dmicroprosthetic implant for the treatment of erectile...
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dMicroprosthetic Implant for the Treatment of Erectile Dysfunction
Robert Douglas and Matthew Schwartz
Biomedical Engineering, Vanderbilt University
Senior Design
Advisor: Franz Baudenbacher, Ph. D.
April 27, 2010
Abstract
Erectile dysfunction is a disease that plagues men across the country. Currently there are many different
treatment options for the problem, but none offer a biomimetic solution. Remedies such as Viagra and Cialis
must be taken far in advance. Implants like penile prosthetics require an invasive surgery that shows no
remorse for the testicular region. In order to restore the quality of life some of these men have lost, we
propose a novel drug delivery device. This device has been fabricated using Medtronic's IsoMed constant flow
pump as the driving force of the drug delivery system. Unfortunately, after designing a circuit and equipping
the pump with two Lee series 120 nozzles it was discovered that the pump's 1ml/day flow rate was not fast
enough. In order to permit for a .2ml injection per erection the pump must deliver at 1728 ml/day. Capillary
tubing within the device is used to restrict flow. It was calculated that by changing the diameter of the capillary
tubing to 1ml I.D., the desired flow rate would be achieved. A microcontroller was programmed and assembled
in order to best demonstrate the basic concept of the device's signal filtering process. The microcontroller
successfully filters out accidental pushes by forcing the user to press the pushbutton three times within ten
seconds in order to activate the valves. If a third push is not achieved within 10 seconds, the device falls to
sleep and becomes dormant, an energy efficient process. Due to the inability to achieve the desired flow rate;
the valves output was significantly different than expected. It took ~4.8 hours to administer a dose that should
have taken 10‐15 seconds.
Figure 1. The corpus cavernosum receives vascular supply from the dorsal artery and the cavernous branch of the internal pubic artery.4
Pathophysiology of Erectile Dysfunction
Erectile dysfunction (ED) is defined as the inability to sustain an erection suitable for
satisfactory sexual intercourse.1 The physiological mechanisms that cause ED are variable and
occasionally complicated by the
fact that causes may overlap.
However, prior to discussing the
causes of ED, it is important to
have an understanding of the
physiology of penile erection and
detumescence (the return of the penis to its natural flaccid state).
The key anatomy in achieving erection is the penile erectile tissue. This includes the
cavernous smooth muscles as well as the smooth muscles of vascular walls in the penis as seen
in Figure 1. In the natural flaccid state, these smooth muscles remain constricted. This
effectively limits blood flow in the penis to be satisfactory for nutritional purposes without
sustaining an erection.2 This low blood flow is evidenced by a blood partial pressure of oxygen
(PO2) of approximately 35 mmHg in the flaccid state.3 Upon sexual stimulation, the release of
neurotransmitters from the cavernous nerve terminals yields relaxation of these smooth
muscles, thus triggering a series of events that leads to erection. With the smooth muscle
relaxation, the arterioles and arteries to the penis dilate, allowing for increased blood flow to
the corpus cavernosum. Simultaneously, incoming blood is trapped by the spongy cavernosum
tissue, and venous outflow is reduced to a minimum level through compression and occlusion
Figure 2. Different views of the autonomic innervation of the penis show the location and physical path of the cavernous nerves.
of the local venous structures. This leads to an increase in PO2 levels to approximately 90
mmHg. The physiology of detumescence involves a reduction in pressure through the return of
venous outflow.2
Knowledge of the neuroanatomy and neurophysiology involved in penile erection is also
crucial to understanding the mechanisms of ED. Penile innervation involves both the
autonomic and somatic nervous systems. The sympathetic and parasympathetic nervous
systems are contained in the cavernous nerves. These nerves enter the corpora cavernosa and
are primarily responsible for the neurovascular events of both erection and detumescence.
Meanwhile somatic innervation of the penis, achieved with the dorsal nerve, is responsible for
sensory function and conscious contraction of the local muscles. The exact paths that these
nerves take to reach the penis are
important because they dictate what sort
of injuries or procedures can adversely
affect erectile function. The autonomic
innervation of the penis stems from the
pelvic plexus as shown in Figure 2.
