A Millimeter Scale Flexural Testing System for Measuring the Mechanical Properties of Marine Sponge Spicules

Michael A. Monn; Jarod Ferreira; Jianzhe Yang; Haneesh Kesari

doi:10.3791/56571

JoVE Journal > Bioengineering

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Bioingegneria

A Millimeter Scale Flexural Testing System for Measuring the Mechanical Properties of Marine Sponge Spicules

Published: October 11, 2017

doi:

10.3791/56571

Michael A. Monn, Jarod Ferreira, Jianzhe Yang, Haneesh Kesari

¹School of Engineering,Brown University

Summary

We present a protocol for performing three-point bending tests on sub-millimeter scale fibers using a custom-built mechanical testing device. The device can measure forces ranging from 20 µN up to 10 N and can therefore accommodate a variety of fiber sizes.

Abstract

Many load bearing biological structures (LBBSs)—such as feather rachises and spicules—are small (<1 mm) but not microscopic. Measuring the flexural behavior of these LBBSs is important for understanding the origins of their remarkable mechanical functions.

We describe a protocol for performing three-point bending tests using a custom-built mechanical testing device that can measure forces ranging from 10^-5 to 10¹ N and displacements ranging from 10^-7 to 10^-2 m. The primary advantage of this mechanical testing device is that the force and displacement capacities can be easily adjusted for different LBBSs. The device's operating principle is similar to that of an atomic force microscope. Namely, force is applied to the LBBS by a load point that is attached to the end of a cantilever. The load point displacement is measured by a fiber optic displacement sensor and converted into a force using the measured cantilever stiffness. The device's force range can be adjusted by using cantilevers of different stiffnesses.

The device's capabilities are demonstrated by performing three-point bending tests on the skeletal elements of the marine sponge Euplectella aspergillum. The skeletal elements—known as spicules—are silica fibers that are approximately 50 µm in diameter. We describe the procedures for calibrating the mechanical testing device, mounting the spicules on a three-point bending fixture with a ≈1.3 mm span, and performing a bending test. The force applied to the spicule and its deflection at the location of the applied force are measured.

Introduction

By studying the architectures of load bearing biological structures (LBBSs), such as shell and bone, engineers have developed new composite materials that are both strong and tough ¹. It has been shown that the remarkable mechanical properties of LBBSs and their bio-inspired counterparts are related to their intricate internal architectures ². However, the relationships between LBBS architectures and mechanical properties are not fully understood. Measuring a LBBS's mechanical response is the first step toward understanding how its architecture enhances its mechanical properties.

However, it is important that the type of test used to measure a LBBS's mechanical response is consistent with its mechanical function. For example, since feathers must support aerodynamic loads, the primary function of a feather rachis is to provide flexural stiffness ³. Therefore, a bending test is preferred to a uniaxial tension test for measuring its mechanical response. In fact, many LBBSs—such as feather rachises ³, grass stems ⁴, and spicules ⁵^,⁶^,⁷^,⁸—primarily deform by bending. This is because these LBBSs are slender—i.e., their length is much greater than their width or depth. However, performing bending tests on these LBBSs is challenging because the forces and displacements that they can withstand before failing range from 10^-2 to 10² N and 10^-4 to 10^-3 m, respectively ³^,⁴^,⁵^,⁷^,⁸. Consequently, the device used to perform these mechanical tests should have force and displacement resolutions of ≈10^-5 N and ≈10^-7 m (i.e., 0.1% of the sensor's maximum measureable force and displacement), respectively.

Commercially available, large scale, mechanical testing systems typically cannot measure forces and displacements with this resolution. While atomic force microscope-based ⁹^,¹⁰ or microelectromechanical systems-based ¹¹ testing devices have adequate resolution, the maximum force (respective displacement) they can measure is smaller than the maximum force (respective displacement) that the LBBS can withstand. Therefore, to perform bending tests on these LBBSs, engineers and scientists must rely on custom-built mechanical testing devices ⁵^,⁷^,¹²^,¹³. The primary advantage of these custom-built devices is that they can accommodate large ranges of forces and displacements. However, the construction and operation of these devices is not well documented in the literature.

