropatterned PDMS are candidate materials for use in present or future robotic capsule
journal of the mechanical behavior of biomedical materials 15 (2012) 24 –32
endoscope designs and are therefore of interest to the authors. The micro-patterned PDMS is manufactured by the authors and is a drive component of a robotic capsule endoscope (Sliker et al., 2011) discussed in previous work (Sliker et al., 2010). The surface of the PDMS is covered with 70 mm tall, 140 mm diameter cylindrical pillars that are equally spaced at 245 mm center-tocenter distance. In addition to testing the adhesivity of mucosa to these engineering materials, the adhesivity of mucosa to itself is also investigated. In Table 2 the randomized adhesivity test matrix for Pig 1 is shown. Similar matrices exist for Pigs 2–4. Tack tests were performed on each permutation of bowel region, material of interest, and porcine model. For example, for the test permutation ‘‘Distal, PC, Tack’’ shown in Table 2, a 6.45 cm2 piece of polycarbonate is gripped by the upper grip of the tensile tester. A segment of small bowel is adhered with cyanoacrylate to the lower platform with the mesentery facing upward. The bowel is cut longitudinally along the mesentery and splayed open, exposing the mucosa. The polycarbonate is brought downward at 1 mm s1 until the material of interest is pressed against the tissue with a force of 0.2 N for 10 s of dwell time. The upper grip is then raised at Table 2 – Randomized adhesivity test matrix for pig 1. ‘‘Proximal’’, ‘‘Middle’’, and ‘‘Distal’’ indicates the test was performed on tissue from that region of the small bowel. ‘‘SS’’, ‘‘PC’’, ‘‘MT’’, or ‘‘Mucosa’’ indicates that stainless steel, polycarbonate, micropatterned tread, or mucosa were in contact with the mucosa during the test. ‘‘Tack’’ and ‘‘Peel’’ are the test protocols. Note that tests are shown randomized. Middle,PC,Peel Middle,SS,Tack Distal,MT,Peel Distal,Mucosa,Peel Proximal,PC,Peel Proximal,SS,Tack Middle,MT,Tack Middle,Mucosa,Tack
Distal,PC,Peel Middle,MT,Peel Proximal,PC,Tack Distal,Mucosa,Tack Middle,PC,Tack Distal,SS,Peel Proximal,MT,Tack Distal,MT,Tack
Proximal,MT,Peel Proximal,Mucosa,Tack Distal,SS,Tack Proximal,SS,Peel Distal,PC,Tack Middle,SS,Peel Proximal,Mucosa,Peel Middle,Mucosa,Peel
27
10 mm s1 until the polycarbonate is fully separated from the tissue. Force and displacement of the upper grip are measured at 500 samples s1. The test is repeated twice (on contiguous, fresh samples) to yield a total of three contiguous measurements per tack test permutation. Similarly, the peel tests are performed on each permutation of bowel region, material of interest, and porcine model. For example, for the test ‘‘Proximal, MT, Peel’’, an 8 cm 2 cm rectangular section of small bowel is excised and placed on micro-patterned PDMS tread with the mucosa facing downward, so that it is in contact with the PDMS without entrapping air bubbles between the two surfaces. One end of the rectangular section of tissue is clamped in the upper grip. The upper grip is then raised at 10 mm s1, which peels the tissue from the PDMS at 901. The section of adhered tissue travels horizontally at the same rate the upper grip travels vertically, thus maintaining the 901 peel angle. The force on the loadcell is recorded at 500 samples s1 throughout approximately 6 cm of travel during peel.
2.3.
Statistical analysis
Four-way analysis of variance (ANOVA) was performed to examine the effects of the factors listed in Table 1 on the mean adhesivity. P-values less than 0.05 were considered statistically significant. The validity of the ANOVA was verified by testing the data for normality using the Shapiro–Wilk test.
3.
Results
3.1.
Validation of test apparatus
In Fig. 2 raw tack and peel data for ten test samples each are shown. The mean tack strength of the 3M 9471LE commercial adhesive to polypropylene per unit area was 87736 mJ cm2. The mean peel strength of the tape to polypropylene per unit area was 4572 mJ cm2. Errors are one standard deviation of
Fig. 2 – Raw tack test data from ten samples of commercial adhesive (3M 9471LE) adhered to polypropylene (left). Notice the high variability beginning around 1 mm of travel. This is due to the variable nature of the location of the abrupt release of the adhesive from the polypropylene. Raw peel data from ten samples of commercial adhesive (3M 9471LE) adhered to polypropylene (right). Notice the tension ramping as the slack is removed from the adhesive tape. Adhesion calculations do not include the ramping region. For both figures, error bars are one standard deviation of the mean.
