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Electrodes are frequently used for measuring medical signals. They are expected to be soft, wettable, and capable of conducting electricity to make them suitable for attaching to living tissue. Metal electrodes have been widely used for medical purposes.

Conventional metal electrodes repel water and are quite hard. The mismatch between living tissue and metal electrodes causes an increase in the electrical resistance and can be uncomfortable to wear.

Conductive polymers are widely used in flexible displays and for antistatic coatings. One polymer, PEDOT-PSS, has been examined as a material for medical electrodes, because it is biocompatible and highly conductive in both wet and dry environments.

However, in wet conditions PEDOT-PSS absorbs water, which reduces its mechanical strength. For medical use, PEDOT-PSS must be improved so that it is more durable when it becomes wet.

Silk fiber is one of the most widely used natural fibers. It has been used in the biomedical field as suture thread due to its excellent strength and biocompatibility.

A series of experiments was done using a microfiber composite material made of PEDOT-PSS and natural silk thread as a biomedical electrode. The addition of glycerol to the PEDOT-PSS silk string was also studied.

Study 1

In a series of trials, researchers took a silk thread and electrochemically fixed PEDOT-PSS to the silk by drawing the thread through a solution containing PEDOT-PSS. The researchers then measured the conductivity of the composite fiber. In some trials, the PEDOT-PSS was fixed to the silk twice, and in other trials other chemicals such as glycerol were added in an effort to improve the fiber's water resistance. The higher the conductivity, the easier the biomedical signals will move through the fiber.

Fixation Level Chemical Additive Conductivity
Single -- 0.000184
Single Glycerol 0.0304
Double -- 0.00135
Double Glycerol 0.102
Double Ethylene Glycol 0.0963

Study 2

The researchers measured how much various fibers would stretch in both wet and dry conditions. The three fibers tested were:


1- Plain Silk
2- Silk with PEDOT-PSS
3- Silk with PEDOT-PSS and Glycerol


1’- Plain Silk
2’- Silk with PEDOT-PSS
3’- Silk with PEDOT-PSS and Glycerol

For use in biomedical applications, greater fiber flexibility makes the fiber feel softer and less irritating.

Study 3

The researchers also measured how durable the threads were through repeated washing by washing composite fibers both with and without glycerol, and measuring the conductivity over repeated washings with water.

Tables and charts adapted from Tsukada S, Nakashima H, Torimitsu K (2012) Conductive Polymer Combined Silk Fiber Bundle for Bioelectrical Signal Recording. PLoS ONE 7(4): e33689. doi:10.1371/journal.pone.0033689

Which of the following statements could be supported by the data in Chart 2 in Study 3?


The addition of glycerol increased the resistance of PEDOT-PSS to being washed off.


Adding glycerol enhanced the conductivity of the silk PEDOT-PSS fibers.


Adding ethylene glycol to the fibers would have made them even more resistant to having the PEDOT-PSS washed off.


Fibers without glycerol had the greatest conductivity.

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