Minimally invasive surgical (MIS) techniques, including laparoscopy, are widely known to provide many benefits for patients [1]. More recently, teleoperated surgical devices have provided clinicians the ability to perform MIS procedures with the advantages of 3D vision and more dexterous instruments, which may provide additional benefits to the patient [2]. Laparoscopic and robotic procedures are done with instruments inserted through cannulas, which create ports from the outside to the inside of the patient (Fig. 1Fig. 1(a) Robotic cannula, and (b) in body wall). Some MIS procedures have resulted in post-operative port site complications including wound infections and hernias [3]. There is minimal data regarding forces applied at the port sites and possible relation to such complications. This paper focuses on a surgical device that may be useful for assessing forces applied at the body wall that could relate to port site tissue injury.A sensorized cannula was developed to monitor body wall forces during MIS procedures performed through the abdominal wall. Existing devices for measuring various forces during surgery have focused on measuring the forces exerted between instruments and tissue, or require a specially sensorized instrument in conjunction with a sensorized cannula [4–6]. It may also be possible to use an instrumented laparoscopic tool and analytically calculate the force at the body wall with well-defined boundary conditions (i.e., static forces). However, our goal was to have a force sensing tool in which any laparoscopic or robotic instrument can be inserted and used naturally.A device for measuring forces at the interface between a cannula and a body wall must be small, robust to anatomic environments, and sensitive within the range of 0–50 N. This range was determined through preliminary porcine studies with a digital force gauge (Chatillon, FL, USA) and an 8 mm metal cannula. The final assembly must have a low profile such that the geometry of the sensorized cannula does not interfere with the surgical procedure or alter the way forces are applied to the body wall.Many sensor options were considered in the design phase. Traditional force–torque sensors were not available in the proper form factor, miniature force sensors lacked the breadth of force range, and resistivity-based thin film sensors failed to provide reliable measurements when curved to the proper contour and introduced to an in vivo environment. Feasibility testing confirmed that all design specifications could be met by a set of tactile array sensors based on MEMS barometers on a printed circuit board (Fig. 2) [7].Four off-the-shelf sensor strips with five barometers per strip [8] were arranged on four sides of an 8 mm diameter long-instrument cannula (Fig. 3) and modified with pliable materials to achieve the proper force range. Barometers measure the ambient air pressure through an air hole in the sensor housing, and therefore they are configured to measure forces on the same order of magnitude as those resulting from air pressure. Higher forces quickly cause the sensors to saturate. This force limit can be increased by using materials to trap air inside the sensor housing, thereby compressing air against the MEMS sensor when a force is applied to the material. Weather-resistant rubber foam was found to tightly seal enough air inside the housing such that the MEMS sensor could measure more than 50 N without saturating. Hard plastic tubing (Teflon FEP, Shore D55) was wrapped around the assembly to distribute the load between sensors and sensor strips. The sensors were sealed with silicone to avoid damage from blood or moisture. Calibration was performed with a digital force gauge at the remote center, which is the location on the cannula that is aligned with the body wall. Figure 4 shows the calibration curve of force versus sensor output normalized to the sensor's max output value (in terms of counts). The nonlinear nature of the curve reflects the nonlinear behavior of the foam, the plastic tubing, and the silicone when forces are applied.The performance of the sensorized cannula was validated through live porcine model MIS experiments. A sensorized laparoscopic tool was outfitted with a force–torque sensor (ATI Nano25, NC, USA) to measure the surgeon’s force input to the tool. A simple static analysis of the tool was then used to calculate the resultant body wall forces. During testing, body wall forces were measured by the cannula sensor and the ATI-sensorized laparoscopic tool simultaneously. Typical surgical tasks that require static interaction with tissue in an upper GI setup were performed. Two independent examples of static gallbladder retraction tests are shown in Fig. 5. Both tasks demonstrate the agreement between the cannula sensor (blue line) and the ATI sensor (green line). Across multiple tasks, the sensorized cannula measurements showed agreement with the ATI sensor to within 98% accuracy on average. Although validation tests were performed quasi-statically due to the limitations of the laparoscopic tool’s static analysis, it is important to note that the sensorized cannula measurements are valid for dynamic forces as well.Currently, one proof of concept for measuring body wall forces has been developed and tested during MIS techniques. Experiments (to be published separately) were designed to measure the changes in body wall forces for MIS tasks ranging in motion speed, size, and forcefulness in tissue interaction. Work is currently being done to reduce the outer diameter of the sensorized cannula assembly and to improve the assembly process for repeatability, calibration, and reliability for in vivo environments. Other methods of modifying the force range, such as casting the assembly in rubber, are also being explored.The success of this device, a proof of concept for a sensorized cannula, shows that it is possible to directly measure body wall forces at port sites during MIS. It should be noted that tissue damage is a function of pressure, and the larger diameter of the sensorized cannula does affect the pressure distribution of typical forces. Nonetheless, this device may enable clinicians to better understand and potentially reduce port site tissue injury.

