Project Acronym BIOLOCH BIO-mimetic structures for LOComotion in the Human … · 2006-02-28 ·...

Project Number IST – 2001 – 34181 Project Acronym BIOLOCH

Project Title BIO-mimetic structures for LOComotion in the Human body

Report: Review Report Due Date: month 13 Delivery Date: 20/06/2003 Deliverable Type Report

Nature Pub Version: 1.0

Status FINAL

Pages: 93

Title: Review Report for First Year Review

Short Description: This document illustrates the work performed from the beginning

of the BIOLOCH project (May 2002) to June 18 2003

Authors: • Scuola Superiore Sant’Anna (SSSA) - Pisa (Italy)

• University of Bath, Department of Mechanical Engineering (UBAH Mech Eng) – United Kingdom

• Centro "E. Piaggio", Faculty of Engineering, University of Pisa (UniPi) - Italy

• Foundation for Research and Technology – Hellas (FORTH) - Greece

• University of Tuebingen, Section for minimally invasive surgery (UoT) – Germany

• Steinbeis Institute of Healthcare Industries (IHCI) – Berlin, Germany

Made available to: Project Officer, BIOLOCH Reviewers, BIOLOCH Consortium

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Responsible Partner: SSSA Name of Deliverable: Review Report Page: 1

Summary

Summary .............................................................................................................................................1 1 Introduction and methodology.........................................................................................................3 2 Mechanisms of friction enhancement and tribological studies .............................................................4

2.1 Adhesion by mechanical mechanisms ........................................................................................4 2.1.1 Replicating the adhesion mechanism exploited by Taenia Solium...........................................5

2.1.1.1 Sucker dimensioning ...................................................................................................6 2.1.1.2 Fluid-dynamic effect....................................................................................................7 2.1.1.3 Surface forces effect ...................................................................................................8 2.1.1.4 Hooks fabricated by µEDM...........................................................................................8 2.1.1.5 Hooks fabricated by fused polymer self-shaping ............................................................8

2.2 Differential friction phenomena ...............................................................................................11 2.2.1 Avena sativa mechanism..................................................................................................12 2.2.2 A demonstrator: Differential friction mechanism of a counter motors system ........................16

2.3 Adhesion by interface modification ..........................................................................................17 2.3.1 Structure of intestinal mucus ............................................................................................17 2.3.2 Mucoadhesion mechanism................................................................................................18 2.3.3 Gluing process in the GI tract ...........................................................................................18 2.3.4 Adhesion by dehydration of the interface...........................................................................21

2.4 Preliminary Evaluation of adhesion mechanisms .......................................................................27 3 Undulatory locomotion ..................................................................................................................28

3.1 Locomotion with longitudinal waves.........................................................................................28 3.1.1 The earthworm model .....................................................................................................28

3.1.1.1 Introduction .............................................................................................................28 3.1.1.2 Aim and hypotheses..................................................................................................29 3.1.1.3 Analysis ...................................................................................................................29 3.1.1.4 Axial and radial waves...............................................................................................31 3.1.1.5 A simple friction model ..............................................................................................32 3.1.1.6 Back to biology.........................................................................................................33 3.1.1.7 Conclusions ..............................................................................................................37

3.2 Locomotion with transversal waves .........................................................................................37 3.2.1 Errant polychaete locomotion ...........................................................................................37 3.2.2 Undulatory locomotion in fluids: The Taylor model .............................................................39 3.2.3 Equations of motion ........................................................................................................41 3.2.4 Undulatory body wave generation.....................................................................................42 3.2.5 The Effect of the Parapodia ..............................................................................................43 3.2.6 Technical Implementation ................................................................................................46

3.2.6.1 Steering ...................................................................................................................47 3.2.6.2 Body and bristle design .............................................................................................48 3.2.6.3 Fabrication ...............................................................................................................49

4 Enabling technologies, design rules and control issues.....................................................................51 4.1 Actuators ..............................................................................................................................51

4.1.1 Bio-like actuators.............................................................................................................51 4.1.1.1 Actuator exploiting amphiphilic gel .............................................................................52

4.1.2 Electro-chemical actuators ...............................................................................................56 4.1.2.1 Wiring...........................................................................................................................58

4.1.3 Shape Memory Alloy actuators..........................................................................................59 4.2 Sensors.................................................................................................................................61

4.2.1 Functional morphology of nereid setae ..............................................................................64 4.2.2 Condylura Cristata sensing system....................................................................................67

4.3 Control and gaits ...................................................................................................................68

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4.3.1 Open-loop control............................................................................................................68 4.3.1.1 The forward gait .......................................................................................................69 4.3.1.2 Other gaits ...............................................................................................................71 4.3.1.3 Gaits employing the parapodia ...................................................................................74

4.3.2 Closed-loop control..........................................................................................................74 4.3.3 Sensor-based control .......................................................................................................76

4.3.3.1 Polychaete sensors ...................................................................................................76 4.3.3.2 Undulatory centering response...................................................................................77

4.3.4 Neural control by central pattern generators ......................................................................81 4.3.4.1 Polychaete neural system ..........................................................................................81 4.3.4.2 Neural control of undulatory locomotion......................................................................82

5 Conclusions and next steps ...........................................................................................................91 6 References...................................................................................................................................92

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1 Introduction and methodology

In the first year of the BIOLOCH project, the Consortium focused their efforts on the identification of a matching between the biological propulsion mechanisms and the range of possible applications, firstly in the endoscopic field, but also in other domains as future possible goal (e.g. for rescue applications). Propulsion mechanisms exploited by biological lower animal forms are very different and it is not trivial to make clear a classification or taxonomy. In particular, existing classifications are essentially based on phenomenological and morphological aspects rather than on “propulsion effectiveness” from an engineering point of view. The approach followed by the Consortium can be summarised in Figure 1 which represents the overall methodology of the BIOLOCH project. The University of Bath provides the “technological partners” of the Consortium (University of Pisa and Scuola Sant’Anna) the taxonomy of the locomotion mechanisms for the lower animal forms which are selected as the most appropriate for locomoting in solid or semi-solid environments, thus excluding wing-based and purely fin-based systems. Therefore, SSSA and UoP must analyse and model the propulsion mechanisms and they must identify the enabling technologies for the realization of preliminary prototypes. In this activity, SSSA and UoP are assisted and guided by the inputs coming from FORTH and the medical partners. The developed prototypes are test-benches for testing by the medical partners and for a “biomimetic” evaluation by the University of Bath. Then, the process is repeated for continuous optimization and improvement of the the preliminary solutions. The first implementation of this process will be represented by the deliverable D3 (compare the Technical Annex).

UT

UoB

FORTHUoP SSSA

Prototypes

Taxonomy of locomotion mechanismsand matching withenabling technologies

Taxonomy of locomotion mechanismsand matching withenabling technologies

Figure 1

The taxonomy of biological propulsion systems has been performed by dividing between “adhesion systems” and “locomotion systems”. This classification has been pursued in order to analyse the problem of locomotion in the human cavities form an engineering starting point. In fact, even the simplest biological creatures exploit quite complex and sophisticated motion mechanisms which are hard to understand and replicate without a sort of a priori systematisation. This approach has been implemented by the coordinator extensively in studying locomotion issues in the human colon for more application driven robotic projects (L. Phee, 2002). These studies have demonstrated that the peculiar anatomy, tribology and mechanics of the human gut and of other human cavities pose many problems in terms of adhesion and stopping rather than in terms of propulsion, which is often a physiological process (in the intestine, in the blood vessels, etc.). This is the reason why the BIOLOCH project has investigated very much adhesion issues.

In the current report locomotion often indicates the displacement of “adhesive” or “high friction” contact points.

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On the basis of this analysis, Section 2 illustrates the activity the Consortium performed to study, design and implement some solutions for the realization of bio-inspired mechanisms for friction enhancement. Section 3 introduces the undulatory locomotion which, after an overall analysis of biological locomotion mechanisms (e.g. legged or polypedal locomotion, serpentine locomotion,) was considered the most appropriate for locomoting robotic devices in solid, semi-solid and dirty environments. Undulatory locomotion is modelled for two different cases and some preliminary technical implementations are presented. Section 4 presents the enabling technologies which the Consortium has studied and is still investigating for the realization of the biomimetic models previously illustrated. The constraints which the project poses in terms of device size, locomotion effectiveness, autonomous control, overall safety are very challenging and traditional technologies are often inadequate to approach them. Section 4 introduces and describes ad hoc technologies which have the potential to solve actuation, control and sensors issues for the biomimetic locomotion problems. Some preliminary technical implementations are illustrated along with theoretical considerations. Finally, Section 5 summarises the most promising alternatives, which will lead to an artificial system with the desired features, and points out the most important technical constraints and challenges.