Branches from the pelvic plexus also
innervate the rectum, bladder, prostate
and sphincters. As such, the cavernous
nerves can be damaged easily in radical
excision of these organs. These
autonomic pathways are responsible for carrying the cerebral impulses to signal erection.2
However, there is also evidence that there is a reflex involving the somatic pathways that can
stimulate erection without cerebral processing.5,6,7 Animal studies have shown the medial
preoptic area as well as areas within the hypothalamus and hippocampus to be major
integration centers for male sexual function.8,9
Erectile function is also dependent on the molecular mechanism of smooth muscle
contraction and relaxation. The level of contraction or relaxation of the smooth muscle is
primarily regulated by cytosolic Ca2+ levels. The binding of free Ca2+ to calmodulin causes on
conformational change that exposes sites to myosin light‐chain kinase. This process catalyzes
phosphorylation of myosin light chains, developing force along the actin filaments. There are
also calcium‐sensitizing pathways that act when the cytosolic Ca2+ returns to basal levels.
Figure 3 shows the detailed signal pathways involved with penile smooth muscle relaxation
along with several ED treatment drugs, such as papaverine and sildenafil, and their respective
pathway effects on the overall signal pathways. These drugs, called phosphodiesterase type‐V
inhibitors, work by circumventing certain parts of this signal pathway, thus forcing smooth
muscle relaxation and increasing the likelihood of achieving erection.2
The pathophysiology of ED is complicated by the fact that there are several causal
mechanisms that can overlap to cause ED. As such, ED is typically broken down into categories
for classification. These classifications primarily include psychogenic, neurogenic, arteriogenic,
and venogenic causes, but there may be other more specific causes for individual cases of ED.2
Figure 4 shows a classification system recommended by the International Society of Impotence
Research.
Figure 3. The chemical signal pathways involved in penile smooth muscle relaxation can be bypassed by certain drugs.10
Psychogenic impotence, being caused by the brain, was previously believed to be the
most common type of ED.11 The mechanism of psychogenic impotence is likely rooted in the
limbic system, the
hypothalamus, and the cerebral
cortex, which control sexual
behavior and penile erection.
Thus in psychogenic impotence,
it is possible that either the
brain is sending excess inhibitory
signals, or there could be a
chemical imbalance issue.12 In
either case, it has been
demonstrated that cerebral control plays an important role in achieving erection.
Estimations of the percentage of ED cases related to neurogenic causes range from 10
to 19%.13,14 There may also be a significant portion of neurogenic ED in the population that is
iatrogenic (caused by a physician). As previously mentioned, the cavernous nerves are in very
close quarters with the rectum, bladder, and prostate. As such surgical techniques performed
in this area on these other organs can cause peripheral damage to the cavernous nerves,
causing varying degrees of ED. Diseases or trauma occurring in the brain or spinal cord can also
cause significant ED depending on type and location. Damage to the peripheral sensory nerves
can reduce the potential for reflexogenic stimulation, which is an important factor in achieving
erection. Being that erection is a neurovascular event, essentially any neurological damage
Figure 4. The medication injected dilates the arteries of the penis and allows blood to flow in. Depending on the drug used, an erection occurs anywhere from 5 to 15 minutes after injection. 33
along the stimulation pathway can cause ED if the stimulation signal is not transmitted
properly.2
Arteriogenic causes of ED reflect the inability to get sufficient blood flow into the penis
in order to achieve erection. Factors such as atherosclerosis or occlusion of vasculature can
limit blood flow into the penis. There are also several diseases and behavioral factors that have
high correlation with ED, likely due to their effect on blood flow such as hypertension, cigarette
smoking, diabetes mellitus, and pelvic trauma.15‐17 When the vasculature of the penis is
compromised, the normal stimulation from the nerve signals may not be enough to cause
satisfactory vasodilation. Similar to arteriogenic ED, if there is not adequate venous occlusion
after blood has entered the penis, there will be venous leak and an erection will not be
sustained. Venous leak can stem from large venous channels, degenerative changes such as
Peyronie’s disease, traumatic injury, and inadequate expansion during the filling stages. The
reason that inadequate filling also leads to venous
leak is because the filled tissue in turn occludes the
venous outflow.2 The treatment methodology of our
proposed device specifically targets the neurologic
and arteriogenic causes of ED. Hopefully, by treating
the ED in this fashion, the venogenic causes will be
overcome through adequate filling which causes
venous occlusion.
