Brent Utter

Application of distractive forces to the small intestine: defining safe limits.

BACKGROUND Distraction enterogenesis is a novel method for increasing small bowel length by the application of linearly directed forces. However, the magnitude of distractive forces that human and animal small bowel can safely withstand is unknown. METHODS Acute ex vivo force-displacement curves for human (n = 5) and pig (n = 6) small intestine (with and without mesentery) were made by applying increasing amounts of distractive forces to bowel immersed in normal saline (39°C). Progressive load was applied until gross disruption of the tissue was detected, or the applied force reached 1000 gram-force (gf). Histology was used to detect evidence of load-induced damage. In vivo blood flow to pig bowel with distractive loads (30-200 gf) was measured by laser Doppler. RESULTS The relationship between the level of force and degree of displacement was linear. The presence of a mesentery increased stiffness of pig bowel, but did not affect human bowel. Gross tissue disruption in pig and human tissue was seen at forces between 235 and 295 gf, respectively. However, in grossly undamaged areas, histology was unchanged even after application of higher loads. With in vivo testing, mesenteric blood flow was present up to 200 gf; however, blood flow to the bowel wall was reduced to undetectable levels at loads exceeding 100 gf. CONCLUSIONS While whole bowel tissue may tolerate greater applied loads, blood flow to the bowel wall was compromised at loads over 100 gf, suggesting that any higher forces place the bowel at risk for ischemia. These measurements will help guide the clinical application of distraction enterogenesis.

Development of an endoluminal intestinal attachment for a clinically applicable distraction enterogenesis device.

PURPOSE Previous methods of distraction enterogenesis have relied upon blind-ending intestinal segments or transmural device fixation, requiring multiple operations and potential bowel injury. We hypothesized that using a novel attachment would allow reversible device coupling to the luminal bowel surface, achieving effective endoluminal distraction. METHODS A telescopic hydraulic device was designed with latex balloon attachments covered with high-friction mesh and a dilating fenestrated elastic mask (DFM attachment), allowing mesh-to-mucosa contact only with inflation. Yorkshire pigs underwent jejunal Roux-en-Y limb creation and device placement via jejunostomy. Devices underwent 3 cycles of balloon inflation and hydraulic extension/retraction per day for 7 days and then explanted and studied for efficacy. RESULTS DFM attachment allowed reversible, high-strength endoluminal coupling without tissue injury or reduction in bowel perfusion. After 7 day implant, distracted bowel achieved a 44 ± 2% increase in length vs. fed, nondistracted bowel, corresponding to a gain of 7.1 ± 0.3 cm. Distracted bowel demonstrated increased epithelial cell proliferation vs. control bowel. Attachment sites demonstrated villus flattening, increased crypt depth, thicker muscularis mucosa, and unchanged muscularis propria thickness vs. CONCLUSION Novel high-strength, reversible attachments enabled fully endoluminal distraction enterogenesis, achieving length gains comparable to open surgical techniques. This approach may allow development of clinically applicable technology for SBS treatment.

Preliminary In Vivo Experimental Validation of SMA Based Bowel Extender for Short Bowel Syndrome

Short Bowel Syndrome is a serious medical condition caused by insufficient small bowel length resulting in significantly high rates of morbidity and mortality. The limited success of current therapies has prompted the investigation of a new treatment approach based on mechanotransduction — the process through which mechanical tensile loading on the bowel induces longitudinal growth. To enable clinically relevant mechanotransduction growth studies in large animals, such as pigs, a fully implantable and instrumented bowel extender device based on a Shape Memory Alloy (SMA) ratchet was developed and validated in benchtop and ex vivo tests. These devices, however, must also be validated against the unique in vivo environment which presents challenges such as sealing, battery life, surgical implantation, signal attenuation from tissue, and isolating the measurement of tensile loading on the bowel wall. This paper extends the earlier development work to in vivo validation experiments within live pigs. A brief summary of the bowel extender architecture and operation is provided along with earlier ex vivo results that established device limits for in vivo testing. The wireless communication rate was updated to extend battery life and new surgical implantation procedures and lengthening schemes were developed. Two bowel extenders were tested in in vivo experiments ranging from 2.5 to 4.5 days with data collected to validate the wireless communication, SMA ratcheting and load/displacement measurements, confirming that the bowel extender successfully operates in vivo. More importantly, the bowel extenders successfully induced significant growth, which is promising for future studies comparing different lengthening schemes for optimal growth and the development of a clinical device for treating short bowel syndrome in humans.Copyright