A protocol is described for performing three-point bending tests using a custom-built mechanical testing device that can measure forces ranging from 10^-5 to 10¹ N and displacements ranging from 10^-7 to 10^-2 m. Technical drawings, including all dimensions, of the components of the mechanical testing device are provided in the Supplementary Material. The primary advantage of this mechanical testing device is that the force and displacement ranges can be easily adjusted to suit different LBBSs. The device's operating principle is similar to that of an atomic force microscope ⁹. In this device, a specimen is placed across a trench cut in a stainless steel plate (see Figure 1A-C). The span of the trench is measured from optical micrographs to be 1278 ± 3 µm (mean ± standard deviation; n = 10). The trench edges support the specimen during a bending test (see Figure 1C, and D). This sample stage is attached to a three-axis translation stage and positioned beneath an aluminum wedge so that the wedge is located midway across the trench's span (see Figure 1C). By moving the stage in the direction (see Figure 1A, and C), the specimen is pushed into the wedge causing the specimen to bend.

We refer to the wedge as the load point tip (LPT) and the component of the device that contains the wedge as the load point (LP). The LP is attached to the end of a cantilever whose displacement is measured by a fiber optic displacement sensor (FODS). The FODS emits infrared light, which is reflected off of a mirror located on the top surface of the LP (see Figure 1B) and received by an optical fiber in the FODS. A ≈5 mm square piece of a polished silicon wafer is used as the LP mirror and is affixed to the LP using epoxy. The FODS measures displacements by comparing the intensities of the emitted and reflected light. The cantilever stiffness and displacement are used to compute the force, , experienced by the wedge due to its interaction with the specimen. The cantilever displacement is also used to compute the displacement of the specimen's cross-section beneath the wedge, . Cantilever-based force sensors have been used in a number of micro- and macro-scale mechanical testing studies ¹⁰^,¹¹^,¹²^,¹³^,¹⁴. The specific design presented here is adapted from a mechanical testing device used for performing adhesive contact experiments ¹⁴. A similar design has also been used in a commercially available micro-tribometer ¹⁵^,¹⁶.

Figure 1: Overview of the custom-built mechanical testing device. (A) A computer aided design rendering of the device. The stage components are highlighted in green. The force sensing subassembly (cantilever, load point (LP)) is highlighted in red. (B) A magnified view of (A). The LP mirror is shown in blue on the top surface of the LP beneath the FODS and is labeled LPM. (C) The coordinate system used to describe the motion of the translation stage. By leveling the stage in step 1.9 of the protocol, the direction is made to coincide with the vector normal to the surface of the LP mirror. (D) A schematic of the three-point bending configuration showing the deformation of the spicule and the measured displacements , and . Please click here to view a larger version of this figure.

The device's capabilities are demonstrated by performing three-point bending tests on the skeletal elements of the marine sponge Euplectella aspergillum⁶^,⁷. This sponge's skeleton is an assembly of filaments, called spicules (see Figure 2A). The spicules are ≈50 µm thick and are composed primarily of silica ⁶. Biosilica-based spicules are found in sponges belonging to the classes Demospongiae, Homoscleromorpha, and Hexactinellida. Sponges, such as E. aspergillum, that belong to the class Hexactinellida are also known as "glass sponges." While the spicules of glass sponges are composed primarily of silica, it has been shown that the silica often contains an organic matrix composed of either collagen ¹⁷^,¹⁸ or chitin ¹⁹^,²⁰^,²¹. This organic matrix plays an important role in silica biomineralization ¹⁸^,²⁰. Furthermore, in some spicules the organic matrix also serves as a template for the biomineralization of calcium ²². In addition to being distributed within the silica, the organic matrix can also form distinct layers that partition the spicule's silica into concentric, cylindrical lamellae ⁶^,²³. It has been shown that this concentric, lamellar architecture can affect the spicules' deformation behavior ⁶^,⁷^,⁸^,²⁴^,²⁵^,²⁶. Consequently, the spicules' mechanical properties are determined by a combination of their chemistry (i.e., the chemical structure of the silica-protein composite) and their architecture ²⁷. Both the chemical structure and architecture of glass sponge spicules are still under investigation ²⁴^,²⁸^,²⁹.

Most of the spicules in E. aspergillum are cemented together to form a stiff skeletal cage. However, at the base of the skeleton there is a tuft of very long (≈10 cm) spicules known as the anchor spicules (see Figure 2A). We describe the protocol for performing three-point bending tests on small sections of the anchor spicules.