Current Treatment Methods
Figure 5. The three‐piece internal penis pump is the most technologically advanced penile implant. Urologists consider the 3‐piece inflatable implant to be the "gold standard" of penile implants. 21
This device implements existing technologies in an attempt to correct a physiological
problem plaguing men all over the world. Utilizing MEMS technology as well as the proper drug
and drug dosage, this device proposes a potentially
non‐invasive biomimetic solution to ED. Today,
there are many different treatments for erectile
dysfunction, but amongst these treatments there
are no devices and or drugs that can initiate an
erection without the patient’s triggering of an
event. Such events include taking drugs, using
injections, using pumps, and receiving penile implants. The most commonly used erectile
dysfunction drugs are Viagra, Cialsis, and Levitra. These drugs are effective in triggering the
onset of an erection, but do not provide an immediate response to sexual stimulation.
Additionally, drugs that are ingested orally require
a much larger dosage compared to the minimum amount needed to stimulate the target. By
utilizing a local drug delivery system, higher concentrations in the form of smaller dosages can
be administered at the site of the target site.18
A common method of local drug delivery comes in the form of injections to the corpus
cavernosum. As shown in Figure 4, the injection is placed in a general location, whereas an
implantable device will be able to administer drugs to a much more specific site. The three
most common injections are papaverine hydrochloride, phentolamine, and prostaglandin E1.
These drugs are very effective, but men do not enjoy the thought of sticking a needle into their
penis. These drugs can also leave scarring at the injection site, as well as contribute to
prolonged and uncomfortable erections. 20
A more invasive strategy for some men is the penile implant or prosthesis. Such devices
are either malleable or inflatable. The simplest form consists of a pair of bendable rods that are
surgically implanted within the corpora cavernosa. This type of implant causes the penis to
have a permanent semi‐rigid state, and requires that the penis be lifted or adjusted into the
erect position in order to proceed with intercourse. Alternatively, many men choose a
hydraulic, inflatable prosthesis. The hydraulic prosthesis involves a serious surgery requiring
general anesthesia, and weeks of recuperation. These prostheses are implanted in 20,000‐
30,000 men annually worldwide. 19,20,21
The hydraulic implant (Figure 5) has a 95% success rate, but involves the use of a
mechanical pump to achieve an erection. A less invasive and more tabooed solution involves a
vacuum constriction device or a penis pump. In contrast to implantable devices and pumps,
some surgical techniques have been performed in order to increase direct blood flow to the
penis. This surgery is referred to as vascular reconstructive surgery, and is technically difficult,
costly and not always effective. 22
Figure 6. This block diagram shows that the dorsal nerve of the penis bears responsibility for transmitting sensory signals. However, these signals can still be used to trigger drug release with this device.24
Our described device incorporates aspects from three existing erectile dysfunction
treatments. The device takes the effectiveness and speed of injection based drugs, resides
within the body like the prosthetic
implant, and delivers a small and
precise quantity of a favorable a
drug to promote an erection just as
Viagra does.
Design:
Proof of concept:
In order to fully understand
the concept of constant drug
delivery, a constant drug delivery device was fabricated. In order to accomplish this, a siphon
bladder was placed within a sealable container. One input of the bladder was connected to a
one‐way valve (bike tire valve) and the other was connected to a latch valve. On the upper
casing of the sealable container existed another one‐way valve that allowed for the device to
be pressurized. In order to refill the siphon bladder a handheld bike pump was use. In order
pressurize the vessel; the same hand pump was used.