We describe a modeling methodology intended as a preliminary step in the identification of appropriate constitutive frameworks for the time-dependent response of biological tissues. The modeling approach comprises a customizable rheological network of viscous and elastic elements governed by user-defined 1D constitutive relationships. The model parameters are identified by iterative nonlinear optimization, minimizing the error between experimental and model-predicted structural (load-displacement) tissue response under a specific mode of deformation. We demonstrate the use of this methodology by determining the minimal rheological arrangement, constitutive relationships, and model parameters for the structural response of various soft tissues, including ex vivo perfused porcine liver in indentation, ex vivo porcine brain cortical tissue in indentation, and ex vivo human cervical tissue in unconfined compression. Our results indicate that the identified rheological configurations provide good agreement with experimental data, including multiple constant strain rate load/unload tests and stress relaxation tests. Our experience suggests that the described modeling framework is an efficient tool for exploring a wide array of constitutive relationships and rheological arrangements, which can subsequently serve as a basis for 3D constitutive model development and finite-element implementations. The proposed approach can also be employed as a self-contained tool to obtain simplified 1D phenomenological models of the structural response of biological tissue to single-axis manipulations for applications in haptic technologies.

Computer-aided medical technologies are currently restricted by the limited understanding of the mechanical response of solid abdominal organs to finite loading conditions typical of surgical manipulation [5]. This limitation is a result of the difficulty in acquiring the necessary data on whole organs. To develop a constitutive model capable of predicting complex surgical scenarios, multiple testing modalities need to be simultaneously obtained to capture the fundamental nature of the tissue’s behavior under such conditions. In vivo tests are essential to obtain a realistic response, but their inherent difficulty and unknown boundary conditions makes them an impractical approach. Ex vivo tests are easy to control, but the response is unrealistic. A perfusion apparatus was previously developed that obtained near in vivo conditions for whole livers while allowing the ease of ex vivo testing [3]. This work presents the results from complete viscoelastic testing of whole-perfused livers with surgically relevant time-dependant indentation loading profiles to 35% nominal strain. These results will aid in the development of a constitutive model for the liver whose parameters can be related to the physical constituents of the tissue. As an intermediate modeling step, a 1D rheological modeling tool was used to identify the form and initial parameters for a constitutive model.

INTRODUCTION Soft biological tissues are complex materials whose heterogeneous makeup contributes to their viscoelastic nonlinear mechanical behavior. Developing a constitutive law for soft tissues to characterize their behavior under large deformations typical of medical manipulations is an area of active research [1-4]. Although these groups have developed nonlinear constitutive laws for the liver, none have taken the fluid flow into account. We recently made large strain creep indentation measurements on porcine liver to determine the effects of perfusion on the viscoelastic response [5]. The results not only indicate that perfusion greatly effects the creep response of the organ but also suggest that a constitutive model containing two time constants is required. A summary of these findings and a physiological basis behind the two time constant model are presented. METHODS To determine the effects of perfusion on the liver, we developed a perfusion system for ex vivo experimentation [5]. We conducted large strain creep tests on porcine livers by applying a 100 g load to the surface while displacement was recorded over 5 minutes. We did this on livers in the in vivo, ex vivo perfused, ex vivo post perfused, and ex vivo excised sectioned states. Empirical models were fit to the responses to quantitatively compare the differences between the conditions. A second order lumped element model was required to describe the behavior of the large strain creep response (Figure 1, Equation 1), (1) where A 0 is the amplitude of the steady state displacement and A 1 and A 2 are the amplitude contributions due to the creep time constants τ 1 and τ 2. The liver is one of the largest organs in the body and receives 30% of the cardiac output resulting in a total blood flow of 1.5 L/min [6]. The portal system is unique in that it receives 75% of its blood volume from the portal vein, which drains deoxygenated blood from the gut at a low pressure of 8-10 mmHg. The remaining 25% is oxygenated blood from the hepatic artery (95-100 mmHg). The Figure 1: Second order lumped element model for creep indentation of liver. intricate structure of the liver's vasculature is comprised of lobular units: a parallel branching organization of the portal triad (portal veins, hepatic arteries and bile ducts) connects to a central vein via a dense microstructure. Specifically, the portal vein and the hepatic artery drain into low-pressure (5 mmHg) fenestrated discontinuous …