2 Mechanisms of friction enhancement and tribological studies

Biological creatures (fishes, frogs, lizards, geckos, but also plants) have elaborated very efficient and various mechanisms to adhere onto different surfaces. Some mechanisms - exploited by several shellfishes - are based on a sort of concrete which makes the detachment process quite impossible. Some other mechanisms are more compliant or they use a combination of active adhesion techniques which can tune the time and the effectiveness of the adhesion phenomenon. For example, it is the morphological variation of octopus suckers which makes variable the tenacity of the adhesion of octopus legs onto stones outside water and in deep water. The criteria which have been selected for the identification of the most promising adhesion methods for the BIOLOCH project described in this section are the following:

- safety for possible applications within the human body. This issue must be evaluated with the medical doctor teams;

- energy saving. If the identified solution must be implemented in a wireless device, the problem of power supply is not trivial. “Passive” solutions based on peculiar geometries or automatic behaviours are preferred on “active mechanisms”;

- possibility of exploiting “natural” structures of human body cavities (e.g. internal mucus).

2.1 Adhesion by mechanical mechanisms Mechanical mechanisms, such us suckers, clampers, needles and pincers, are widely exploited in medicine for sutures and for fixing medical devices. Extensive studies and practical experiments have been performed in order to evaluate risks and damages produces by these adhesion methods and devices. Generally, if needles for biomedical usage are sufficiently tiny and sharp, negligible damages are generated to living systems. In the same way, also suction forces do not produce any problems is the suction time is short and if the suction area is large. Possible damages, such as local bleeding, are recovered very speedily thank s to the fast turn-over of the living tissues. Many biological creatures exploit similar mechanical solutions for adhering on to the human skin without visible damage: it is the case of the leech, which produces a vacuum volume on the human body skin and insert a small and sharp needle under the skin without pain for the host. This paragraph illustrates theorethical models and technical implementations for this type of adhesion systems, starting from biological examples.

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2.1.1 Replicating the adhesion mechanism exploited by Taenia Solium Among the studied lower animal forms, Taenia Solium, being the only parasite of the human intestine not microscopic in size which actually clamps itself firmly to the slippery intestinal wall, shows the most interesting features. Taenia Solium clings to biological tissue with a two-step strategy involving:

- Mechanical clamping using hooks penetrating inside the outer layer of the mucosa; - Suction.

Figure 2 : The head (scolex) of the Taenia Solium (left). On the right the umbrella-like disposition of the hooks on the scolex; vacuum is provided by four circular suction pads (two clearly visible on the right).

A number of hooks are distributed over a circular crown around the scolex (head) of the parasite (Figure 2). During attachment the Taenia approaches its scolex (head) to the duodenum wall; after contact, it moves its hooks outward until the tissue is pierced. During the suction phase Taenia Solium exploits suckers located over four sides of the scolex in order to stabilize adhesion through vacuum. The replication of this structure involves significant technological difficulties, due to:

complex 3D geometry small dimensions (hooks are about 100 µm in diameter) implementation of functions (hooks outstretching and suction) in a small volume.

Figure 2 represents the concept devised by SSSA. Figure 3 shows in detail the location of hooks. A cross section is shown in Figure 5. During mounting the membrane is slightly stretched in order to eliminate mechanical gaps and to assure a good adherence to the mobile part. The hooks are located in the proximity of the lateral wall of the sucker in order to avoid the contact of hooks tips with tissue when the device is not actuated. In the developed prototype (see Paragraph 2.1.1.5) no vacuum is generated: it must be provided by an external equipment. When the device is actuated the mobile part moves outward, thus stretching the membrane and pushing the hooks outward (Figure 5). The hooks, embedded in the membrane during the fabrication process, are mounted so that their tips are bent radially against the movement direction for a more effective penetration into the mucosa.

Figure 3: Schematic drawing of the device designed at SSSA to mimick the Taenia Solium clamping mechanism.

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Figure 4: Detail on the location of hooks. When the elastic membrane is stretched the hooks move radially and compenetrate the outer layer of the mucosa.

In the final prototype the movement of the mobile part will generate a light vacuum pressure in the cylindrical chamber between fixed and movable parts. The vacuum will be transferred to the tissue by the hollow sucker.

2.1.1.1 Sucker dimensioning The aim of this paragraph is to provide a simple model for the understanding of the effects of all the phenomena involved in the interaction of the sucker with the tissue. When the sucker leans onto the intestinal surface it sweeps a theoretical volume imposedV∆ . We will denote by realV∆ the ratio of the geometric volume imposedV∆ that effectively is produced in the process of vacuum generation. We denote

by 0V the volume of air trapped by the sucker. In general, 0V is the volume occupied by the sucker before actuation. During suction the volume 0V of air is expanded to the whole available volume. The progressive generated depression allows the water to evaporate up to 100% of relative humidity.

Figure 5 : Schematic of the device mimicking the Taenia clamping mechanism. The device has 1 degree of freedom and requires a linear actuator that 1) stretches the membrane, causing the outward movement of the hooks; 2) produce a vacuum pressure that is transmitted to the tissue by the sucker.

The very compliant intestinal wall, under the action of the atmospheric pressure, penetrates inside the sucker where a slight vacuum pressure is present. The following simplifying hypotheses are supposed:

Perfect adherence to the intestinal wall during and after suction; Negligible inertial effects; The volume of sucked tissue is a paraboloid;

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The tissue can be modelled using a quadratic constitutive equation: 2ε⋅χ=σ When the diameter of the sucker is less than about 5 mm, it is experimentally observed that

realimposed VV ∆≈∆ . By approximating air to ideal gas and using Dalton’s law we have:

satreal0

00 PVV

V1PP −

∆+

−=∆ ⋅

where Psat is the pressure of water at 37°C (body temperature at physiological condition) and P0 is the atmospheric pressure. When a relative negative pressure of magnitude ∆P is produced, a volume ∆Vdef of tissue is attracted into the sucker:

P3z2

V 02

def ∆⋅ς⋅⋅⋅ζ⋅χ⋅π

=∆

where ζ is the radius of the sucker and z0 is a length comparable to the thickness of the tissue. The tissue is attracted with a force Fsuc given by:

PF 2suc ∆⋅ζ⋅π= The deformation of the tissue partially fills the volume in the sucker chamber. In addition, a volume of mucus fills a fraction of the theoretical volume. The real increment of volume due to the actuation of the sucker is ∆Vreal:

mucus2

defimposedreal hVVV ⋅ζ⋅π−∆−∆=∆ where hmucus is the height of the layer of mucus trapped between the sucker and the dry tissue. Conservatively we assume that hmucus ≅ 1 mm (in physiological conditions hmucus is generally less than 1mm).

We used the following values:

• 390 m10V ⋅=−

• Pa101325P0 ⋅= • ( ) Pa2.6266C37TPsat ⋅=°= • Pa100.5 6 ⋅⋅≈χ • m102z 30 ⋅⋅≈

− The intensity of the suction force is given by (ζ is the radius of the sucker):

142139

219152

suc 1014.31042.51000.51075.81016.1257.1F

⋅+ζ⋅⋅+⋅−ζ⋅⋅+⋅−

⋅ζ⋅=

It is noteworthy that the radius of the sucker cannot be as large as possible. In particular it must be less than about 2.7 mm otherwise the detrimental effect of the mucus becomes dominant and the suction force becomes negligible. The suction force must be stronger than the resultant of all repulsive forces arising in the attachment process. In particular, we must take into consideration antagonistic forces due to fluid-dynamic and surface effects:

2.1.1.2 Fluid-dynamic effect We must assume that the vacuum pressure beneath the sucker attracts mucus from outside.

Indeed, the penetration of the mucus under the edge of the sucker produces a repulsive fluid-dynamic lift that can be evaluated assuming that the pressure drop along the thin sucker edge is linear (from the external pressure p0 to the internal value psat). A simple integration over the edge provides the following expression for the lift (r is the radius of curvature of the edge):

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( )[ ]202 rprp3pr6p332F ⋅∆++ζ⋅∆⋅ζ⋅−ζ⋅⋅⋅+∆⋅ζ⋅⋅π⋅=

2.1.1.3 Surface forces effect Surface tension hampers the penetration of mucus until the liquid film between sucker and dry

tissue is broken. In the worst case the liquid film is not broken and a repulsive force due to surface force must be taken into account. The force due to surface tension is described by the equation:

( ) ζ⋅⋅+⋅= − r89.221014.9S 4 The total resistant force FR is:

SFFR += It has been found numerically that the radius of the sucker must be larger than 0.4 mm to overcome fluid-dynamic lift and surface tension effect. The simple dimensioning criteria can be summarised in the following design rules:

The radius of the sucker has to be: o larger than 0.4 mm so that the suction force overcomes the resistance of the surface

tension and that of the fluid-dynamic lift; o smaller than 2.7 mm, so the mucus trapped inside the sucker does not compromise the

correct working of the sucker; o directly proportional to the required force of adhesion; nevertheless the radius cannot be

increased indefinitely since the presence of the mucus reduces its effectiveness starting from about 2.3 mm when the suction force is 2.95N.

imposedV∆ must have the largest possible value, compatibly with the bulk specifications; Curvature radius of the sucker edge: a small radius is less affected by surface tension (linear trend)

and by fluid dynamic lift (quadratic trend).