In order to properly seal all leaks and holes extra strength epoxy was used.
Prototype:
In an effort to reduce costs and FDA testing time, Medtronic's IsoMed pump was utilized
for constant drug flow delivery. Attached to the device is .04" silicone tubing. The tubing splits
into two by means of a polycarbonate Y‐connector with 1/16 openings. The tips of the Y‐
connector are each fastened with a single 5V series 120 Lee valve. The valves are controlled via
a parallax bs2 Microcontroller, a 6V battery and a silicone‐coated push button.
The microcontroller is inherently in a dormant state, optimizing power consumption.
Once the use clicks the pushbutton the processor awakens and searches for more impulse. If
the user presses the pushbutton three times within ten seconds, the microcontroller sends an
impulse to both valves. When the valves receive an impulse, the pressurized fluids are released
to the site of target.
The microcontroller is embedded on a 2" OD PCB board, and has one input (the button)
and two outputs (the valves). The embedded chip is housed within a 3" aluminum casing.
Two separate microcontrollers were built, one that most accurately depicts the filtering
process and another that can be used for demonstration purposes. The parallax controllers
were programmed in basic and contained roughly 50 lines of code. The microcontroller has
basic functions that make programming 7 segment displays fairly simple. In addition to the
mechanical design of the system, the IsoMed pump was filled with a potent and effective drug
called papavarine. Papavarine's shelf life and effectiveness make it a prime candidate for use in
the device.
Results:
Because the prototype devices that we obtained did not match our specified flow rate,
it would not have been helpful to determine the accuracy of this prototype. Instead we
focused on calculations that would help show that it would be feasible to modify the IsoMed
pump to meet our requirements. To do so we used the Hagen‐Poiseuille equation.
This equation is crucial to the flow rate of the pump because there is a long piece of capillary
tubing inside the pump that restricts the flow. We wanted to show that removing or altering
this piece of tubing could theoretically increase the flow rate to our desired dosing level. To do
so, we called Medtronic to determine the specifications of the capillary tubing. They were able
to provide us with the length of the tubing, but we had to estimate the radius. The results of
these calculations are in the table below:
As can be seen, by dramatically reducing the length of the capillary tubing and slightly
increasing the radius, the flow rate of the IsoMed pump could be easily adjusted to meet our
specifications. The other feasibility process we completed was to estimate the total energy
consumption of the device over ten years. By estimating the microcontroller’s energy
consumption during both sleep and run times and the energy draw of the two Lee valves, we
were able to determine that over the course of 10 years the device would consume
approximately 80,000 J. We believe this shows the device is feasible for a few reasons. First,
these calculations were approximated with devices that are not optimal for long term low
power consumption. Second, we think that a lithium battery could potentially provide this long
term power source. Third, there are currently implantable devices such as the implantable
pacemaker that consume similar amounts of energy and have roughly 10 year implantable
lifespans.
Costs:
This project requires the use of a Medtronic IsoMed pump ($5900), two Lee company
series 120 valves ($150/each), a parallax BS2 Microcontroller ($40) and various miscellaneous
tubing (<$20), amounting to a total cost of $6220. In order to significantly cut costs we
contacted medtronic and tried to convince them that we could use their technology to solve
other worldly issues. Medtronic liked our hypothesis and sent us two IsoMed models, one new
and one used. We used this same tactic when contacting The Lee Company, and to our surprise
they sent us the valves for free. Additionally, Dr. Milam from Vanderbilt's Urological Center
gave us a mechanical prosthesis ($7000) in order to demonstrate the benefits of our device. We
received roughly $19,100 if you take into consideration all of the products that were sent to us
for free.
