The effects of b radiation and
low temperatures on the
strength of Spider Silk
E. M .Pogozelski
SUNY Geneseo
Undergraduate Research Assistants:
Dave Abramo
Lee Papasergi
Tina Kieffer
Brendan See
Bill Becker
Motivation:
Inertial Confinement Fusion
(ICF)
Fuel/Target has a diameter of 1 mm.
Over 60 beams are
incident on the target.
Fuel may be Deuterium,
or a Deuterium-Tritium
mixture.
DT Cryo-Targets
4 dragline pairs
~1 mm diameter
~2 cm long
DT Target < 1 mm
Irradiation During
the Filling Process
Requires 1
month.
Spherical
plastic shell is
Pressure is
initially
hundreds of
empty…
atmospheres.
Total dose =
100 Mrad
Our Goals:
1.
2.
3.
4.
Dragline Mechanical Properties:
Stress-Strain Behavior, Strength,
Elasticity, Toughness
Physical Diameter
Response to b Radiation
Response to low
temperatures
Silk Harvesting
Technique
Silk Harvesting
Technique
Silk Mounting
Technique
Mechanical Testing System
Loading Samples
for Mechanical
Testing
Run Sequence
Run Sequence
Toughness = Area under plot
 J/m3
Toughness 
90 MJ/m3
Unirradiated Results: Trends
Determination of Strand Diameter
Both stress and dose depend on accurate
measurement of the strand diameter.
D
Diameter Determination
SEM with gold-plated silk
Diameter Determination
SEM Imaging with gold-plated silk and calibrated
0.80 mm latex microspheres
SEM Errors?
• The SEM indicates that the microsphere
diameters are about 1.09  0.07 mm,
rather than the known value of
0.80  0.01 mm.
• A systematic error of +35% in the SEM
measurements
• Diameter Measurements
adjusted by 35%...
Unirradiated Results:
Comparison Using
Corrected SEM Length Scales
Result
Diameter (mm)
0.93  0.14
Area (mm2)
1.38  0.40
Max Force (mN)
1460  320
Max. Stress (MPa) 1166  450
Max Strain (%)
0.27  0.07
Our b source: SEM
Tritium average: 6.5 keV
Possible SEM
energies:
• 5 keV
• 10 keV
• 15 keV
Scanning Electron Microscope
Computation of Dose
Rads: Energy absorbed
mass target
bb-
1 rad = 0.01 J/kg
b-
Measured: r
b-
b-
bb-
b-
w
Measured: w
A
Measured: I
Known: E
Measured: t
2IEt
Dose =
rprew2
SEM
Current Corrections
Unfortunately, the measured
current is the net current
passing through the lower
plate. We want the current
exposing the silk directly.
The difference is due to
electronics scattered from the
base material, creating an
apparent false current.
This formula tells us
The incident dose.
But how much of
this energy is
absorbed by the
silk?
2IEt
Dose =
rprew2
CASINO Simulations
“Monte CArlo SImulation of ElectroN Trajectory in Solids”
CASINO output
5 keV
360 beams: results 5 keV
Black:
High absorption
White:
Low absorption
CASINO Results
Assuming Spider Silk Composition is:
51.2% Hydrogen
23.5% Carbon
16.6% Oxygen
8.5% Nitrogen
With a density of 1.2 g/cm³…
Results
Energy
Deposition
5.0 keV
100%
6.5 keV (3H)
96.5%
10.0 keV
47.6%
15.0 keV
18.6%
For our SEM irradiation…
Since incident 5keV electrons deposit 100%
of their energy into the silk…
It is reasonable and conservative to
approximate the energy deposition from
Tritium decay electrons using 5 keV SEM
electrons.
Visible Damage from
b radiation
Cumulative Dose: 0.0 × 1012 rad
Visible Damage from
b radiation
Cumulative Dose: 0.4 × 1012 rad
Visible Damage from
b radiation
Cumulative Dose: 1.6 × 1012 rad
Visible Damage from
b radiation
Cumulative Dose: 3.4 × 1012 rad
Visible Damage from
b radiation
Cumulative Dose: 10.8 × 1012 rad
Visible Damage from
b radiation
Cumulative Dose: 15.7 × 1012 rad
Unirradiated Samples:
Average maximum
force: 1105 mN
0 Mrad
Irradiated Samples:
Average maximum
force: 798 mN
Relative Strength:
72.2% ± 4.4%
1620 Mrad
Radiation Results
1.2
Relative Strength =
0.613*exp(-0.0055*dose)+0.387
1.0
Relative Strength
0.8
0.6
0.4
0.2
0.0
1.0
10.0
100.0
Dose (Mrad)
1000.0
10000.0
Low Temperature Testing
Dipping Linear
Actuator
Low Temperature
Testing
Straining Linear
Actuator
Force
Transducer
Upper Pin
s
Nylon Strip
“S” Hook
Spider Silk
Liquid
Nitrogen
Lower Pin
(stationary)
E
Typical Results:
1400
D
1200
Room Temperature
C
B
Stress (MPa)
1000
Liquid Nitrogen
800
600
400
200
A
0
0.00
0.05
0.10
0.15
Strain
F
0.20
0.25
Low temperature results still
indicate double-strandedness:
1400
1200
Room Temperature
Stress (MPa)
1000
Liquid Nitrogen
800
600
400
200
0
0.00
0.05
0.10
0.15
Strain
0.20
0.25
0.30
Low Temperature Results
Average
LN
Air
LN / Air
σmax
(MPa)
1157 ± 26
734 ± 16
1.642 ± 0.032
T (MPa)
95.0 ± 4.4
100.0 ± 2.6
1.105 ± 0.068
Why is colder silk stronger?
One hypothesis: water is absorbed
from the atmosphere…
250
LN Strength %
200
150
100
50
0
35
40
45
50
55
60
65
Humidity (RH% )
Doesn’t seem to be water…
70
Primary Hypothesis:
Low temperature bonds require
greater net energy input to
overcome electrostatic attraction.
Acknowledgements
• LLE & DOE
• Craig Sangster, Mark Bonino of LLE