2.1.1.4 Hooks fabricated by µEDM The adhesion mechanism presented in this section involves the use of miniaturized hooks in combination with volumetric suction. The geometry of the hook is intrinsically 3-dimensional and, because of the small dimensions, their fabrication is not trivial. At SSSA two different approaches have been attempted in order to show the feasibility of fabrication of hooks about 600 µm long and 200 µm large. The direct method uses stainless steel as a substrate material and Micro Electro Discharge Machining as fabrication technology (µEDM). The geometry of a single hook array is shown in Figure 6. 6 arrays (18 hooks overall) have been embedded in the polymeric membrane by positioning them in the mold before polymer curing. In the current prototype the sucker has not been integrated and the actuation is manual by acting on a rod (Figure 7). The external diameter of the prototype is about 15 mm. The effect of actuation is shown in Figure 8.

2.1.1.5 Hooks fabricated by fused polymer self-shaping The second method for the hooks fabrication is illustrated below. In order to further reduce the dimension of the hooks, a custom technology has been developed. It involves the use of cylinders of a hard polymer (Nylon 6, having melting temperature of 135°C) with a small diameter (500µm in the example shown in Figure 9). The cylinder is in contact with two high temperature metal tips (brass in Figure 9,a) that conductively warm up the polymer, up to its melting temperature. When the polymer is fully melted, it is kept in place by surface tension thus realizing a liquid bridge between the metal parts. When the metal tips are moved apart the polymer liquid polymer bridge is spontaneously shaped by surface forces (Figure 9,b). When the strain exceeds a critical value, the fused polymer volume splits into two hook-shaped parts (Figure

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9,c). In order to design the fabrication set-up a simple model has been developed to take into account the effect of material properties and geometry on the shape of the resultant hook. The model is based on the following hypotheses:

Axial symmetry; Negligible mass forces due to the small dimensions.

Figure 6: footprint of a 3 needles array fabricated by µEDM from a 1 mm thick stainless steel foil. All dimensions are in mm. The shape of the base of the piece allows a secure embedding of the metal part into the silicon membrane.

Figure 7: Left: Schematic of the prototype assembly. The actuation rod is manually displaced in order to stretch outward the membrane. The hooks are embedded in the polymeric membrane before curing. Right: overview of the assembled device. The outer diameter is about 15 mm.

Figure 8: Left: Membrane in the rest position. Right: Deformed membrane. The extension of the membrane pushes the micro hooks against the tissue. The central hole where the sucker will be mounted is well visible. We will choose a z-axis coincident with the axis of revolution of the polymeric liquid bridge (its orientation is not important because gravity is negligible).

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The relevant equations are:

(1)

+

ϕ⋅γ=

1r1

rsenp

equation of normal equilibrium (Laplace); p is the trans-membrane pressure. (2) 02 2 =−⋅⋅−⋅⋅⋅⋅ Frpsenr πϕγπ

equation of axial equilibrium

(a)

(b)

(c)

Figure 9: Three steps of the fabrication process. (a) a small cylinder of Nylon 6 is conductively warmed up between two hot metal parts. (b) It is kept in place by surface forces and stretched by moving apart the metal parts. (c) The break down of the bridge corresponds to the fabrication of two hooks with an average width less than 200 µm.

(3) ( )

II

23

2I

1 r

r1r

+

−=

bending radius

(4) 2

)r(arctg I π+=ϕ

angle between z and the normal to revolution surface

(5) ∫ ⋅⋅π=⋅⋅π0h

00

22 hRdz)z(r

constancy of volume The meaning of the symbols used in the previous equations is the following:

• γ : surface tension • 1r , 2r principal bending radiuses • z: axial coordinate • Ir , IIr first and second derivate of r with respect to z • r: bending radius of the parallel • 1r : bending radius of the meridian • 0h : initial height of the cylinder of polymer (before the stretching) • F: axial resultant of all the reactions on the liquid bridge - substratum interface

500 µm

200 µm

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The system of equations (1)-(5) leads to a non-linear 2nd order differential equation:

(6) ( )

( )

+−

π⋅γ⋅

+

=2I

2

23

2I

II

r1

rFr

r1r

Eq. (6) must be integrated with the two boundary conditions: Rr 0z ==

π−ϕ==

2tgr A0z

I

Where Aϕ is the contact angle of the liquid polymer over the metal hot tip. Eq. (6) has been solved numerically (MatLAB).

Figure 10: Numerical simulation of the shape of the liquid bridge before breaking.

The simulations have shown that it is not possible obtain a biomorphic geometry (hooks with an aspect ratio higher than 10) by using an almost static fabrication process. In order to get the desired profile a dynamic process is required where a rapid movement of the two metal tips is accompanied by a progressive cooling of the polymer in correspondence of the central, thinner section. In this case, the shape of the bases of the two hooks, fabricated in a single step, is controlled via a static process that can be optimised using the presented model, while the tips are obtained by quickly moving apart the metal tips. Since the modelling of the dynamic process would require a large number of complex phenomena to be simultaneously taken into account, some of which depending on the non Newtonian behaviour of the rapidly cooling down polymer, the optimisation of the full process must rely on extensive experimentation.

2.2 Differential friction phenomena Although adhesion and friction are different concepts - depending the second one by the normal force – some friction phenomena can contribute to the generation of temporary adhesion conditions which can enable locomotion. Some plants, fish spines, snake surfaces have peculiar profiles and geometry which prevent the displacement in a direction and allow it in the opposite direction, thus producing effects similar to adhesion [Scherge, 2001]. In general phenomena of differential friction in different directions are visible and effective when a structure with an opportune profile or geometry is put in vibration. This paragraph illustrates an example taken by the vegetal domain and describes a technical implementation which can be consider a simple robotic demonstrator of such a principle. This can be considered as an example of “senseless motion”.

Senseless motion is the movement in absence of external systems of perception, like as the vision and the tact for the human beings. Oscillating circuits in the central nervous system of many living species,

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produce rhythmic movements, finalized to locomotion. Researches completed on some bugs of fruit have shown that the neural circuits of these insects produce stimuli for the movement, also in absence of external sensory systems for the feedback of the information to the central nervous system. In nature there are various examples of senseless motion and even human beings show a series of involuntary movements that happen especially during the period of the gestation but also when our nervous system still is not completely developed. In order to develop a complex problem as the movement of a miniaturized endoscopic system in the gastro-intestinal tract an approach based on the senseless motion appears useful, since it is difficult to realize systems extremely complicated that not only move in an extremely hostile atmosphere but also perceive the external environments and interact with it at high level. Various studies have been completed on many species animals, in order to investigate as they can move in absence of senses. In particular, the motion of Drosophila has been analysed; in adult species, rhythmic movements are produced by a group of neurons in the central nervous system, which allow these bugs to fly. These centres of stimulus are able to command the movement also in absence of external systems of perception.