Flow Through a Mechanical Distraction Enterogenesis Device: A Pilot Test

BACKGROUND We tested the coupling portion of a prototype intraluminal distraction enterogenesis device to allow flow-through of simulated enteric contents (SEC) in both pig and human jejunum. MATERIALS AND METHODS SEC was made using 80% corn syrup. Ten-cm pig and human intestinal segments had a spoke-shaped 2.2 cm coupling adaptor sutured in place, intraluminally. The adaptor had a flow-through area of 33.6 mm(2). SEC was pumped into the proximal part of the intestinal segment at 0.083 mL/s. The times to first passage of SEC through the coupler (first drop), 10 mL, and 20 mL of SEC eluted from the distal end were recorded. RESULTS Mean time to first drop elution was 155 ± 38 s with pig, and 149 ± 22 s with human bowel (P = 0.8). This corresponded to a hydrostatic pressure of 37.5 mmHg before the initial drop passed through. Mean flow rates were 0.094 mL/s in pig bowel and 0.084 mL/s in human bowel (P = 0.09). To account for occlusion from luminal debris, a 75% occlusion of coupler holes was studied in the smaller pig bowel to investigate if reductions in flow-through area could be tolerated. Mean time to first drop increased slightly to 171 ± 15 s, but the elution rate stayed the same (P = 0.5). CONCLUSIONS After a physiologic level of initial pressure buildup allowing the first drop of SEC to pass the coupling adaptor, our prototype intestinal coupling adaptor did not obstruct flow-through of SEC, even after a 75% decrease in flow-through area. This type of attachment represents a viable approach to placing a device in-continuity without obstructing flow of enteric contents.

Design and operation of a fully implantable SMA actuated implant for correcting short bowel syndrome

Short Bowel Syndrome (SBS) is medical condition characterized by insufficient small intestine length, leading to improper nutrient absorption and significant mortality rates. The complications of current treatment methods have encouraged the development of a novel treatment method based on mechanotransduction, the process through which mechanical tensile loading induces longitudinal growth of intestine. Animal based studies with simple extension devices have demonstrated the potential of the treatment to grow healthy bowel, but an implantable device suitable for clinical use remains undeveloped. This paper presents the development of an instrumented fully implantable bowel extender based upon a shape memory alloy driven linear ratchet that can be controlled and monitored remotely. The overall bowel extender system is described with respect to specifications for pig experimental tests. The functionality of the mechanical and electrical subsystems of the device are detailed and experimentally validated on the bench top, in segments of living bowel tissue removed from a pig, and in cadaveric pigs. Mechanical loading characteristics and safe load limits on bowel tissue are identified. Results from these experiments establish the readiness of the device to be tested in living pigs, enabling studies to move one step closer to clinical studies.

Design of an SMA Actuated Mechanotransductive Implant for Correcting Short Bowel Syndrome

Short bowel syndrome is a serious medical condition afflicting an estimated 20,000 to 200,000 people in the United States with mortality rates as high as 40%, despite current treatments. Recent research on mechanotransduction, the process through which mechanical load induces tissue growth, has successfully demonstrated permanent growth of healthy, functional bowel in small animals. Unfortunately, the underlying technological approaches limit further research of growth under different load profiles and extension to safe clinical devices. This paper presents a fully implantable bowel extender which expands via a unique Shape Memory Alloy (SMA) driven ratcheting mechanism, measures the bowel tension and load, and enables studies of mechanotransductive bowel tissue growth where the displacement or load may be controlled wirelessly in real-time. The architecture and operation of the bowel extender is illustrated, focusing on the SMA driven ratcheting mechanism that incrementally expands the device. To help visualize the SMA wire and reset spring design, an alternative graphical method is outlined which transforms the SMA material curves into a Reset View based on predictions of the system forces. An analytical model predicts the ratchet mechanism force with tooth and pawl geometry selected based on packaging, load-bearing, and kinematic constraints. Force limits to maintain tissue health are established from ex vivo and in vivo porcine small bowel loading experiments. The Reset View methodology is applied to design a bowel extender prototype which is used to experimentally validate the ratchet force model. The functionality device is demonstrated, operating against loads much larger than specified, validating the device’s ability to enable new studies of mechanotransductive bowel growth in pigs.Copyright