Over the past four years, we have redesigned Harvard's introductory mechanical engineering course to introduce the principles, practices, and pleasures of mechanical engineering in an accessible format. The main goals of the course are to provide experience in the design process, demonstrate the connection between engineering science and design early in the curriculum, and build student enthusiasm for engineering, serving to attract and retain students. Unlike most introductory mechanical engineering courses, we cover strength of materials and machine elements, material usually presented much later in the curriculum, in order to provide tools for the students to quantitatively evaluate their designs. By providing just enough of this background knowledge to allow for analysis of designs, we demonstrate the connection between engineering science and design early in curriculum and motivate in-depth coverage of these topics in later courses.The laboratories for the course build enthusiasm for engineering by incorporating exciting design projects and introducing students to some of the most attractive mechanical engineering tools. Students learn 3-D solid modeling with CAD software, create prototypes from CAD models using manual and CNC machining, and reverse engineer common consumer products. Using these tools, students build their own hardware prototypes for both a cantilever beam catapult and a model all-terrain-vehicle. These exercises, carefully chosen to reinforce the strength of materials and machine elements concepts, culminate in design contests that enhance the visibility of engineering within the larger university community and increase student interest in the field.

Accurate characterization of soft tissue material properties is required to enable new computer-aided medical technologies such as surgical training and planning. The current means of acquiring these properties in the in vivo and ex vivo states is fraught with problems, including limited accessibility and unknown boundary conditions in the former, and unnatural behavior in the latter. This paper presents a new testing method where a whole porcine liver is perfused under physiologic conditions and tested in an ex vivo setting. To characterize the effects of perfusion on the viscoelastic response of liver, indentation devices made force and displacement measurements across four conditions: in vivo, ex vivo perfused, ex vivo post perfused, and in vitro on an excised section. One device imposed cyclic perturbations on the liver's surface, inducing nominal strains up to 5% at frequencies from 0.1 to 200Hz. The other device measured 300s of the organ's creep response to applied loads, inducing nominal surface stresses of 6.9–34.7kPa and nominal strains up to 50%. Results from empirical models indicate that the viscoelastic properties of liver change with perfusion and that two time constants on the order of 1.86 and 51.3s can characterize the liver under large strains typical of surgical manipulation across time periods up to 300s. Unperfused conditions were stiffer and more viscous than the in vivo state, resulting in permanent strain deformation with repeated indentations. Conversely, the responses from the ex vivo perfusion condition closely approximated the in vivo response.

Although most soft tissues do not have loadbearing functions, understanding their mechanical behavior is of great interest to the medical simulation, diagnostic, and tissue engineering fields. Obtaining these properties is a formidable challenge due to soft tissue’s mechanical and geometric nonlinearities, multi constituent heterogeneity, viscoelastic nature and poorly defined boundary conditions.

Accurate characterization of the mechanical behavior of soft tissues is needed for medical simulation, diagnostic, and tissue engineering purposes. Determining the complex behavior of soft tissues requires mechanical testing in their natural state. Such in vivo tests are wrought with ethical, accessibility, physiological noise, and uncontrolled boundary condition issues. Several groups have developed means for in vivo mechanical testing, but the interpretation of their results remains to be understood. Conversely, testing biological tissues under ex vivo conditions is desirable because it allows for precise control of the boundary conditions, ease of accessibility, and use of fewer animals. However, the behavior and properties of the tissues are drastically altered once removed from their natural state. This study seeks to quantify the differences in the viscoelastic response of soft tissues between four different test conditions. We introduce a new ex vivo set-up that allows for near in vivo testing.