2.2.1 Avena sativa mechanism In nature many plants or animals use a system based on differential friction to improve their motion and adhesion to the ground, without a sensorial feedback. In particular avena sativa (wild oats) is known to feature a particular movement for its seeds to find an appropriate place to develop [McLean R. 1989]. Avena sativa is composed of a central body covered by millions of setae and two lateral structures which are L-shaped and covered by setae themselves (Figure 11). The two lateral structures are like a sheet wrapped around the longitudinal axis; in presence of a gradient of humidity, they unroll. This unrolling generates an elicoidal motion of the central part, that rotates the central part and their relative L-structures. When they are in contact with the ground they act as a hinge for the avena and give direction of movement for the seed. Thanks to the setae present on the body and on the L-structure the avena does not revert its movement and adhere to the ground or to the walls of a hole present in the ground, assuring the seed to develop its plant. Following this strategy, it should be possible to improve the adhesion of a robot which moves inside a human cavity by covering its external surface with microfabricated setae. In this manner it is possible to generate a differential friction that improves the motion of the robot.

Figure 11: Left: Avena sativa. Right SEM picture showing the setae

In order to reproduce these structures two different microfabrication techniques, developed at Centro ”E. Piaggio”, University of Pisa have been adopted: microstamping, a technique derived from soft lithography methods, and Pressure Assisted Microsyringe (PAM) system. Microstamping can be obtained in three ways: a) simple casting: A polymer solution is placed on the mold, which is permeated through micro channels (i.e. setae molds) under application of a vacuum. The excess of polymer is then removed with a glass-slide

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rinsed in the solvent of the polymer. The stamp is placed in an oven at low temperature in order to allow the evaporation of the solvent in excess. Then the polymeric pattern can be easily removed from the mold with tweezers (Figure 12a); b) spin-coating: the polymer solution is placed on the master and then spin-coated to fill up all the microchannels. After the removal of the excess of polymer, the structure can be removed from the mold like in the previous case (Figure 12b); c) based on microfluid dynamic technique: the PDMS mold is sealed on a substrate (ex. glass) and the polymer solution is placed at one extremity. At the opposite side a vacuum is applied so that the microchannels of the mold are filled up. Thus the system is placed in an oven in order to eliminate the excess of solvent, the pattern is lifted-off by removal of the mold from the substrate (Figure 12c).

Figure 12: Schematic representation of the microstamping technique.

For this application the microstamping process begins with the realisazation of a master on a silicon wafer using a negative photoresist. Then, a polydimetilsilossano solution has been casted on the master, thus obtaining, after baking in an oven, a mold. On this mold a polymer solution has been casted. Currently biocompatible polymers are used (Poly-lactic acid, polycaprolactone and poly-lactic-co-glycolide), but a biocompatible polymer with better mechanical properties will be selected for dynamic applications. The layout of the masks needed for the process have been realized with a graphic software, and then printed using a printer with a resolution of 10-20 µm (Figure 13a and b).

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(a)

(b)

(c)

(d)

Figure 13: (a) Design of a typical mask used by Centro Piaggio for the microstamping process and (b) details of its second line. (c) Picture of the master obtained with soft lithographic process, and (d) optical picture of details of the structure.

Once the master has been obtained (Figure 13c and d), the molds have been fabricated (Figure 14) using poly-dimetilsilossano solution which, after baking in a oven at 60° C for 4 hours, solidifies.

Figure 14: Photos of molds of PDMS

A polymeric solution has finally been casted in the mold; printing the mold on a glass-slide the microsturctures showed in Figure 15 have been obtained.

100 µm 300 µm

2 mm

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Figure 15: Photos of microstructure obtained with soft-lithographic techniques

PAM has been developed purposely for application in Tissue Engineering. It is composed of a stainless steel syringe of 10 ml fixed on the z axis of a 3 axis micropositioner moved by stepper motors with resolution of approximately 0.1 micron. The tip of the syringe is a glass needle with an inner diameter of around 5-20 µm, from which the polymer is extruded by the application of a pressure controlled between 0-300 mmHg. The system is interfaced to a computer, that controls the positioning of the syringe and the applied pressure. The system also includes a CAD software purposely developed that allows to design the structure layer for layer. In this study, the bioerodable polymer used was a mixture of poly-l-lactide (PLLA, molecular weight 300,000), and polycaprolactone (PCL, MW 65,000). The realization of the polymeric structure is obtained moving the deposition substrate relative to the z axis on which the syringe is mounted. Once a layer is realised, the syringe is raised of a space dz; then the system restarts to micro-fabricate the new layer. Using this system it is possible to quickly obtain two-dimensional and three-dimensional structures with complex geometries, and with lateral resolution between 5-20 µm (Figure 17). The system can deposit a polymer soluble in an organic volatile solvent, such as chloroform.

Figure 16 : PAM system used at Centro E. Piaggio – University of Pisa

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Working with PAM system and opportunely regulating the pressure applied to the microsyringe, structures thinner than those obtained with soft-lithographic methods have been successfully obtained. As shown in Figure 17, however, these structures present a shape as a drop at the tip, and so they must be improved.

(a)

(b)

Figure 17: (a) Structures obtained using the PAM technique, and (b) details of the structures.

2.2.2 A demonstrator: Differential friction mechanism of a counter motors system The possibility to realise a mini-robot based on a similar system of motion has been investigated by Centro Piaggio; the robot must be endowed of “an unique nervous system” able to produce pulsed movements, without the use of external sensors of perception and a complex system of control of the motion.

In order to optimise the rectilinear movement of the realized prototype, it has been necessary to cover the external surface with polymeric microstructures (such as the artificial setae presented above) that could guarantee to the robotic system to transform the motion senseless in an ordered motion.

To reproduce this type of motion two prototypes have been realised by Centro Piaggio. The first (figure 7a) is composed of two counter motors on which an eccentric mass is placed. In this manner an asymmetrical motion, directed by asymmetrical skates under the platform on which the motors and the voltage supply are mounted, is reproduced. The system has a total weight of 70 g, and a length of 7 cm. The second prototype (figure 7b) is completely covered by asymmetrical skates, thus allowing to analyze the robot motion in 3D.

(a)

(a)

Figure 18: Prototypes of system that features an asymmetrical motion.

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The second prototype has the external part made out of PVC. A silicone version of the second prototype is currently being developed, where a stainless steel piece will represent the weight, the electronic and sensing part. The motion of this system is expressed by the equation:

⋅−=⋅⋅−⋅⋅=⋅

)tanh()()()cos(

xbaxgmxtFxm

&&

&&&µ

µω

where a, b and F are parameters to be obtained experimentally. This equation has been obtained supposing that: 1) a sinusoidal excitation, Fcos(ωt), acts on the prototype; 2) the friction Coefficient is asymmetric. In order to increase the directionality of the movement of this microrobot, and its adhesion, its external surface can be covered with scales, as presented in the section above. To realise these scales avena sativa was used as a reference model. As already described, in fact, the presence of small setae on its surface allows to the inflorescence to grip the surface and to move only in one direction.

2.3 Adhesion by interface modification Some materials or conditions exist which can catalyse the adhesion process. When special materials are added in proximity of an interface to produce adhesion, a typical “gluing phenomena” is obtained. When some parameters of the surface where the attachment must be produced are changed, an interface modification which enhances the adhesion process is generated. The difference between the two processes is not very sharp, but this classification makes clearer the following paragraphs which illustrate some typical examples of these phenomena. Both procedures have advantages and drawbacks: the process of addition of gluing material generates the need for a tank of material large enough to fuel, for example, an entire endoscopic process. The second process can exploit the materials already available on the locomotion surface (e.g. the intestinal mucus), but requires some methods to warm, dry, wet or change in some ways the parameters of the interface. Obviously, both methods require a detachment agent, if detachment is not produced by a sort of time-dependant effectiveness of the gluing agent. Biological examples of animals that secrete both gluing and detachment liquids have been already studied (e.g. Limpet Lottia Limatula [Smith, 1999] and Asterias Rubens [Flammang, 1998]). In this scenario, the motion of snails acts as inspiration: in fact snails move by crawling or floating with currents. Land snails crawl on the ground, creeping along on their large, flat foot; a special gland in the foot secretes mucus (a slimy fluid) that helps the snail to move. The snails, thanks to the presence of the glands that secrete mucus, can walk on surfaces with many obstacles, and also on vertical surfaces. Mucus is a substance able to adhere to different surfaces and allows motion through difficult cavities. An interesting approach is to use mucoadhesive substances in order to allow a micro robot to move through intestinal cavity. The term “bioadhesion” is used to describe attachment of synthetic or natural polymers to a biological surface. If the adhesion surface is a mucous coated with a thin layer of mucus, then the term “mucoadhesion” is employed. The use of mucoadhesive polymers and copolymers as platforms for the local or systemic controlled delivery of therapeutically active drugs has been very investigated in the past two decades.