In step 1 of the protocol, the procedure for assembling and aligning the components of the custom-built mechanical testing device is described. Steps 2 and 4 of the protocol provide instructions for generating calibration data used to compute forces and displacements in the bending test. The steps taken to prepare a section of a spicule and mount it to the test fixture are described in step 3. The procedure for conducting the bending test on the spicule section is described in step 5. Finally, in the Representative Results section the calibration data obtained in steps 2 and 4 are used along with the bending test data obtained in step 5 to compute and .

Figure 2: Procedure for sectioning and inspecting E. aspergillum spicules. (A) The skeleton of E. aspergillum. The tuft of free-standing anchor spicules is shown at the base of the skeleton. The scale bar is ~25 mm. (B) A single anchor spicule is held in place on a microscope slide using a #00000 red sable brush and sectioned using a razor blade. The scale bar is ~12 mm. (C) A section of an E. aspergillum spicule placed across the trench on the sample stage. The trench edges and trench ridge are highlighted in teal and orange, respectively. The spicule is pushed against the trench ridge to ensure that its axis is perpendicular to the trench edges. (D) A micrograph of a spicule that passes the inspection procedure described in step 3.4 of the protocol, which describes how to determine if a spicule section is damaged and should be discarded. (E) A micrograph of a spicule containing many cracks and missing large sections of silica layers that would fail the inspection procedure described in step 3.4 of the protocol. Scale bars = 250 µm (C), 100 µm (D), and 100 µm (E). Please click here to view a larger version of this figure.

Protocol

1. Assembly and Alignment Choose a cantilever whose stiffness is appropriate for the intended experiment. Attach the LP to the cantilever using #4-40 socket head cap screws (SHCSs) (see Figure 3A). Take care to not plastically deform the cantilever arms while attaching the LP. Figure 3: Procedure for …

Representative Results

The most basic outputs of any mechanical test are the magnitude of the force applied to the specimen and the displacement at the location where the force is applied. In the case of a three-point bending test, the goal is to obtain the magnitude of the force applied by the LPT, , and the displacement of the specimen's cross-section beneath the LPT in the <img alt="Equation 4" src="/files/ftp_upload/56571/56571eq4.jpg"…

Discussion

Several steps of the protocol are particularly important for ensuring that forces and displacements are measured accurately. While some of these critical steps are universal to all three-point bending tests, others are unique to this mechanical testing device.

In step 1.2 of the protocol the LP mirror is cleaned and inspected for scratches, and in step 1.6 of the protocol the FODS gain is set. It is important for the gain and the LP mirror reflectance to be co…

Divulgazioni

The authors have nothing to disclose.

Acknowledgements

This work was supported by National Science Foundation [Mechanics of Materials and Structures Program, grant number 1562656]; and the American Society of Mechanical Engineers [Haythornthwaite Young Investigator Award].