Table 1. Both the prevalence and incidence estimations for ED are promising for growth in the ED treatment market. It is also important to consider that there may be a general underreporting of ED both in the United States and worldwide.18
Market Size and Potential Market Share Capture
Treatments for erectile dysfunction (ED) fall under the healthcare sector of men’s
health, which in 2005 had a total world market size of $19.5 billion. Additionally, this market
size is predicted to grow to $26.1 billion, or a 5.9% increase, by 2010. Factors such as a
generally increasingly older population, men’s health awareness, and societal factors, may all
play a role in this trend. Table 1 shows the estimated prevalence and yearly incidence of ED in
the world and specifically the United States, and the figures are staggeringly large. Drug
therapies such as Viagra, Cialis, and Levitra have dominated the ED treatment market, which in
2005 totaled $3.1 billion. This market size is expected to grow at a rate of 6.5% annually
through 2010.33
With the three primary treatment methods currently in existence being oral pills,
injection therapy, and penile implants, it is important to identify the market share that each of
these treatments has in the ED treatment space. This will help us estimate what sort of market
share our device could potentially capture. As of 2005, oral pills, specifically phosphodiesterase
type‐V inhibitors (PDE‐5 inhibitors) such as Viagra, Cialis, and Levitra, dominated the market.
These PDE‐5 inhibitors represent roughly 90% of the ED treatment market, or approximately
$2.8 billion in 2005. However, as new therapies arise in ED treatment, this percentage is
expected to decline, but these products still clearly dominate the market. In 2005, other
therapies such as injections and implants totaled only $339 million, and it is likely that injection
therapies represent a larger portion of this than do penile implants.33
As with any new product, the keys to capturing market share are implementation of
bringing the product to market, marketing strategies, cost considerations, and competitive
advantages. While we are not currently at a development stage where we can consider
product implementation or marketing strategies, we can identify some cost considerations and
our potential competitive advantages. In terms of cost considerations, our device will benefit
from several things. First, gaining coverage from insurance companies and Medicare will likely
increase the number implantations performed by reducing the costs to the consumer. Second,
our product will see increased adoption of the total cost to the consumer of receiving the
implant is less than the cost of paying for prescription PDE‐5 inhibitors over the course of the
lifetime of the implant. For example, if the yearly cost of a Viagra prescription is $500, and our
device has a lifetime of 5 years, then ideally the cost for a consumer to receive the implant
would be no greater than $2500 ($500 x 5 years). Obviously from this relationship, the longer
the device lifetime can be extended, whether through increased reservoir size or easy refillable
access, the more cost advantageous the device becomes.
We do believe that our device, if it works as planned, offers several competitive
advantages over all other treatment options. When compared to PDE‐5 inhibitor pills, this
implant would improve patient compliance by removing all patient responsibility to administer
their dosage. Additionally, the implant would improve patient’s experience compared to pills
as erection is achieved without any preplanning. However, there is a major drawback with
transitioning from pill treatment to this implant in that implantation requires a significant
surgical procedure. Transitioning from injection therapy to this implant would likewise improve
patient compliance and reduce risk from injection injury. Still, there is the drawback of surgery.
Current penile implants also require a surgical procedure, which is generally more invasive than
our proposed implantation procedure. Additionally, our implant acts as a biomimetic therapy
for ED, while current penile implants require mechanical manipulation to achieve erection.
Thus, provided our implant works as designed, the only foreseeable drawback of our device
compared to current penile implants is a possibly shorter device lifetime. In all cases, it is our
belief that this implant will offer the most natural treatment of ED by internally using the body’s
nervous signals as input and outputting a local vasodilator that will cause erection.
Under ideal circumstances, this device will be available to the consumer at cost
advantageous prices compared to oral pill therapy, meaning roughly the same as the yearly cost
of PDE‐5 inhibitors multiplied by the yearly lifetime of the implant. If this is the case, we
believe that this implant offers considerable advantages over all other therapies with the one
major drawback being surgery. As such, it is our feeling that this device has the potential to
capture market share from all competing therapies. Assuming the device is produced
successfully, obtains FDA approval, and is adopted by surgeons as an acceptable treatment, we
can estimate initial revenue streams. In order to capture just 2% of the market share from the
PDE‐5 inhibitor market, 120,000 implants would need to be performed. This comes from the
assumption that a year’s supply of PDE‐5 inhibitor drug costs $500 and the total PDE‐5 inhibitor
drug market is approximately $3 billion. This makes for approximately 6 million patients, and
2% of 6 million is 120,000. A 2% market capture of the PDE‐5 inhibitor drug market also
represents yearly revenues of roughly $300 million ($500 yearly cost of prescription drugs x 5
year device lifetime x 120,000 implants). If 10% of the remaining injection and current implant
market, valued at roughly $400 million, can be captured, then this would translate to an
additional revenue stream of approximately $40 million. The reason that we assume greater
market capture in this part of the market is because the switch from injection or current
implant to our implant is less dramatic than the switch from oral pills to a surgical implant.