Mechanical properties of biological tissues are needed for accurate surgical simulation and diagnostic purposes. These properties change postmortem due to alterations in both the environmental and physical conditions of the tissue. Despite these known changes, the majority of existing data have been acquired ex vivo due to ease of testing. This study seeks to quantify the effects of testing conditions on the measurements obtained when testing the same tissue in the same locations with two different instruments over time. We will discuss measurements made with indentation probes on whole porcine livers in vivo, ex vivo with a perfusion system that maintains temperature, hydration, and physiologic pressure, ex vivo unperfused, and untreated excised lobes. The data show >50% differences in steady state stiffness between tissues in vivo and unperfused, but only 17% differences between in vivo and perfused tests. Variations also exist in the time-domain and frequency domain responses between all test conditions.

Mathematically describing the mechanical behavior of soft tissues under large deformations is of paramount interest to the medical simulation community. Most of the data available in the literature apply small strains (<10%) to the the tissue of interest to assume a linearly elastic behavior. This paper applies a nonlinear hyperelastic 8-chain network constitutive law to model soft tissues undergoing large indentations. The model requires 2 material parameters (initial modulus, locking stretch) to reflect the underlying physics of deformation over a wide range of stretches. A finite element model of soft tissue indentation was developed and validated employing this constitutive law. Ranges of the initial shear modulus and locking stretches were explored based on values found for breast tissue [17, 25]. Results of the model are shown with a lookup table containing third order polynomial coefficient fits. This work serves as an initial method to determine the unique material parameters of breast tissue from indentation experiments.

Accurate real-time models of soft tissue behavior are key elements in medical simulation systems. The need for fast computation in these simulations, however, often requires simplifications that limit deformation accuracy. Validation of these simplified models remains a challenge. Currently, real-time modeling is at best validated against finite element models that have their own intrinsic limitations. This study develops a physical standard to validate real-time soft tissue deformation models. We took CT images of a cube of silicone rubber with a pattern of embedded Teflon spheres that underwent uniaxial compression and spherical indentation tests. The known material properties, geometry and controlled boundary conditions resulted in a complete set of volumetric displacement data. The results were compared to a finite element model analysis of identical situations. This work has served as a proof of concept for a robust physical standard for use in validating soft tissue models. A web site has been created to provide access to our database: http://biorobotics.harvard.edu/truthcube/ (soon to be http://www.truthcube.org).

Mammals use the elastic components in their legs (principally tendons, ligaments, and muscles) to run economically, while maintaining consistent support mechanics across various surfaces. To examine how leg stiffness and metabolic cost are affected by changes in substrate stiffness, we built experimental platforms with adjustable stiffness to fit on a force-plate-fitted treadmill. Eight male subjects [mean body mass: 74.4 +/- 7.1 (SD) kg; leg length: 0.96 +/- 0.05 m] ran at 3.7 m/s over five different surface stiffnesses (75.4, 97.5, 216.8, 454.2, and 945.7 kN/m). Metabolic, ground-reaction force, and kinematic data were collected. The 12.5-fold decrease in surface stiffness resulted in a 12% decrease in the runner's metabolic rate and a 29% increase in their leg stiffness. The runner's support mechanics remained essentially unchanged. These results indicate that surface stiffness affects running economy without affecting running support mechanics. We postulate that an increased energy rebound from the compliant surfaces studied contributes to the enhanced running economy.

Accurate and real-time models of soft tissue behavior are key elements in the development of medical simulation systems. However, precise validation of these models remains a challenge. Currently, real-time modeling is at best validated against FEM models that have their own intrinsic limitations. This study is the first in a series that will develop physical standards resulting in a database of relevant information used to validate real-time soft tissue deformation. In this first study, a simple 8cm cube of silicone rubber with an exact pattern of embedded Teflon beads was tested in uniaxial compression while CT images were taken. The known material properties, geometry, and carefully controlled boundary conditions resulted in a complete set of volumetric displacement data. The results were also compared to an FEM analysis of an identical situation. This work has served as a proof of concept for a robust physical standard for use in validating real time models. A web site has been created to provide access to our database at: http://biorobotics.harvard.edu/truthcube /.