Week 14/15: The spine tingler

What if the scary book you were reading could really shock you?

This piece of biofeedback-based experimental book art detects the rising fear or excitement in a reader and then amplifies it, by causing tingling in the hands through to mild electric shocks. This effect is created by a wrap-around book cover that can sense the physiological changes that occur when a person sustains fear or other arousal, and then responds by injecting a small electric current through the book jacket and into the hands.

Important note: Those who suffer from heart conditions, who wear a pacemaker, are epileptic, pregnant, or undergoing cancer treatments should not use this device. (It works at low enough levels that it should be safe for those people but I am not familiar enough with all makes of cardiac implants and other therapies to guarantee safety.)

The Spine Tingler in use

In essence, the book jacket acts as a lie detector. (Lie detectors don’t detect lies, but they do effectively measure increased activity of the sympathetic nervous system—the fight or flight response.) When you read something that excites you, your sympathetic nervous system ramps up leading to the galvanic skin response, in which the electrical resistance of the skin changes. The response kicks in with fear, anger, being startled, or sexual feelings.

When the device measures the increase, it switched mode to send an electrical current proportional to the level of sympathetic nervous system response through the book cover. It starts out feeling as a very slight tingling, increasing to enough of a shock that most people would naturally recoil from it, probably dropping the book.

How it works

The first stage of the Spine Tingler algorithm is to detect the galvanic skin response. This is achieved through use of a home-made combination resistive and capacitive touch panel. It consists of one sheet of conductive polymer and two sheets of striated conductive polymer—conducting plastic—separated by a very thin sheet of non-conductive plastic. (The capacitive touch panel is similar in principle to the iPhone touch screen but not quite the same.)

The stripes of conductive polymer are oriented different directions so that when a finger touches the plastic, the capacitance changes in the region of the finger, and because there are two sets of lines in perpendicular directions, the location of the touch can be indexed. We must know the locations of touches because measuring the galvanic skin response, or GSR, requires a measurement of resistance, which is confused by the potentially varying amount of contact between the reader’s hands and the book cover.
The first few minutes of holding the book are used for calibrating the resistance, which depends on the person and changes on time scales of hours or days. Just hope the literary shocks don’t come during calibration in the first few pages or the proper effects won’t be felt!

Once calibrated, the galvanic skin response is measured in detail, looking for small but consistent changes in resistance. The level of GSR is determined based on the resistance measured through the conductive polymer outer layer, scaled by the positions of the hands measured through the capacitive touch panel. As there are many possible hand configurations, this calculation is based on a heuristic algorithm, trained by the measurements of various hand positions during a control period in which there is no changing activity of the sympathetic nervous system.

With the measurement in place, the conductive polymer outer can now be turned to a different purpose: delivering a small current to the hands. When an increased GSR implies increased activity of the sympathetic nervous system, the electric shock can be applied to the reader.

The front and back cover of the book are set to opposite voltages so that touching both covers (as is normal for most book holding positions), will cause a small alternating current to flow through the skin. The applied voltage difference is determined by the measured GSR so that a stronger response to the book corresponds to a stronger current delivered to the reader. A series of tests allowed the device to be calibrated for appropriate levels of current, and is scaled by the baseline knowledge of the GSR measured in the first few minutes of holding the book.

Construction details

I’ll keep the construction description just in outline here but could send detailed construction plans on request once I finish writing them up.

In the prototype, the layers of plastic are simply laid on top of each other and smoothed down against the book tightly, folded over the outer edges of the covers and taped in place while both covers are splayed open. That means when the book is anything less than fully open there is sufficient tension in the plastic to hold the layers tightly together. This also means that the device only works well for hardcover books. A future version will include a proper way to combine the layers of plastic so they are more flexible and can work on paperback books.

The electronics and power supply are housed in a block that sits against the spine, which results in an increased effective thickness of the spine, but the size could be minimized by using surface mount components. The biggest challenge is the rechargeable battery supply, which can only hold enough charge for one or two decent shocks before it needs recharging. I used a lithium-ion polymer battery for best capacity to volume ratio. The electronics could be mounted off the book, but it seemed that the device should be portable and not need anything connected externally to the book.

The control system is operated via the ATmega328 chip, which is the basis of some varieties of the Arduino open-source computing platform, which I already had experience with and have found quite powerful for various applications. The programming of the chip, etc. occurred through a modified Arduino board in which I could insert the ATmega328, program it, and then remove it for mounting in my own circuit. It was much easier to upload the code, do the testing and calibration, etc. that way. The electronics could be made smaller using a surface mount version of the ATmega328, but that would make it more difficult to program the chip so I used a DIP package.

The alternating current is provided by using a hacked transcutaneous electrical nerve stimulation (TENS) device—Medical Products Online, Inc. model MPO2800—that is intended for physical therapy application. This control of the alternating current could be all done within the chip, but hacking an existing device made it very easy to produce the appropriate waveforms suitable for inducing the tingling needed.

Send me an email for further details of the electronics and full circuit diagrams, code, etc. I could supply pre-programmed ATmega328 chips at cost for people who don’t have the capacity to set it up and program it themselves.


In essence, the device works okay as a prototype, with the ability to deliver a tingle strong enough to make me want to release the book, activated by a sufficiently strong reading stimulus. Future versions will minimize the spine size; find a battery solution that is rechargeable and has a longer life, while still delivering enough current to provide a shock of sufficient size to be surprising; develop an improved version of the homemade conductive/capacitive touch cover; and develop improved algorithms for correlating an appropriate sympathetic nervous system response to an appropriate level of current through the cover. Suggestions for improvements are very welcome and I can supply more details of this project on request.