2.3.1 Structure of intestinal mucus A short overview of the main fuatures of mucus in nthe gut is essential to interpret muco-adhesion processes. In the colon, mucus forms a continuous, insoluble adherent gel layer which protects the underlying mucosa from the hostile environment of the lumen. The protective properties of the mucus gel are directly correlated with the polymeric structure of the component mucins. The polymeric structure is

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stabilized by interchain disulphide bridges. The mucin protein core consists of highly glycosylated regions (resistant to proteolysis) and regions sparsely or non-glycosylated (susceptible to proteolysis). Mucus is a translucent, visco-elastic hydrogel consisting of water-insoluble glycoproteins. Its main functions are to lubrificate and to protect the delicate epithelium against mechanical, chemical, and microbial challenges. Gastrointestinal mucus is secreted by specialized epithelial cells (globlet cells), which may be either interspersed between the common enterocytes or organized in glands opening to the mucosal surface. The mucus consists of about 95-99% water and 5-1% glycoproteins, plus a large number of other components such as electrolytes, lipid, proteins and nucleic acids. The turnover time of the mucus gel layer appeared to be no longer than approximately 1-4h [Lehr, 1990], although this number is based on several assumptions and was obtained from isolated rather than normal gut segments. Mucins are secreted from epithelial cell of the gastrointestinal tracts to form mucus gel protecting mucosal surfaces from damaging effects and agents. Mucous gel is not pure mucin but contains other components secreted into the gel, e.g. IgA, protein, lipid and nucleic acid from epithelial cell. When measuring the rheological properties of mucus/mucin gels, it is essential to consider the effect of these “nonmucin” components. Mucin alone is the gel-forming component of mucus.

2.3.2 Mucoadhesion mechanism Bioadhesion (or mucoadhesion) defines the ability of a biological or synthetic material to “stick” to a mucous membrane, resulting in adhesion of the material to the tissue for a long period. So, mucoadhesion involves the attachment of a natural or synthetic polymer to a biological substrate. Numerous theories have been developed to explain the phenomenon of bioadhesion. According to the electronic theory, there is a double layer of electrical charge at the interface between the bioadhesive and the tissue, due to a transfer of electrons upon contact. This electron transfer occurs because of the difference in structure between the bioadhesive and the glycoprotein chains in the mucus. Bioadhesion in this case is due to an attraction across the electrical double layer [Derjaguin, 1977]. The adsorption theory has been developed over a period of many years and suggests that bioadhesion is due to secondary forces such as Van der Waals forces and hydrogen bonding. The fracture theory of bioadhesion relates the force necessary to separate two surfaces to the adhesive bond strength [Kinloch, 1980]. The wetting theory, which is applied mainly to liquid bioadhesive systems, analyzes the ability of liquid to spread over a biological surface. This theory uses analysis of the spreading coefficient of a liquid bioadhesive over a tissue by displacement of the surrounding gastric fluid. The diffusion theory was proposed by Voyutskii [Voyutskii, 1963] and involves interpenetration of the polymer chains in the interfacial region. In bioadhesion, once the polymer is placed in contact with the mucus, it generates a gradient of concentration across the interface and causes the diffusion of the chains of the bioadhesive into the mucus layer and also the diffusion of the glycoprotein chains of the mucus into the bioadhesive polymer. The rate of the diffusion is dependent on the concentration gradient and on the diffusion coefficient of a macromolecule through a crosslinked network. The chains that have diffused across the interface serve as anchors to aid in securing semipermanently the bioadhesive device in place. The interpenetration distance necessary for good bioadhesion is approximately equal to the end-to-end distance of the macromolecular chains [Voyutskii, 1963].

2.3.3 Gluing process in the GI tract A biomimetic robot able to locomote inside the gastrointestinal (GI) tract requires dedicated mechanisms to safely and firmly adhere on the GI walls for the time required to complete a programmed operation. Experiments have been performed using three approaches involving:

- biocompatible glues - mucus-adhesive polymers - physiological mucus purposely altered

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The activity on muco-adhesive polymers is on going with an experiment set up prepared by Scuola Sant’Anna and Centro Piaggio. In this paragraph we will address essentially the gluing process via biocompatible glues and we will briefly describe the preliminary experiences with mucoadhesive polymers. Among the available biocompatible glues the following have been found in literature:

- GLUBRAN 2 N-Butyls (2) cyanoacrylate + metacrilossisofolan. It is used in general surgery for many applications;

- Tissucol (Glue formed by human fibrin, steam treated). It has adhesive properties, but there are not many applications in which it is utilized;

- Neoprene Sterile (used for the closing of the pancreatic stump after duodenocefalopancreasectomy) - Sheets formed by collagen dusts. They have not adhesive properties by themselves but they are

usually used together with one of the two prevoius glues. Cyanoacrylate presents most interesting characteristics, because it is suited for different surgery applications and it has a very rapid effect. Therefore the cyanoacrylate has been characterized to the purpose to eventually use it as glue in an endoscopic robot. In-vivo tests on pig colon mucosa have been carried on by using the device of Figure 19.

Figure 19: overview of the device used to measure the performance of different gluing strategies in in-vivo experiments

The test device is composed by an aluminum frame, pulled during the tests with a wire connected to a load-cell, onto three pads are fixed. Two pads, closest to the pulling wire, work as passive support; the third, rear pad hosts the adhesive surface whose properties should be investigated. Evidently, the third pad must be often renewed during the tests, following a fixed protocol.

Figure 20: overview of the test pad used to test the behavior of the cyanoacrylate on the intestinal

mucosa

5 mm

5 mm

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Every pad leans on the tissue on a circular pad (3 mm of diameter); each face is carefully rounded on the edge in order to avoid interlocking effects during the test (as shown in Figure 20). The test has been conducted in the following way:

- a drop of cyanoacrylate glue is put on the aluminum pad; - the device is put on the intestinal surface, having care to have it lying on all the pads, so that the

device is perfectly parallel to the test tissue surface; - after about 10 seconds the cyanoacrylate has stuck the pad to the tissue; - the wire is progressively pulled till the tangential solicitation provokes the first slight movement of

the device in the direction parallel to the surface of the bowel; - the maximum value, which the load cell indicates, represents the force of incipient motion; - this test is repeated three times, in three different tracts of the intestinal surface in order to have a

static value. During test, aluminum and Teflon pads have been used in order to investigate cyanoacrylate behavior in the interface with different materials. The results of the in-vivo tests are the following (data expressed in grams):

Teflon 1° measure 2° measure 3° measure

Maximal normal force 0,91 0,98 1,07

Maximal tangential force 1,49 1,65 1,43

Aluminum 1° measure 2° measure 3° measure

Maximal normal force 1,59 1,68 1,49

Maximal tangential force 3,48 3,75 3,78 The glue interface detached from the Teflon in the first case, and from the tissue in the second one. In fact the cyanoacrylate attaches quite strongly the aluminum, but not the Teflon. The interesting thing is that, in the case of Teflon, the glue residue on the tissue could be detached from it with a soft traction (about 10 grams) about after only 30 seconds; it has the shape of a small rigid plate and its removal does not cause any visible damage to the intestinal surface. The ability of mucoadhesive polymers to produce a large increase in the resistance to deformation when incorporated into a mucus gel, relative to when the mucus gel and test materials are evaluated separately at the same concentration, has been reported in several previous studies. It has been proposed that this phenomenon, called “rheological synergism”, can be used as a measure of the strength of the mucoadhesive interaction. The viscosity of the mucus layer is largely determined by the type and amount of the glycoproteins present. The combination of the mucous layer and the epithelial layer provides a substantial barrier for sufficient mucoadhesion time of the dosage form.

Considering this aspect, one possible method to promote the adhesion of endoscope system to the gut cells without damage it, is to cover our system with mucoadhesive polymers. The materials that are currently under investigation at Centro Piaggio are Carbopol 974P, 971P and Noveon AA-1. They all are high molecular weight, acrylic acid-based polymers crosslinked with polyalkenyl polyethers, which include a great number of carboxylic groups; hydrogen bonding, in fact, has been found to be a key part of the mechanism of the mucoadhesion, and the presence of a great number of carboxylic groups provides the ability to form hydrogen bonds. Although these polymers are very mild acids – weaker than acetic acid – they readily react to form salts. Aqueous dispersions of Carbopol polymers have an approximate pH range of 2.8 to 3.2 depending on

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polymer concentration. The greater the concentration, the higher the carboxyl concentration and, therefore, the lower the pH.

