Local distortion of the uniformity of the B-field component of the RF field will give rise to flip angle variation and creates contrast and signal-to-noise ratio SNR inhomogeneity. The specific absorption rate, which is defined as the RF power absorbed per unit of mass of an object, can exceed legal limits. If the specific absorption rate exceeds legal limits, images cannot be made using magnetic resonance scanners.
As frequencies increase, conduction begins to move from an equal distribution through the conductor cross section toward existence almost exclusively near the surface. Depending on the conductor bulk resistivity, at sufficiently high frequency all the RF current is flowing within a very small thickness at the surface. Lower bulk resistivity results in shallower electromagnetic EM skin depths in the conductor,.
For this reason, when EM skin depth is shallow, the solid conductor can be replaced with a hollow tube with no perceivable loss of performance. Choice of a plating material can degrade performance increase attenuation if its bulk resistivity is greater than that of the body of the wire. If such a conductor is placed inside the E field of an MRI RF transmit coil, there will be RF energy deposition in the tissue surrounding the wire resulting in elevated temperatures that may result in physical injury to the patient.
There also may be current flowing into the tissue at tips of the wire. The antenna may be used for powering the implanted device or for unidirectional or bidirectional communication with an external device. This container also serves as an electromagnetic interference EMI shield, protecting the contained electronics from external electrical or magnetic noise. Such noise can potentially interfere with the function of the device as it may cause corruption of the physiological data that is being gathered.
The signal levels of physiological data tends to be very small, e. Ambient electrical noise EMI field strengths in home, store, office or industrial environments can be anywhere from one volt per meter to hundreds of volts perimeter and set up induced noise levels in the body that can easily be many times larger than the signal of interest. A disadvantage of this method is that in order for a power or communication antenna to work, the antenna has to be positioned outside of that enclosure, as an internal antenna would not be able to receive or transmit effectively through the Faraday shield.
The electrical lead comprises a first coil that includes at least one first insulated conductor wound along a length of the lead. The first coil has a first inductance and a first capacitance, wherein the first inductance and the first capacitance cause the first coil to act as a first parallel resonator tuned to the Larmor frequency.
The first parallel resonator mitigates signals at the Larmor frequency that otherwise would travel along the first coil. The electrically conductive layer serves to dissipate or quench radio frequency energy that enters the electrical lead. The second coil has a second inductance and a second capacitance that cause the second coil to act as a second parallel resonator tuned to the Larmor frequency.
The second parallel resonator mitigates signals at the Larmor frequency that otherwise would travel along the second coil. The first coil and second coil may be wound in the same or opposite directions along the length of the electrical lead. As a consequence, the medical device incorporates one or more mechanisms that offer high impedance to currents induced by the MRI electromagnetic fields or prevent such currents from forming in the first place. These mechanisms comprise non- ferromagnetic components which have a magnetic susceptibility close to that of the surrounding tissue; electrical leads with traps for MRI induced currents, and a housing formed by a plurality of electrically conductive segments that combine to provide RF shielding of internal circuit while not providing large enough areas for formation of eddy currents.
As used herein, a "trap" is a circuit element that either blocks current induced by the MRI fields or significantly attenuates those currents to a level at which the current does not cause an adverse effect to the animal being scanned. One embodiment employs parallel resonant networks, such as bazooka baluns, to prevent standing waves on the shield of the cable.
The reason for putting rigid pieces in is to keep the resonance frequency of the balun constant. In a flexible structure the frequency of the balun would move around too much, especially if it is a self resonant structure. As an alternative to a balun, at least one PfN diode is placed along the cable and selectively forward and reverse biased by a DC control voltage to act as a switch. The PIN diode is rendered conductive during stimulation pulses produced by the medical device and is non-conductive at other times.
A micro electro-mechanical system MEMS is another type of switch that can be used. The DC leads also need to present high impedance at the RF frequency, which can be accomplished via chokes, or resistors, if the diode or MEMS switch uses low current. The parameters to characterize the lead's electrical characteristic include varying pitch, turn to turn distance, coaxial radial spacing, permittivity of dielectric and number of layers. Having more turns per unit of length increases inductance and capacitance. Increasing turn to turn spacing will decrease parasitic capacitance.
Adding a second coaxially wound layer creates a classic coax transmission line topology. The resultant circuit structure resembles a chained LC network, with the primary inductance being in the layers and the capacitance between the layers. In this arrangement, turn to turn capacitance will also be present. The effect of "global" capacitance rather than turn to turn capacitance is enhanced by winding the second layer opposite from the inner layer, i.