Damper-controlled switch for SMA motion smoothing

While the use of SMA-actuated devices continues to grow in many industries, current device limitations pose a challenge to successful adoption for certain classes of applications. SMA-actuated devices typically demonstrate motion with non-constant velocity due to the non-linear thermo-mechanically coupled behavior of SMA material transformation, and motion sensitivity to external factors such as voltage and load. This variation in motion can lead to the perception of poor device quality, limiting SMA-actuated devices to applications hidden from the sight of the product user, or requiring them to be augmented with higher cost controls to improve the motion quality. Therefore, a need exists for simple, passive, low-cost device technologies that enable the designer to prescribe desired motion characteristics with relative insensitivity to fluctuation in operating conditions. This paper presents a Damper Controlled Switch (DCS) mechanism that delivers constant velocity and relative insensitivity to operating conditions when combined with a standard SMA wire actuator. The DCS includes a damper that acts against a spring to open a switch when the velocity exceeds a tunable threshold. To validate the ability of the DCS to provide the desired motion quality, experiments were conducted comparing the normal motion of the SMA actuator to the motion produced when the same actuator was fitted with a DCS prototype. The addition of the DCS produced nearly constant actuator velocity, performing significantly better than the SMA actuator alone. The tunability of the DCS was demonstrated producing a wide range of attainable constant velocities. Finally, a set of experiments explored the DCS’s sensitivity to voltage and load, indicating a low sensitivity to a wide range of operating parameters for which the operating limits were identified. The DCS represents a simple, compact technology based on passive, low-cost components, providing a very practical solution that will enable integration of SMA-actuated devices into a broader class of applications.

Design of SMA Helical Actuators: An Experimental Study

The Shape Memory Alloy (SMA) helical actuation architecture overcomes the typical strain limitations of straight wire SMA actuators by producing larger stroke in a small package size. SMA Helical actuators also provide design tailorability where the tightness of the coil can be used to make tradeoffs between force and displacement, along with the coupled tradeoff between package length and diameter. These are very attractive for industrial settings such as the automotive industry, but require models and design processes to support the needs of the quick, early design cycles typically required. This paper presents an experimentally-based parameter study from which a streamlined design process is extracted. SMA helical actuators with a range of wire diameters and coil diameters were fabricated and experimentally characterized. The functional dependence of performance metrics such as austenite stiffness, force level of the martensite plateau, and recoverable strain were evaluated. A two-step decoupled design procedure is presented based on the resulting empirical model where the kinematic design is first undertaken to select the spring index which provides the required strain within the available package space, and then the non-dimensionalized force is used to scale the design such that the required actuation force is achieved. The streamlined, systematic design framework presented in this paper provides the means for the design of compact, large stroke helical actuators in industrial settings.Copyright

Model-Based Shape Memory Alloy Wire Ratchet Actuator Design

Shape Memory Alloy (SMA) wire ratchet actuators overcome SMA wire strain limitations by accumulating actuation stroke over multiple cycles. The underlying architecture is effective for producing large strokes from a small package, creating continuous rotation or extended displacement, and precise. It also provides discrete positioning with zero-power hold. While there have been several successful implementations of SMA ratchet actuators, most are designed ad-hoc since limited models exist to predict the stroke and force interaction during actuation cycles. Since the SMA wire actuation is highly dependent on the forces experienced through the ratchet mechanism, a model requires the prediction of the force interaction between the rack and pawl teeth along with friction in the device, and of the external force variation over actuation cycles due to the relative position change between the external system and the SMA wire. This paper presents a model-based systematic design methodology for SMA ratchet actuator which actuates position-dependent external systems. A generalized ratchet mechanism and operation sequence is introduced along with a force balance model for both austenite and martensite equilibrium to address the mechanical coupling changes. Analytical kinematic and kineto-static rack and pawl interaction models are reviewed, which feed into the force balance models. The effective stroke is evaluated by subtracting backlash from the SMA wire stroke, found through equilibrium with the mechanism and external system. This effective stroke accumulates to produce the overall actuator motion. A design methodology is suggested along with visualization methods to aid design decisions. Parametric studies expose the effects of design parameters on the SMA ratchet actuator to gain further design insight. This model-based design foundation and parametric understanding enable the synthesis of SMA wire ratchet actuators.Copyright