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

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 i....

Protocol

1. Assembly and Alignment

1. 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: Pr....

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.......

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.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
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