The velocity is the inverse of the square root of the product of permittivity and permeability of the tissue. Preferably the electrical length of the lead is an odd multiple of a quarter wavelength of interest for a 1. The same applies to any other frequency, although 1.
It is farther designed to be electrically open or high impedance typically the driven end of the lead at one end and almost shorted at the other end typically the stimulating end of the lead. In this context, the term "almost shorted" refers to low impedance of ohms at 64 MHz. These considerations may include avoiding loops in the lead at all times unless the distance at the crossover point between the two ends of the lead forming a loop, is larger than approximately ten lead diameters.
The result is a housing that offers high impedance for signals up to KHz and acts as a continuous shield for RF signals. Since traps are narrow band devices, they need to be tuned to the Larmor frequency of the tissue of the animal being imaged by the MRI scanner. The MRI scanner responds to that Larmor frequency. The RF shielding is due to the capacitance coupling between the electrically conductive segments. A cylindrical electrically conductive shield 16 that surrounds the cylindrical enclosure 14 and is encased in an insulating outer cover The traps impose high impedance to the common mode current induced in the cable by the E-field of an MRI radio frequency body coil.
The modified tri-axial cable 20 comprises a central, first conductor 22 surrounded by a first tubular insulator 24 of a conventional dielectric material. A tubular second conductor, or inner shield, 26 extends around the first tubular insulator 24 to form an inner shield and is in turn surrounded by a second tubular insulator 28 of the dielectric material. The resultant triaxial structure is encased in an insulating outer cover. Those sections form traps 30 for common mode current induced in the cable by an MRI scanner.
In the embodiment of Figure 3, each trap 30 comprises a bazooka balun 31 connected to the remaining cable layers, thereby forming a parallel resonant network connected to a two conductor coaxial cable. This is an RF frequency emitted by the magnetic resonance imaging scanner. As will be described, the cut sections of the outer shield 32 form networks each having an inductor connected in parallel with a capacitor, wherein the LC networks are tuned to different MRI frequencies. One end of each outer shield section is shorted by shunts 34 to the tubular second conductor 26, and the opposite section end is disconnected from the first and second conductors 22 and This forms a standard bazooka balun 31 that is attached to the remaining cable elements which function as a coaxial cable.
The second tubular insulator 28 now also serves as the outer covering of that coaxial cable. The insulating outer cover 36 encloses the tubular outer shield 32 and preferably has its ends sealed to the second tubular insulator 28 to prevent short circuits.
However, other types of baluns could be used as the traps depending on the intended location of the cable. Examples of other baluns include a cable trap balun, where the cable is looped as a solenoid, and a parallel capacitance connects the grounds before and after the solenoid, thus forming a parallel resonator with high impedance at the frequency of interest.
The bridge or lattice balun consisting of a network of two capacitors and two inductors also may be used. The medical device 40 has electronic circuitry contained in a housing 42 from which a modified tri-axial cable 44 extends. That cable 44 has a plurality of bazooka baluns 45, 46 and 47 with coaxial cable sections 48 and 49 located there between. At the remote end of the cable 44 from the housing 42, the central, first conductor 22 and the second conductor 26 are exposed to form bipolar electrodes for applying DC stimulation pulses to the tissue of the animal in which the device is implanted.
Alternatively the central, first conductor 22 and the second conductor 26 can be connected to other forms of electrodes that are adapted for placement in or against particular anatomical features of the animal. The chokes impose high impedance at radio frequencies, but low impedance to DC. This embodiment requires additional cable conductors that are decoupled by chokes 72 and consume power from the medical device to bias the PIN diodes during long time periods. However, additional cable conductors and decoupling chokes 78 still are required. The above two solutions require extra wires that now will also need to be decoupled.
In addition, the quarter wavelength transmission line is shorted at the end, and, therefore, forms high impedance at the other end.
Generally, the pitch, layer diameters and wire size are determined by electrical and mechanical design consideration to make the lead mechanically flexible and durable. Note that a dual bifilar is discussed here, but other combinations are possible as well to increase the total number of conductors.