Hopefully over time though, the surgical technique will be perfected and shown to be minimally
invasive and effective, and then the market capture of the PDE‐5 inhibitor drug market will
grow.
References
1 National Institutes of Health Consensus Development Panel on Impotence: Impotence. JAMA, 270: 83, 1993. 2 Robert C. Dean, MD and Tom F. Lue, MD. Physiology of penile erection and pathophysiology of erectile
dysfunction. Urol Clin North Am. 2005 November ; 32(4): 379‐v.
3 Sattar AA, Salpigides G, Vanderhaeghen JJ, et al. Cavernous oxygen tension and smooth muscle fibers: relation and function. J Urol 1995; 154: 1736.
4 Human Anatomy. Gray’s Anatomy on www.theodora.com/anatomy. <http://www.theodora.com/ anatomy/the_penis.html>.
5 Burnett AL, Tillman SL, Chang TS, et al. Immunohistochemical localization of nitric oxide synthase in
the autonomic innervation of the human penis. J Urol 1993; 150: 73.
6 Carrier S, Zvara P, Nunes L, et al. Regeneration of nitric oxide synthase‐containing nerves after cavernous nerve neurotomy in the rat. J Urol 1995; 153: 1722.
7 Fiuliano F, Rampin O, Jardin A, et al. Electrophysiological study of relations between the dorsal nerve
of the penis and the lumbar sympathetic chain in the rat. J Urol 1993; 150: 1960.
8 Sachs B, Meisel R. The physiology of male sexual behavior. New York: Raven Press, pp. 1393‐1423, 1988.
9 Marson L, Platt KB, McKenna KE. Central nervous system innervation of the penis as revealed by the transneuronal transport of pseudorabies virus. Neuroscience 1993; 55: 263.
10 Lue TF: Erectile dysfunction. N Engl J Med 2000; 342: 1802—1813. Copyright 2000 Massachusetts
Medical Society. All rights reserved.
11 Masters, W Johnson, V. Human Sexual Response. Boston: Little Brown, 1970.
12 Steers WD. Neural control of penile erection. Semin Urol 1990; 8: 66.
13 Abicht J. Testing the autonomic system. In: Erectile Dysfunction. Edited by U. Jonas, W. Thoh, C. Steif. Berlin: SpringerVerlag, pp. 187‐194, 1991.
14 Aboseif S, Shinohara K, Borirakchanyavat S, et al. The effect of cryosurgical ablation of the prostate on erectile function. Br J Urol 1997; 80: 918.
15 Goldstein I, Feldman MI, Deckers PJ, et al. Radiation‐associated impotence. A clinical study of its
mechanism. Jama 1984; 251: 903.
16 Levine FJ, Greenfield AJ, Goldstein I. Arteriographically determined occlusive disease within the hypogastric‐cavernous bed in impotent patients following blunt perineal and pelvic trauma. J Urol 1990; 144: 1147.
17 Rosen MP, Greenfield AJ, Walker TG, et al. Arteriogenic impotence: findings in 195 impotent men
examined with selective internal pudendal angiography. Young Investigator’s Award. Radiology 1990; 174: 1043.
18 "Cialis, Viagra and Levitra to Treat Erectile Dysfunction." WebMD ‐ Better information. Better health. Clevland Clinc. Web. 13 Nov. 2009. <http://www.webmd.com/erectile‐dysfunction/cialis‐viagra‐levitra‐treat‐ed>.