28
journal of the mechanical behavior of biomedical materials 15 (2012) 24 –32
Fig. 3 – Mucosal tack (left) and peel (right) tests.
Fig. 4 – Mucosal tack separation force versus separation position for mucosa adhered to polycarbonate (left). Mucosal peel separation force versus separation position for mucosa adhered to polycarbonate (right).
the mean. Notice the large standard deviation of the tack modality as compared to peel. This is due to the randomness of the location of the abrupt separation of the adhesive from the polypropylene. Also note that the first 1.5 mm of peel data is highly transient, which is due to tensioning of the tape, prior to actual separation, therefore peel adhesivity measurements do not include this region. Based on this data, separation of the commercial adhesive from polypropylene via the tack modality requires approximately 190% more energy per unit area than by peel. The larger energy required for separation via tack is probably explained by the contribution bulk deformation of the adhesive as explained by Josse et al., 2004 in their work measuring interfacial adhesion between a soft viscoelastic layer and a rigid surface.
3.2.
Mucosal tack and peel adhesivity tests
In Fig. 3 the mucosal tack and peel test setups are shown. In Fig. 4 representative raw tack and peel data for the test permutations ‘‘Middle,PC,Tack’’ and ‘‘Middle,PC,Peel’’ from Table 2 are shown. As described in Section 2.2, the tack data
consists of the three contiguous samples (indicated by Trials 1–3 in the figure), which are averaged, and peel data is derived from a single sample. As shown in Fig. 4 (right), the raw peel force (FT) increases due to the increasing weight of the tissue hanging from the upper grip. The weight is subtracted from the raw data resulting in Fa from (4). Note that the transient regions of the peel data are not used to calculate Fa, hence they are truncated as shown in the figure. In Figs. 5 and 6 the summaries of the full factorial tests for the mucosal tack and peel strength, respectively are shown. Each of these figures contain 11 box plots. The plots show the median, the 25th and 75th percentiles of the data (the box), and the extents of the data (the whiskers). Outliers are denoted by plusses. The bars are grouped first by pig, then bowel region, and finally material. For example, the four leftmost boxes indicated by the labels ‘‘Pig 1’’ through ‘‘Pig 4’’ compare the median adhesivity of all materials and bowel regions of each pig. The next three boxes labeled ‘‘Proximal’’, ‘‘Middle’’, and ‘‘Distal’’ compare the median adhesivity of all pigs and materials for each bowel region. The last four boxes labeled ‘‘SS’’, ‘‘PC’’, ‘‘MT’’, and ‘‘Mucosa’’ compare the median adhesivity of all pigs and bowel
journal of the mechanical behavior of biomedical materials 15 (2012) 24 –32
29
Fig. 5 – Summary of the adhesivity tack strength. From left to right, boxes 1–4 show the strength per porcine model for all regions and materials. Boxes 5–7 show strength per region of the small intestine for all pigs and materials. Boxes 8–11 show strength per material tested against the mucosa for all pigs and regions: SS is stainless steel, PC is polycarbonate, MT is micropatterned PDMS tread, and Mucosa indicates the tack strength of mucosa adhered to itself.
Fig. 6 – Summary of the adhesive peel strength. From left to right, boxes 1–4 show the strength per porcine model for all regions and materials. Boxes 5–7 show strength per region of the small intestine for all pigs and materials. Boxes 8–11 show strength per material tested against the mucosa for all pigs and regions: SS is stainless steel, PC is polycarbonate, MT is micropatterned PDMS tread, and Mucosa indicates the peel strength of mucosa adhered to itself.
regions for each material. Note that Figs. 5 and 6 are both interpreted in this way. Overall, the mean tack strength of the mucosa per unit area was 0.19870.070 mJ cm2. The mean peel strength of the mucosa per unit area was 0.05570.016 mJ cm2. This result follows the same trend as the commercial adhesive where separation via tack requires more energy per unit area than by peel.
3.3.
Statistical analysis
The Shapiro–Wilk test confirmed normal distributions within the tack and peel tests for both the commercial adhesive and mucosal testing but rejected the null hypothesis for lumped tack and peel means. Therefore, the two adhesive modalities were analyzed separately. For both tack and peel tests,
30
journal of the mechanical behavior of biomedical materials 15 (2012) 24 –32
Fig. 7 – Live porcine small intestine of a fasting animal (left). Notice the intestine has collapsed on itself and is flat. Schematic of an RCE traveling through the collapsed small intestine (right). Examples of adhesivity are indicated by the dotted regions. The RCE will expend energy overcoming adhesivity on its leading edge as the mucosa peels away from itself, and also on the trailing edge as the mucosa is peeled from the tread material.