A molecule of these polymers in the dry powder state is tightly coiled, thus limiting its thickening capability. When dispersed in water, the molecule begins to hydrate and uncoil slightly, generating an increase in viscosity. However, to achieve the highest possible performance with the polymer, the molecule must be completely uncoiled. There are two mechanisms by which the molecule can become completely uncoiled, providing maximum thickening, emulsion formation, or bioadhesion performance. The most commonly used mechanism is accomplished by neutralizing the polymer with a suitable base. Neutralization ionizes the Carbopol polymer, generating negative charges along the polymer backbone. Repulsions of these negative charges cause the molecule to completely uncoil into an extended structure. This reaction is rapid and gives instantaneous thickening, emulsion formation or bioadhesion. A micro robot can therefore be covered with this kind of polymer, to increase its adhesion to the internal surface of the gut.

2.3.4 Adhesion by dehydration of the interface The property of the mucus of sticking on substrates when dried can be exploited to use mucus itself as a biological glue. To the purpose the force adhesion provided by the mucus has been studied, in particular when its water content at the interface between the intestinal tissue and the surface of an appropriate test device is changed (Figure 21).

Figure 21: overview of the test device used to investigate adhesive property of the mucus when

dried. The outer diameter is 20 mm.

Figure 22: overview of the CAD model of the test device shown in fig. 21.

The drying air is aspired through an axial outlet on top of the distributor and it comes through eight lateral holes (Figure 22).

20 mm

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Figure 23: dimensions of the test device

In the lower part of the device, a medical gauze (Figure 24) has been applied. In the figure below the gauze weft is represented:

Figure 24: Weft of a medical gauze

Drying is due to air that flows on the mucus, and that removes the water vapor progressively formed. By using a wire-netting surface (or also a porous surface), an air flow through the centre of such surface is guaranteed and therefore a more effective adhesion can be obtained; in such way, every interlaced wire forms a sort of cord of adhesion. In the test device, the medical gauze has been stretched and fixed using an elastic ring, as illustrated in Figure 25.

Figure 25: overview of the wire – netting surface stretched and fixed using an elastic ring

5 mm

20 mm

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In – vitro test Friction measurements were made on the internal surface of a portion of pig colon, carefully cut and stretched on a plain substrate. The external part of the gut was perfectly cleaned by traces of fat and mesentery tissue, to prevent dishomogeneity, while the internal part was maintained moisturized to mimic physiological conditions (Figure 26). The device was pulled by using a flexible wire that does not transmit moments, which passes through a pulley assembled on a ball bearing (to reduce wire friction). To the other end of the cotton wire a disposable paper cup was fixed: by increasing the quantity of liquid (water) inside the glass, a variation of tangential force on the device – gut interface has been achieved.

Figure 26: overview of the in – vitro test set – up

The measurements were performed maintaining a constant air-flow through the device valve (10 cc/sec), for four fixed intervals of time:

1. 0 sec: the gut wall was perfectly wet (physiological conditions) 2. 90 sec: the gut walls began to be slightly dry 3. 180 sec: the gut walls was dry 4. 900 sec: the proteins contained in the gut mucus bonded together

The device weight (normal force on the friction surface) was 13.3 g (0.130 N), while the frictional forced generated by the pulley was estimated to be about 0.15 g (1.5 10-3 N). An increasing tangential force was applied to the friction surface by increasing the weight of water inside the glass. The force that causes the detachment of the friction surface was evaluated in different points. The test results are reported in the table below (the values are forces expressed in grams):

1° Measure 2° Measure 3° Measure Mean Value 1 5.4 5.5 6.2 5.7 2 33.3 29.8 24.1 29.1 3 52.9 44.7 52.1 50.0 4 >70 >70 >70 >70

The equivalent friction coefficient, defined as adhesion force divided by maximum tangential load, is:

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Friction Coefficient 1 0.436 2 2.230 3 3.826 4 > 5.363

To appraise the adhesive effect of the mucus on the gauze, it is useful to calculate the tangential force per unit of length of wire (the overall length of the wire exposed to the adhesive action of the intestinal mucus). The gauze is formed by an interlacement of wires that overall forms a regular stitch composed by a net of identical squares. Given a number “n” of squares (each with side long “l”), it is possible to calculate the overall length “L” of the wire that forms the stitch, given by the following approximated expression:

( )nnlL +⋅⋅= 2 The used gauze is composed by squares of side 1.1 mm long. The exposed surface is 200 mm² (n= 180 squares). Therefore the overall length of the wire used for the surface is 425 mm. It can be now calculated, for the four introduced cases, the magnitude of the mean adhesive force per unit of length (soaked of dried mucus) (N/m):

Drying time Specific adhesion force (N/m)

0 Mean Value

90 0.13

180 0.68

900 1.18 In-vivo test The in-vivo test has been performed using the experimental set-up shown in Figure 27. In particular compressed air is taken from the high-pressure line. Its pressure is regulated by a valve; the compressed air comes in a vacuum generator which aspires air from the outlet of the test device. The aspired air comes through the relative humidity sensor, which measures the magnitude of the aspired air dampness (data are sent to the electronic circuit). The magnitude of the adhesion force is measured by a load cell, which sends data to the electronic circuit. The electronic circuit sends data to a laptop, which elaborates the information.

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Figure 27: in–vivo test set – up

The in – vivo test protocol is the following: - The test tissue is put on a horizontal plan (aluminum plate) that can be easily lied on abdomen of

the pig - the surface is plugged with a gauze to eliminate traces of bolus - the mucous is allowed 15-20 seconds to regenerate the thin layer of mucus - the device is lied on the intestinal wall

Figure 28: overview of the device and of pig bowel mucosa during the in – vivo test

- the valve is held open till the desired magnitude of relative humidity is observed; to measure the air

relative dampness , a humidity sensor lapped by the air coming from the test device has been used

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Figure 29: relative humidity sensor

- the cotton wire is progressively pulled till the tangential solicitation provokes the movement of the

device - the maximum value measured with the load cell is recorded as the value of the incipient motion

force - test is repeated three times, in three different tracts of the intestinal surface (and eventually

engraving ex-novo other ones) Results In the in-vivo test two types of stitch have been used:

- medical gauze - silk

In the first case the net is formed by squares of side 1.1 mm long. In the second case the squares are 0.25 mm long. The equivalent lengths of wire are respectively:

- Gauze: 425 mm - Silk: 1628 mm

1° weft (medical gauze) Adhesion forces in grams

Relative humidity

100%: physiological condition

73,40%: softly dried mucus

Environmental humidity: hardly dried mucus

Environmental humidity: Fully dried mucus

1° measure 0,85 1,56 2,04 2,26 2° measure 0,58 0,95 2,01 2,17 3° measure 0,49 0,88 1,74 2,90 Mean 0,64 1,13 1,93 2,44

2° weft (silk) Adhesion forces in grams

Relative humidity

100%: physiological condition

73,40%: softly dried mucus

Environmental humidity: hardly dried mucus

Environmental humidity: Fully dried mucus

1° measure 0,67 1,10 1,46 1,98 2° measure 0,46 1,10 1,59 2,07 3° measure 0,49 0,98 1,56 2,20 Mean 0,54 1,06 1,54 2,08

20 mm

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It has been observed that the magnitudes of the adhesion forces obtained in-vivo are always less than those obtained in- vitro. The reason for this behavior lays probably on the fact that living mucosa uses active mechanisms for water secretion in order to continually restore its physiological condition when the external device tries and drying it. In fact in the colon, mucus forms an adherent gel layer which protects the intestinal mucous from the aggressive environment of the lumen. The protective properties of the mucus are directly correlated with its chemical content (and therefore with its percentage water content). From the measurements shown in the two tables above, the mean force per unit of length (N/m) can be calculated for each of the 4 measurements conditions and for both silk and gauze:

Gauze 0,015 0,026 0,045 0,057

Silk 0,003 0,006 0,009 0,012 The capability of adhesion of the mucus does not depend only on the overall wire length that characterizes the weft of the contact surface. The adhesion capability also depends on other parameters such as the weft shape and wire material. The study of the influence of aforesaid parameters requires a further investigation.