19 "Penile Implant Types." Impotence Guide ‐ All about Impotence. Web. 13 Nov. 2009. <http://www.impotence‐guide.com/penile‐implant‐types.html>.
20 Francois, J. "Penile Injection Therapy ‐ Treatment of Erectile Dysfunction (ED)." Penile prosthesis, penile implant, erectile dysfunction (ED), BPH, and urinary incontinence treatments ‐
J. Francois Eid MD ‐ http://www.UrologicalCare.com. 2008. Web. 13 Nov. 2009. <http://www.urologicalcare.com/other‐ed‐treatments/penile‐injection‐therapy//>.
21 "Erectile dysfunction treatment Newark Delaware." Delawareurologic.com. Web. 13 Nov. 2009. <http://www.delawareurologic.com/erectile_dysfunction/>.
22 "Vascular Reconstructive Surgery to Treat Erectile Dysfunction." Erectile Dysfunction: Vascular Reconstructive Surgery. Microdex. Web. 13 Nov. 2009. <http://www.webmd.com/erectile‐dysfunction/guide/vascular‐reconstructive‐surgery>.
23 Rudloff, David A. Penile Implant. United States Patent and Trademark Office. <
http://www.google.com/patents/about?id=w7s8AAAAEBAJ&dq=4664100>. 19 Nov 1984.
24 De Groat WC and Steers WD. Neuroanatomy and neurophysiology of penile erection. Sexuality and Disability, Vol. 12, No. 1, 1994.
25 Steers WD, Mallory B, and de Groat WC. Electrophysiological study of neural activity in penile nerve of the rat. Am J Physiol Regulatory Integrative Comp Physiol. 254:989‐1000, 1988.
26 Mallela, Venkateswara Sarma. "Trends in Cardiac Pacemaker Batteries." Trends in Cardiac Pacemaker Batteries 4.4 (2004): 201‐12. Print.
27 "CODMAN® 3000." Codman Pumps. 15 Oct. 2009. Web. <http://www.codman.com/DePuy/products/Products/neuromodulation/pump/index.html>.
28 Romanowski, Perry. "Pacemaker: How Products are Made." ENotes ‐ Pacemaker. Web. 13 Nov. 2009. <http://www.enotes.com/how‐products‐encyclopedia/pacemaker>.
29 Zhang, John X.J. "Biomedical MEMS Micro‐Injection." Department of Biomedical Engineering ‐ University of Texas at Austin. Web. 13 Nov. 2009. <http://www.bme.utexas.edu/research/zhang/flash/loader.html>.
30 "Papaverine Facts and Comparisons at Drugs.com." Drugs.com | Prescription Drugs ‐ Information, Interactions & Side Effects. 2008. Web. 13 Nov. 2009. Micromedex.Web <http://www.drugs.com/cdi/papaverine.html>.
31 "Papaverine Injection (Papaverine Hydrochloride Injection) Drug Information: Uses, Side Effects, Drug Interactions and Warnings at RxList." Papaverine Injection. WebMD. Web. 13 Nov. 2009. <http://www.rxlist.com/papaverine‐injection‐drug.htm>.
32 "Complete Alprostadil (PGE1; Prostaglandin E1) information from Drugs.com." Drugs.com | Prescription Drugs ‐ Information, Interactions & Side Effects. Microdex. Web. 13 Nov. 2009. <http://www.drugs.com/ppa/alprostadil‐pge1‐prostaglandin‐e1.html>.
33 Elder, Melissa. Men’s Health: The Worldwide Market for Current and Emerging Drug Therapies, 2nd ed. Kalorma Information. May 2006.
34 Sacral Plexus of the Right Side. Gray’s Anatomy.
35 Feldman HA, Goldstein I, Hatzichristou DG, Krane RJ, McKinlay JB. Impotence and its medical and psychosocial correlates: Results of the Massachusetts Male Aging Study. Urology 1994; 151: 54‐61.