ANOVA identified significant differences between porcine models (p ¼0.05 and 0.00 for tack and peel, respectively). The differences in adhesivity between bowel regions were not significant (considering all pigs and materials), although there may be a weakening trend from proximal end to the distal end. The adhesive strength of the mucosa to itself is significantly stronger than that of mucosa to the engineering materials (p¼ 0.02 and 0.01 for tack and peel, respectively). Although not significant, the mean adhesive strength of the engineering materials to mucosa varied from least to greatest as follows: stainless steel, polycarbonate, and PDMS micropatterned tread. This trend is manifested in both the tack and peel tests.
4.
Application
Understanding the adhesivity of the mucosa to RCE materials leads to a more comprehensive knowledge of the in vivo forces experienced by such a device. For example, in Fig. 7 (left) is a section of live porcine small intestine that illustrates the flat, collapsed condition of the bowel in a fasting animal. In this condition, the mucosa is adhered to itself, and the RCE will expend energy at its leading edge to separate the two adhered surfaces as it actively travels through the collapsed bowel (Fig. 7, right). In addition, the RCE will expend energy at its trailing edge as the tank-style tracks (the drive mechanism for the authors’ RCE) peel away from the mucosa. Following is an example of how the adhesive properties of the mucosa measured in this work can be used to make a simple estimation of the energy requirements for overcoming it. The authors’ RCE (Fig. 8) is used in the following analysis as an example, however the effects of mucosal adhesivity should be considered for any RCE design. An estimation of the work required to overcome adhesion at the leading edge is Wle ¼ ðC=2ÞLWpeel; mucosa
ð6Þ
where C is the circumference of the inner surface of the small intestine, L is the axial length traveled, wpeel,mucosa is the peel adhesivity of mucosa adhered to mucosa (indicated in Fig. 6,
Fig. 8 – Front and side views of the authors’ RCE. The device has a rectangular cross section with tank style treads on the four sides. It is also equipped with a video camera.
right-most box plot). An estimation of the work required to overcome adhesion at the trailing edge is Wte ¼ NdLWpeel; MT
ð7Þ
where N is the number of treads on the RCE, L is the axial length traveled, Wpeel,MT is the peel adhesivity of mucosa adhered to PDMS micropatterned tread (indicated in Fig. 6, 2nd box plot from the right), and d is the width of a tread. Consider, for example, a typical small intestine with a nominal inner diameter of 3 cm, the authors’ RCE with tread width of 6 mm, a travel distance of 1 m, and a travel speed of 10 mm s1. An estimate of the energy required to overcome mucosal adhesivity at the leading and trailing edges due to mucosa–mucosa separation and mucosa–tread separation are Wle ¼ ð3p=2Þ100 :055 ¼ 25:9 mJ
ð8Þ
and Wte ¼ 4 0:6 100 0:054 ¼ 13:0 mJ
ð9Þ
respectively. Energy requirements for other adhesive modalities (like tack) can be calculated in a similar manner by understanding the interfacial geometries and then deriving simple equations such as those presented in (6) and (7). Note that RCE speed was given in the above example (10 mm s1) because that is the speed at which all peel and tack testing were conducted.
5.
Discussion and conclusions
To the authors’ knowledge, this is the first study of porcine mucosal adhesivity to engineering materials. As such, the test
journal of the mechanical behavior of biomedical materials 15 (2012) 24 –32
apparatuses are used to first quantify the adhesivity of a commercially available, environmentally stable, geometrically uniform adhesive. Several attributes of the tack and peel test were learned by first using commercial adhesive. For example, tack adhesivity measurements are highly variable due to the abrupt and random nature of the release of the adhesive from the adherend. Also, the adhesivity of the mucosa is several orders of magnitude less than that of the commercial adhesive, which offers an intuitive feel for the adhesive strength of the mucosa. Separation via the tack modality requires much more energy, which provides an additional consideration for RCE designers who are interested in design optimization. The relative tack and peel strengths show similar trends for both the commercial adhesive and the mucosa. Estimates of energy (and power) requirements for overcoming adhesivity appear to be well within the range of the capabilities of an untethered RCE. When applying these results, however, recall that mucosal adhesivity is a function multiple variables such as separation rate, temperature, dwell time, and postprandial conditions. This work considers a simple subset of those conditions which were of interest to the authors and that should be of general use to others pursing similar work. Future work will investigate adhesivity as a function of these variables. Also note that this is a preliminary study, and although care was taken to maintain hydration of the tissue, future work will investigate in situ measurements that are more representative of the RCE environment.
Acknowledgments This work was funded in part by a Junior Faculty Pilot Award from the Colorado Clinical and Translational Sciences Institute (CCTSI). This publication was supported by NIH/NCRR Colorado CTSI Grant no. UL1 RR025780. Its contents are the authors’ sole responsibility and do not necessarily represent official NIH view.
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