2.4 Preliminary Evaluation of adhesion mechanisms The results obtained in the previous paragraph allow us to do a preliminary evaluation of the mechanism of adhesion and friction that can be implemented in the future work. Table 1 summarizes the mean features of the different mechanisms of friction enhancement or adhesion. Although not exhaustive, these preliminary results demonstrate that some additional issues must be studied and approached before selecting the most adequate mechanism(s) to be implemented in the Biomimetic Locomotion Unit (BLU). In particular, by considering a future "totally on-board configuration" for the BLU, energy issues, integration of dispensers, need for external adhesion agents must be taken deeply into account.

Table 1: A comparison of the investigated adhesion enhancing mechanisms

Specific adhesion

force (N/mm2) Advantages Drawbacks

Suction 2.0 10-4 Biocompatibility Low force Mechanical hooking not available Stability Difficult fabrication

Adhesion by dehydration

3.0 10-4 Biocompatibility Low force Technical difficulty

Biological Glue (cyanacrylate)

5.3 10-3

High force Irreversibility Technical difficulty

Muco-adhesive polymers

not available Biocompatibility Slowness

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3 Undulatory locomotion

During the first two months of the BIOLOCH project, the Consortium has been investigating different locomotion mechanisms both exploited in the animal domain and in the artificial domain (i.e. robotics). By considering the typology of the substrates where the BIOLOCH device has to move, undulatory locomotion has been considered the most promising in terms of adaptability to different substrate conditions and possible artificial replication. Two different types of undulatory locomotion have been considered:

- locomotion with longitudinal waves; it consists of waves which propel in parallel with the direction of motion. This motion is exploited by earthworms and leeches. The difference between the earthworm locomotion and the leech locomotion (called also inchworm locomotion) is that the first consists of continuous waves and the second of discrete waves, as illustrated in Figure 30.

Figure 30: Earthworm and leech locomotion. In both cases the motion wave is parallel to the motion direction

- Locomotion with transversal waves; it consists of waves which are transversal to the motion

direction. These locomotion mechanisms will be analysed and modelled in the following paragraphs. A hardware model of the locomotion with transversal waves will be also described.

3.1 Locomotion with longitudinal waves

3.1.1 The earthworm model

3.1.1.1 Introduction

Peristaltic locomotion is generated by the alternation of longitudinal and circular muscle contraction waves flowing from the head to the tail. The sites of longitudinal contraction are the anchor points; body extension is by circular contraction.

Longitudinal contraction

Circular contraction

Step length Step length

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The pattern of movement is initiated by anchoring the anterior end. As the longitudinal contraction wave moves posteriorly, it is slowly replaced by the circular contraction wave. The anterior end slowly and forcefully elongates, driving the tip farther over the surface. The tip then begins to dilate and anchor the anterior end as another longitudinal contraction wave develops. This sequence is repeated, and the worm moves forward. Reversing the direction of the contraction waves enables the worm to back up.

3.1.1.2 Aim and hypotheses The Earthworm (E) locomotion relies on two kinds of waves along the slender body: longitudinal (axial strain) and transversal (radial strain). We want to find which relation occurs between axial and radial strain, assuming that:

- each cross section remains planar - the earthworm is incompressibile

Because of these two hypotheses, the relation we are looking for is but a continuity equation. Notably, no reference to the smallness of the deformation is done.

3.1.1.3 Analysis Referring to Figure 31, we will define the displacement functions over the domain [0,L], which is fixed in time, e.g. t = 0 (reference configuration). The axial coordinate will be x. We will denote the axial displacement of a generic point, which is at coordinate x in the reference configuration, by ∆(x,t). The radius of a generic cross section will be denoted by r(x, t).

Figure 31: Three configurations of E. Reference configuration (top); two generic configurations at t and t+dt (center and bottom).

Note: Although the length of E changes in time, because of the axial displacement ∆(x,t), the reference domain [0,L], which refers to the reference configuration at t = 0, is fixed. Let consider, in the reference configuration, two points on the axis of E, respectively A0 in x and B0 in x+dx.

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In the generic instant t the points have moved to A1 and B1. After an infinitesimal time, at t + dt, the points will be in A2 and B2. Using the axial displacement function, ∆(x, t), we can write the locations of the six points:

xA =0 dxxB +=0

),(1 txxA ∆+= ),(1 tdxxdxxB +∆++=

),(2 dttxxA +∆+= ),(2 dttdxxdxxB ++∆++=

Since the volume of the segment between Aj and Bj (j=0, 1, 2) must be constant, we can write: 2

,112,22 )()( txdttx rABrAB ⋅−=⋅− + (1)

We must now evaluate the terms B1 - A1 and B2 - A2. By eliminating the higher order infinitesimals, we find:

dxx

AB ⋅

∂∆∂

+=− 111 (2)

where ∆ means ∆(x, t). With the same simplification:

dxtxx

AB ⋅

∂⋅∂∆∂

+∂∆∂

+=−2

22 1 (3)

Substituting (2) and (3) into (1), we get a first relation between ∆(x, t) and r(x, t):

0222

=⋅∂⋅∂∆∂

⋅+∂∆∂

⋅∂∂⋅+

∂∂⋅ dx

txr

xtr

tr

(4)

We can simplify (4 by multiplying both members by r: ( ) 022 =

∂∆∂

⋅∂∂

+∂∂

xr

tr

t (5)

Integrating (5) in time, we find

( )xf

xr =

∂∆∂

+⋅ 12 (6)

where f(x) is an arbitrary function. The displacement ∆(x,t) is identically zero at t=0, because the displacements are evaluated from the reference configuration. Hence:

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( ) ( )xrrxf tx 202 0, == = (7)

Substituting (7) into (6) we finally have:

2

,

2, 1 tx

txtx rx

r =

∂∆∂

+⋅ (8)

Note: The second member of (8) does not depend on t: the relation between ∆(x, t) and r(x, t) is time-invariant.

3.1.1.4 Axial and radial waves The parametric description o the trajectory of a point on the surface of E that has coordinate x in the reference configuration is given by:

=∆=∆

),(),(txrrtx

(9)

Using (8) it is possible to calculate r(x, t) from ∆(x,t), or vice-versa. There is no a-priori reason to choose one approach instead of the other, except for computational convenience. If we derive r(x) from ∆(x,t) we have:

xtx

xrtxr

∂∆∂

+=

),(1

)(),( 0 (10)

Eq. (10) takes into account that both r(x,t) and r0(x) are always greater than zero. The use of (10) is limited to the functions ∆(x,t) having not too steep slopes. In fact, the choice of the formulation (10) implies that we are making use of axial displacements ∆ so that (j∆(x,t)/jx)> -1. We actually would have fewer constraints using the other approach, i.e. by deriving ∆ from r. In fact in this case one has:

∫

−+∆=∆

x

dxtxrxrttx

02

20 1

),()(),0(),( (11)

The choice of the function ∆(0,t) is not arbitrary because, from a physical point of view, the centre of mass of E must have a constant velocity, and in particular it must be at rest if the initial velocity was zero, as we assume. So it must be:

∫ ∫ ∆+=L L

dxxrtxxdxxxr0 0

20

20 )()),(()( πρπρ (12)

or

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∫ =∆L

dxxrtx0

20 0)(),( (13)

If we introduce the quantity:

∫=L

dxxr0

20 )(ψ (14)

and substitute (11) in (13) we have:

∫ ∫

−−=∆

L x

dxxrdktkrkrt

0

20

02

20 )()1

),()((1),0(

ψ (15)

3.1.1.5 A simple friction model The description of the deformation of E is not sufficient to describe the locomotion, because the ability to propel the body depends strongly on the tribological properties of the contact. In fact, deformations are only able to provide internal stresses, which cannot modify the motion of the centre of mass of E (this condition is satisfied thanks to the choice of the arbitrary function ∆(0,t) (15)). On the contrary net locomotion can be achieved when E elongates and deforms itself in contact with a substrate, on which frictional forces are exerted. In this paragraph we present a simple friction model based on the following assumptions:

1) the frictional force is proportional to the normal force; 2) the proportionality coefficient (friction coefficient) depends linearly on the radius of the segment; this hypothesis takes into account the effect of the setae that a segment pushes out during the contraction of longitudinal muscles; 3) friction does not depend on the contact area; 4) the frictional force does not depend on the velocity; 5) E is pushed against the substrate by its own weight; 6) the weight of each segment is counterbalanced by a normal reaction provided by the ground, while adjacent segments do not apply shear forces.