The examples for the purpose of clarity will show a dual bifilar configuration. As shown in the schematic of Figure 9 A, this lead configuration has an air core to allow for a guide wire in one embodiment. A first conductor layer containing bifilar conductors 1 and 2 is separated from a second conductor layer containing bifilar conductors 3 and 4 by a suitable dielectric material e. The second conductor layer is covered by an electrically insulating biocompatible material e. A biocompatible material is a substance that is capable of being used in the human body without eliciting a rejection response from the surrounding body tissues, such as inflammation, infection, or an adverse immunological response.
In one embodiment, the insulating material is applied around the layers of bifilar conductors as shown in Figure 9B. The thickness of the insulating material is an important design consideration. In a preferred embodiment, the thickness of the insulating material is less than 0. This design not only improves the structural integrity of the lead but also provides ample space for an air core for allowing insertion of a guide wire. However, care should be taken in this design to prevent any body fluid from entering at the ends of the lead. It should be noted that electrical properties of the lead are dependent on the inner insulation thickness as well as the permittivity of the insulating material.
Further, it should be noted that the inductance of the lead increases with increased diameter of the helix of bifilar or multi-filar conductors. In practice, however, this diameter cannot be arbitrarily varied since it is fixed due to the restriction imposed on the dimensions of an intravascular lead structure. In any case, inner layer of conductors may be used for delivering stimulation in one embodiment while the outer layer of conductors may be used for transmitting back physiological parameters and other relevant data.
This design requires more insulating material and is mechanically less robust. However, this design completely insulates the conducting wire from coming in contact with body fluids. As shown in Figure 10, the control electronic circuit module has bandstop i. The length of lead 1 and Iead2 on opposite sides of the control electronic circuitry may be unequal in physical length but they need to be an odd multiple of quarter wavelength corresponding to 90, , The actual model is far more complex due to various coupling paths inductive and capacitive.
However, this design may provide a better level of control over the parameters of interest. First, a conventional model is derived to determine locations and values of the resonant circuits along the lead. Second, components are created from lead topologies to create equivalent values. In an alternative method, one can create a plurality of physical models based on various parameters mentioned and measure the output to derive appropriate resonant circuit using well known statistical techniques.
As another example, if conductive stents are needed they can be rendered MRI compatible using a layer of semiconducting material during the stent forming process. Such a semiconducting material may be formed using various methods described below.
Additional biocompatible layer may be formed as in a traditional implantable electrical lead. Additionally, an additional biocompatible layer is formed as in a traditional implantable electrical lead. The thickness of the layer may be adjusted based on the frequency of MRI application and may be based at least in part on the skin depth of the semiconducting material. It is also desirable that the RF energy dissipation material does not break off during chronic use and cause bio-hazard or other complications.
In the following, a number of suitable candidate materials are described. For this material to work, the carbon particles embedded in the polymer need to be touching each other to create a conductive layer. The density of the carbon particles may have to be adjusted to achieve the desired objective, Alternatively, graphite embedded in a rubber compound may be sprayed on to the conductors for a desired thickness. Alloys that have conductivity properties similar to that of graphite can be used as well. Semiconducting materials such as germanium can be used as a coated layer around the conductor.
Braiding carbon is a well understood process and produced in industrial quantities by several companies including SPT Technology, Inc. Their relatively large conductivity, light weight and flexibility are just some of the factors that make conducting polymers much more desirable than metals in certain applications.
Of the various conducting polymers studied, polyaniline PANi has been investigated the most due to its ease of synthesis, relatively high conductivity and good stability. See for example, Pure and Applied Chemistry Vol. Thus, a key aspect of the invention is achieving simultaneous electrical, mechanical and biological compatibility.
One approach to achieving this goal is that the lead circuit is self-resonant wherein an inductance is formed in parallel with the lead's parasitic capacitance.
The inductance is defined by the expression:. The lead's inductance and parasitic capacitance cause the lead to act as a parallel resonator, at the MRI resonant frequency. A parallel resonator has a high impedance across it. R, where Q is the quality of the resonance, and R is the series resistance in the parallel resonator.
Bleaney and B. When stretching a coil, both capacitance and the inductance reduce, but the capacitance reduces faster than the inductance in a region where the pitch of the windings is such that the windings are tightly packed due to the log approaching zero.
The self resonance frequency is inversely proportional to the square root of inductance times capacitance, so at some pitch of the conductor coil, the self resonance frequency crosses the Larmor frequency of the MRI system. The lead can be constructed to provide local parallel resonance presenting very high impedance at the resonant frequency inhibiting current patterns to be established on the lead when exposed to an excitation field,. This method is useful for a higher MRI field strength, such as 3.
