Estimating time of death: a foray into forensics
(I was really tempted to title this entry, "If you want to fool the cops, stick the body in the freezer for while." But I decided not to, because that seemed a bit capricious even for me.)
Time of death (TOD) is an important indicating factor in forensic investigation. The oldest, and easiest, method of estimating TOD involves core body temperature (usually obtained with a liver probe, just like they show on CSI) and/or observed progression of rigor mortis.
The TOD estimation based on temperature is really kind of an iffy thing. The equations for this include many corrective factors to account for variables such as body weight (and fat content), average environmental temperature (which can be unreliable, as it may be subject to large variations over time), degree of exposure (e.g. clothes or other coverings on the body), air or water movement around the body, and substrate (pertains to conductive heat transfer through adjacent surfaces). This involves a lot of educated guesswork by the investigators. Henssge et al. recommended that additional methods of estimating TOD be employed where possible.
(Also, the body temperature is only really useful with relatively "fresh" bodies, because after a while the temperature comes to equilibrium with the environment, after which point the body temperature can't tell you much at all. Another major skewing factor can throw off the entire method when the body is moved from the site of death prior to the start of an investigation. Then, of course, there are cases where the fatal event, constituting the actual crime that the investigators want to solve, doesn't immediately precede death; this opens up a whole other can of worms.)
It is generally accepted that the "average" case of rigor mortis lasts 2-4 days post-mortem. It is also generally accepted that lower-than-"average" temperatures cause this time period to increase. Varetto et al. performed a study in which human cadavers were stored at a constant temperature of 4 degrees Celsius, representing average winter outdoor temperatures in temperate regions. They found that complete rigor lasted at least 10 days and up to 16 days, which is much longer than is usually indicated in forensic medical texts. Partial rigor was observed up to 27 days post-mortem, with complete "relaxation" seen on day 28. Based on these results, Varetto et al. concluded that at this temperature, it can be expected that complete rigor will persist 11-17 days post-mortem, with rigor disappearing altogether by day 28.
Henssge et al.: "Experiences with a compound method for estimating the time since death."
Varetto & Curto: "Long persistence of rigor mortis at constant low temperature."
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Labels: science
More general pondering on the pet project
I'm really intrigued by the whole design concept for the plug-in prosthesis. It's a possibility for arms and legs, but because we started with the leg, we'll continue along that thread. Let's assume here that we want the prosthesis to be easily removable; ideally, the user can take it off and stick it next to the door at bedtime, and just as easily reattach it in the morning. Convenience is key; this includes comfort, good mechanical performance (security, reliability, & durability), non-invasive & low-frequency maintenance, and power efficiency & self-sufficiency (just recharge the batteries overnight!).
Now, the exact attachment point would depend largely on the location of the original amputation. Consider the issue, for example, of how many surviving muscles have retained functionality. We will consider the two general cases of amputation (1) below the knee and (2) above the knee.
Case 1: If the amputation was below the knee, let's assume that the muscles in the upper leg, which mobilize the knee joint, and their attachments to the lower leg are intact. In this case, the natural motion occurring throughout the knee joint is unimpaired; the prosthesis will only have to account for the structures & functions of the lower leg & foot. The prosthesis must also attach securely & comfortably to the termination of the limb. The implanted cybernetic interface (ICI, for simplicity's sake) could be anchored in the distal terminations of the tibia and fibula, assuming these bones are intact down to the point of amputation, with supporting structures placed radially outward through the tissue to support non-central, non-axial loading about the edges of the implant.
Case 2: If the amputation was above the knee, many of the surviving muscles in the upper leg have no purpose other than to serve as cosmetic padding to preserve the natural shape of the leg. The potentially useful muscles, namely those that move the upper leg relative to the torso, may or may not still serve this function; if the amputation was sufficiently high on the limb to disrupt the attachments of these muscles, further surgery should be performed to re-attach them higher up on the femur so as to restore functionality. The missing knee joint, its movement, and the previously discussed structures & functions of the lower leg must be replaced by the structures & functions of the prosthesis. The prosthesis must attach securely & comfortably to the termination of the upper leg, and also must move the entire lower leg independently of muscle action in the upper leg. The ICI would probably be anchored in the distal termination of the femur, with a supporting structure similar to that described above.
In either case, because we don't want miscellaneous protrusions on the limb surface of the ICI, the corresponding surface of the prosthetic will probably have the protruding/moving components of various mating/latching mechanisms to securely hold the prosthesis in place when it is attached. The distal terminations of the surviving nerve branches could be permanently wired via a highly-discriminatory electrode cuff, such as the FINE (discussed in an earlier post), to one or more "outlets" on the surface of the ICI, corresponding to electrical connectors on the attaching surface of the prosthesis.
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Labels: engineering, science
Neuroelectric interface example
Here's yet more supporting work for the plug-in prosthesis idea.