A generic segment in the reference configuration, comprised between the sections x and x+dx, has a constant mass:

dxxrdm )(20πρ=

Although the shape of E changes in time, a generic segment provides a frictional force that has constant modulus, while its sign depends on the velocity, v, of the segment (for simplicity we consider the ground fixed):

dmvgdFa )sgn(µρ=

In a generic instant t the total frictional force is

∫−=L

a dxvxrgF0

20 )sgn()(πµρ (16)

We will indicate with A the point of E that is at x=0 in the reference configuration. The velocity of a generic point P is:

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PAAP vvv +=

where vA is the velocity of A and vPA is the velocity of P relative to A. Evidently:

),0(),( ttxvv AP ∆−∆+= && (17)

The total frictional force (16) can be written as:

∫ ∆−∆+−=L

Aa dxttxtvxrgF0

20 )),0(),()(sgn()( &&πµρ (18)

In (18) vA is the only unknown function, and it must be evaluated through the dynamic equilibrium of E:

ainertia FF =

The inertial force is

[ ]∫ ∆−∆+=L

Ainertia dxttxtvxrF0

20 ),0(),()()( &&&&&πρ (19)

If we expand the dynamic equation (Finertial = Fa) using (13) and using the quantity y defined in (14) we finally get:

[ ]∫ ∆−∆+−=∆−L

AA dxttxtvxrgtv

0

20 ),0(),()(sgn)(),0( &&&&& ψ

µ (20)

3.1.1.6 Back to biology The main results of the previous paragraphs are collected in the following set of equations:

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[ ]

[ ]

=

∆−∆+−=∆−

∆−∆+−=∆

=

−+∆=∆

∫

∫

∫

∫

0)0(

),0(),()(sgn)(),0(

),0(),()(sgn)(1),0(

)(

1)(),0(),(

0

20

0

20

0

20

02

20

A

L

AA

L

A

L

x

v

dxttxtvxrgtv

dxttxtvxrt

dxxr

dxrxrttx

&&&&&

&&

ψµ

ψ

ψ

(21)

The function r=r(x, t) must replicate the actual profile of a living earthworm during normal locomotion. In order to define this function it is useful to introduce the following biomechanical parameters:

L Earthworm length

T Earthworm period

R0 Earthworm radius at rest

min

minmax

lll

z−

=ε Mean longitudinal strain

max

minmax

ddd −

=ϑε Mean circumferential strain

cc

zIccz t

εε =, Mean longitudinal strain rate during circumferential contraction

cl

zIlcz t

εε =, Mean longitudinal strain rate during longitudinal contraction

cc

Icc t

ϑϑ

εε =, Mean longitudinal strain rate during circumferential contraction

cl

Ilc t

ϑϑ

εε =, Mean longitudinal strain rate during longitudinal contraction

where lmax and lmin are respectively the maximum and minimum segment axial length (that occurs respectively during circumferential contraction and elongation), dmax e dmin are the maximum and minimum diameter of the earthworm segment (that occurs respectively during longitudinal contraction and elongation), tcc and tcl

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the time required respectively by the circumferential and longitudinal muscles to contract. The values for the relevant parameters, required to shape the function r(x,t) following experimental evidence, are reported in Table 2 (from [Quillin, 1999]).

Table 2: experimental value of some kinematics variable for the earthworm Lumbricus Terrestris

Variable Experimental value εz 0.6 εθ 0.25

εz,ccI(s-1) 0.98 εz,lcI(s-1) 0.75 εθ,ccI(s-1) 0.41 εθ,lcI(s-1) 0.58

T(s) 4m0.7

L(mm) 102m0.34

r(kg/m3) 1000

( )

−−

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Equation (21) was solved using Eulero’s direct method of integration. During longitudinal muscles contraction the earthworm segment shorten and setae come out to increase friction. This behaviour has been modelled using a linear equation:

+

−= 3),(

2),(

0

00

νµ

µR

Rtxrtx (24)

Where µ0 is the friction coefficient between the earthworm (at rest) and the substrate. Figure 32 shows the plots of calculated mean velocities (mm/s) vs. earthworm mass (g) for different µ0 (0.1, 0.2, 0.3, 0.5). In the same figure the experimental curve (25) is also shown.

33.08.3 mv = (25)

Figure 32

Calculated and experimental mean speed

0,00

2,00

4,00

6,00

8,00

10,00

12,00

0,5 1 1,5 2 2,5 3 3,5 4 4,5 5 5,5 6 6,5 7 7,5 8 8,5 9 9,5 10

mass(g)

mea

n sp

eed

(mm

/s)

friction-0.2friction-0.1experimentalfrction-0.5friction-0.3

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3.1.1.7 Conclusions The comparison between experimental data and the numerical results shows that the model provides a good description of earthworm locomotion and, therefore, it is an useful tool for the dimensioning of miniaturized robot exploiting longitudinal waves. In particular, the calculated mean velocity depends on the choice of the friction coefficient µ0, although only slightly. Nevertheless, when µ0 is higher than 0.5 the mean error is less than 25%.

3.2 Locomotion with transversal waves

3.2.1 Errant polychaete locomotion The polychaete marine worms are members of the Annelida phylum (the segmented worms), and can be found living in the depths of the ocean, floating free near the surface, or burrowing in the mud and sand of the seashore. Their length varies from less than 1mm to over 3m, and they present a great diversity of structural types [Brusca & Brusca, 1990]. True to their Class name, the great majority have many setae (bristles), usually extending from lateral appendages (called parapodia) distributed along their body. The parapodia are multipurpose structures involved in swimming, crawling, digging, breathing, and excretion. Parapodia are also equipped with sense organs (mainly touch receptors) spreading as far as the distally attached setae. Along with the musculature and hydrostatic skeleton, they function together in moving the animal through or over sediment. Based on the degree of dependency on a burrow, polychaete are characterised as either sedentary or errant. Sedentary polychaete occupy fixed simple burrows excavated in the substratum and lined with mucus, utilising peristaltic contractions to move through them. Errant polychaete are on the other hand much more active, and are characterised by an elongated body equipped with a large number of parapodia, which are arranged in pairs for each segment (Figure 33).

Figure 33. Segment cross section from the body of a typical errant polychaeta (Nereis diversicolor) showing the laterally projecting parapodia and supporting musculature.

The large range of locomotion modes employed by errant polychaete is related to the diversity and structural complexity of their habitat environment. One species that is generalised locomotion-wise and has been extensively studied in the literature is Nereis diversicolor, a common intertidal polychaete. It inhabits muddy substrata, living in a more-or-less permanent burrow, but also actively moving over the substratum and rather rarely swimming. Its body can reach 60-120 mm and consists of around 200 segments, each bearing a pair of parapodia. In his detailed account of Nereis locomotion, Gray [Gray, 1939] identified three locomotion modes: (i) slow crawling, (ii) rapid crawling and (iii) swimming. During slow crawling only the parapodia are active, describing elliptical trajectories in two-stroke cycles. In the power stroke, the parapodia are generating propulsive force pushing against the substratum as they move backwards with respect to the body. The setae are extended to provide more contact points with the substratum, with the traction further increased

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by the action of passive joints on the setae [Merz & Edwards, 1998]. During the recovery stroke, the parapodia are lifted from the ground and the setae are retracted back. The parapodial action occurs in waves that move from posterior to anterior along the two sides of the worm. For each segment, the two parapodia perform these movements exactly out of phase. In addition to parapodial movements, rapid crawling also involves body undulations, generated by waves of contraction in the longitudinal muscles of the body wall. These waves of contraction travel from the posterior to the anterior of the body and are timed to coincide with the alternating waves of parapodial activity: the power stroke of a single parapodium occurs at the crest of the body wave, while the recovery phase occurs in the trough of the body wave (Figure 34). Depending on the species, this combined use of body flexions and parapodial movements may occur at different speeds and at different places on the body as the speed increases [Merz & Edwards, 1998]. During swimming, the Nereis exhibits the same combination of parapodial action and body undulations, but the amplitude, frequency and wavelength of the body wave are greatly increased, particularly in the anterior region of the body. This is reflected in the number of segments involved in the generation of a full body wave (11-15 segments per wave for rapid crawling, 40 segments per wave for swimming). In parapodial movements, thrust generation is by the drag associated with its movement through the water. The parapodium must therefore move backwards relative to the wa

Project Acronym BIOLOCH BIO-mimetic structures for LOComotion in the Human … · 2006-02-28 ·...

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