Materials

TMC 36" x 48" isolation table with 4" CleanTop breadboard	TMC	63-563	Isolation Table
Diffeential Screw Adjuster	Thorlabs	DAS110	For stage leveling plate
1" Travel Micrometer Head with 0.001" Graduations	Thorlabs	150-801ME	For stage leveling plate
Right-Angle Bracket for PT Series Translation Stages, 1/4"-20 Mounting Holes	Thorlabs	PT102	For microscope mount
1" Dovetail Translation Stage, 1/4"-20 Taps	Thorlabs	DT25	For microscope mount
1" Translation Stage with 1/4"-170 Adjustment Screw, 1/4"-20 Taps	Thorlabs	PT1B	For microscope mount
12" Length, Dovetail Optical Rail	Edmund Optics	54-401	For microscope mount
2.5" Width, Dovetail Carrier	Edmund Optics	54-404	For microscope mount
0.5" Width, Dovetail Carrier	Edmund Optics	54-403	For microscope mount
InfiniTube Mounting C-Clamp with ¼-20	Edmund Optics	57-788	Microscope component
Standard (with no In-Line Attachment), InfiniTube	Edmund Optics	56-125	Microscope component
Standard In-Line Attachment (Optimized at 2X-10X), InfiniTube	Edmund Optics	56-126	Microscope component
Mitutoyo/Achrovid Objective Adapter (M26 to M27)	Edmund Optics	53-787	Microscope component
5X Infinity Achrovid Microscope Objective	Edmund Optics	55-790	Microscope component
0.316" ID, Fiber Optic Adapter SX-6	Edmund Optics	38-944	Microscope component
¼" x 36", Flexible Fiber Optic Light Guide	Edmund Optics	42-347	Microscope component
115V, MI-150 Fiber Optic Illuminator w/IR Filter and Holder	Edmund Optics	55-718	Microscope component
Allied Vision Manta G-223 2/3" Color CMOS Camera	Edmund Optics	88-452	Microscope component
Power Supply for Manta/ Guppy Pro/ Stingray/ Pike	Edmund Optics	68-586	Microscope component
1/4" Travel Single Axis Translation Stage	Thorlabs	MS1S	FODS micrometer
Analog Reflectance Dependent Fiber Optic Displacement Sensor	Philtec	D20	FODS
30V, 3A DC Power Supply	Agilent	U8001A	Power supply for DAQ and FODS
14-Bit, 48 kS/s Low-Cost Multifunction DAQ	National Instruments	USB-6009	DAQ for FODS
Three Axis Motorized Translation Stage	Thorlabs	Thorlabs T25 XYZ-E/M	Translation stage
T-Cube DC Servo Motor Controller	Thorlabs	TDC001	Motor controller for stage
T-Cube Power Supply	Thorlabs	TPS001	Power supply for motor controller
National Instruments LabVIEW (2013 SP1)	National Instruments		Used for running software
National Instruments LabVIEW Vision Acquisition Software (2016)	National Instruments		Used for running software
Nikon Eclipse Ci-POL Main Body	MVI	MDA96000	Polarized light microscope
Nikon Pi Intermediate Tube with Analyzer Slider	MVI	MDB45305	Polarized light microscope
Nikon Dia-Polarizer	MVI	MDN11920	Polarized light microscope
Power Cord – 7'6"	MVI	79035	Polarized light microscope
Nikon P-Amh Mechanical Stage	MVI	MDC45000	Polarized light microscope
Nikon Lwd Achromat Condenser	MVI	MBL16100	Polarized light microscope
Nikon LV-NBD5BD-CH Manual Quint Nosepiece ESD	MVI	MBP60125	Polarized light microscope
Nikon C-TF Trinocular Tube F	MVI	MBB93100	Polarized light microscope
Nikon CFI 10X Eyepiece FN 22mm NC	MVI	MAK10110	Polarized light microscope
Nikon TU Plan Flour BD 10x Objective	MVI	MUE42100	Polarized light microscope
Venus Flower Basket Sponge	Denis Brand	N/A	Sponge skeleton
3.5X Headband Flip-Up Magnifier	McMaster Carr	1490T5	Used for spicule sectioning
Ø1" Silicon Wafer, Type P / <100>	Ted Pella	16011	Used for load point mirror
Low Lint Tapered Tip Cotton Swab	McMaster Carr	71035T31	Used for cleaning LP mirror
Rubber grip precision knife	McMaster Carr	35575A68	Used for sectioning spicules
Microscope Slides, frosted end, 75 x 25 x 1mm	Ted Pella	260409	Used for sectioning spicules
Sable Brushes, #00000, 0.08mm W x 4.0mm L	Ted Pella	11806	Used for handling spicules
PELCO Pro High Precision Tweezers, extra fine tips, superior finish	Ted Pella	5367-5NM	Used for handling spicules
Dual Axis Linear Scale Micrometer	Edmund Optics	58-608	Used for calibrating the microscopes
FLEX-A-TOP FT-38 CAS	ESD Plastic Containers	FT-38-CAS	Used for storing spicules
Plastic Vial Bullseye Level	McMaster Carr	2147A11	Used for leveling the stage
Analytical Balance	Mettler Toledo	MS105DU	Used to mass calibration weights

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Monn, M. A., Ferreira, J., Yang, J., Kesari, H. A Millimeter Scale Flexural Testing System for Measuring the Mechanical Properties of Marine Sponge Spicules. J. Vis. Exp. (128), e56571, doi:10.3791/56571 (2017).