Usually, a single nerve supplies several different muscles. Within the nerve are several fascicles, or bundles of nerve fibers; each fascicle supplies a different muscle. (Bear in mind that this is kind of an over-simplification; the nervous system is incredibly complex, and very cool, but we don't need to get into the nitty-gritty details here.) The flat interface nerve electrode, or FINE, is a discriminatory electrode cuff that can interact with selective fascicles within a nerve. More invasive electrodes, ones that are inserted directly into nerve fascicles, perform well but at a high risk for nerve damage. Ideally, for our two-way neuroelectric interface, each electrode will communicate selectively with a small group of nerve fibers within the nerve without damaging the nerve or stimulating non-target fibers ("signal slop").
With the FINE, the working concept is that the cuff puts pressure on the nerve, forcing it to deform into an elongated oval shape, which is a favorable geometry for selective stimulation of fascicles. This effectively increases the nerve's surface area, thereby allowing more electrode contacts to be placed around the nerve; this also causes central nerve fibers, which would otherwise be relatively inaccessible, to move closer to the surface, where they can be more easily stimulated by the contacts.
Leventhal & Durand conducted two separate studies with the FINE. They demonstrated that the FINE was able to selectively activate portions of individual fascicles, and that in general the FINE acted as a stable and selective interface for stimulating peripheral nerves.
Tyler & Durand showed that only the most constrictive (narrow) cuff geometry used in their study was associated with signs of nerve damage (changes in the nerve's function), and that these signs had disappeared by 21 days post-implantation. Their data supported the hypothesis that the constrictive FINE causes an acute initial reaction (due to the high force applied by the cuff) that is resolved over time as the nerve reshapes and intraneural pressure (within the nerve itself) returns to normal. In general, they found that the FINE cuff was able to reshape the nerve and fascicles without significant changes to the nerve's structure and function long-term. Leventhal, Cohen, & Durand found that the "wide" and "medium" cuffs caused little to no damage to nerve fibers and supporting cells; the narrow cuffs were found to cause some damage, which was recovered over the course of the study, which supported Tyler & Durand's findings.
Tyler & Durand:
"Chronic Response of the Rat Sciatic Nerve to the Flat Interface Nerve Electrode."Leventhal & Durand:
"Subfascicle stimulation selectivity with the flat interface nerve electrode.""Chronic measurement of the stimulation selectivity of the flat interface nerve electrode."Leventhal, Cohen, & Durand:
"Chronic Histological Effects of the Flat Interface Nerve Electrode."--
Labels: engineering, science
More on the pet project
In the broadest sense, a cybernetic interface is one that provides a communication between man and machine. A PC keyboard falls into this category. When most people think of cybernetics, however, what usually comes to mind is actually a neuroelectric interface, which serves as a sensing and processing system that uses signals from the brain or peripheral nerves as control input for some piece of hardware and/or software.
What we'd really like for the purposes of this "plug-in prosthesis" is a two-way neuroelectric interface. Ideally, this will allow "outgoing" motor nerve signals to be sent to the prosthesis, and in addition the interface will translate "incoming" signals from sensors in the prosthesis and transmit them back up the sensory nerve pathways to the brain. This is not as "sci-fi" as it may sound.
An important goal in realizing a fully-functional, two-way neuroelectric interface is the first step in a bottom-up approach: the development of a functional two-way interface between an electromechanical "terminal" and individual neurons (nerve cells) or a neural network (e.g. an individual nerve bundle leading to one muscle). Some important issues that need to be addressed in the design of such an interface are as follows:
- Neuronal and/or network plasticity ("recovery") following stimulation
- Biocompatibility of interface materials (material degradation, toxicity; how the body responds to the material, i.e. does it cause irritation or inflammation in the surrounding tissues?)
- Insulatory and discriminatory behavior (it should prevent signal "slop" to unintended recipient neurons)
- Separation/isolation of sending and receiving pathways (it should prevent feedback and interface-generated noise)
Two separate groups approached this design problem. Stett et al. designed, constructed, and successfully tested a silicon micro-structure that allowed two-way communication with an individual neuron; Reiher et al. used a titanium-gold electrode-based interface to simultaneously stimulate a large area of a neural network. If you're so inclined, you can find the articles through the links below.
Stett et al.: "Two-way silicon-neuron interface by electrical induction." [PDF, 180 K]
Reiher et al.: "In vitro stimulation of neurons by a planar Ti-Au-electrode interface." [PDF, 153 K] (
HTML version)
I realize that to many people this may sound like a bunch of technical mumbo-jumbo. If you don't understand all of it, well, the only thing you really need to glean from this is the fact that for the purposes of my little conceptual design exercise, I consider the neuroelectric interface problem to be "solved." However, if you have questions about any of this or simply think it's cool, feel free to comment. I'll be happy to explain things, discuss further, and/or otherwise "get my geek on."
I'm weird enough that I actually enjoy killing time by thinking about this kind of stuff.
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Labels: engineering, science