A second approach involves placing RF blocking networks in the lead at least quarter wavelength locations as described earlier. Note that for 3. Thus, to address the issue for these two field strengths, the networks would be placed at greater or equal to the quarter wavelength of the highest frequency approximately MHz encountered.
For the lower 1. However, this redundancy does not adversely affect the blocking function. A third approach involves using a combination of the first and the second approaches. A fourth approach involves reducing the ability of the lead to be an antenna, i. If the lead could be presented to the surrounding field as a low quality antenna, the amount of energy absorbed would be reduced.
This quality reduction can be accomplished by adding damping to the lead, or a way to dissipate the absorbed energy in such a way that no focal spots in the E field or high voltage points will exist. Since focal spots in the E-field can be created by concentration of E-field, such as at tips or ends of wires or components, any sharp edge or point is avoided. For non-sharp objects such as lead conductors, further energy damping can be provided by coating the lead conductor with a medium grade conductor to provide sufficient resistivity to dissipate energy. Second, the inventive approach uses a layer around the lead that absorbs the energy associated with any remnants of the induced standing wave.
The current approach is quite different from the methods that conduct energy of the standing wave to the surrounding tissue. This approach is further described with the following three configurations. The insulated, coiled conducting wires are coated by a tubular damping layer of medium conductive material mentioned before. That entire structure is covered by a biologically compatible electrical insulating layer The distal part of the lead, that is less than one eighth of the wavelength, need not be coated with a medium conductive damping layer The conductive damping layer plays a role in dissipating some of the energy from the MRI fields evenly into the surrounding animal tissue over the entire length of the lead, such that there are no hot spots.
Because the outer biocompatible layer serves as a capacitance to the surrounding tissue, that layer should be as thin as possible to maximize that energy dissipation. One such terminus is shown in the Figure Note that if there are multiple coils, each of them will have a terminus similar to Further, note that in this configuration, all the coiled conductors are in one layer.
Different variations of this fundamental lead configuration provide the following multiple implementations. A short section e. For example, different conductor pitches can be used near the signal generator e. This choice depends on a trade-off between desired mechanical flexibility and degree of lead damping required. Mechanical flexibility is generally higher if the pitch is lower. RF damping of the lead, on the other hand, is higher with larger pitch. There are other effects that need consideration while increasing the pitch.
Protecting Implantable Medical Devices From Electromagnetic Interference
One effect involves the lowering of the self-resonance of the effective LC circuit formed by the lead with larger pitch due to the inverse relationship between the LC-product and square of the self-resonance frequency. Therefore, changing the lead pitch also causes the MRI frequency that will be blocked to change. Since MRI compatibility, site stimulation, data sensing, lead longevity and mechanical flexibility are all requirements that must be simultaneously met, a combination approach may provide optimal lead configuration in the implementation of an MRI safe lead.
An example of this type could be a bifilar configuration in which there are two coiled conductive wires with each layer. The outer and inner conductor layers are separated by a spacer layer of an dielectric material. In the above-mentioned bifilar configuration, two termini and from the outer layer and two termini and from the inner layer are available for connecting to the electrode elements not shown. It should be noted that the multi-layer, multi-wire lead configuration can approximate a single layer, multi-wire configuration described previously if the spacing between the inner and outer layers of conductors and is less than an empirically determined thickness.
For example, in one embodiment, if the spacing is less than 0. It should be further noted that the spacing might be due to an insulating material of a particular design choice. Those insulating materials are previously described. Combined the coiled inner and outer conductors form a central structure of the lead. The insulated, coiled outer coil is coated by a surrounding layer of a medium conductive material as mentioned before, which layer covered by a biologically compatible and electrically insulating layer The distal part of the lead, that is less than one-eighth of the wavelength of the MRI signal, need not be coated with the conductive layer Each conductive wire in the outer layer of conductors has a terminus or at which an electrode not shown usually is located to contact the tissue to be stimulated.
Similarly each conductive wire in the inner layer of conductors has a terminus or In particular the outer layer of conductors is encased in the layer of conductive material. In this example, it should be noted that both inner and outer layers have multiple insulated conductors wound on each layer. The number of insulated conductors for these two layers may be the same or they may be different.
In one embodiment, Figure 16A shows three insulated conductors A, B and C are wound on the outer layer and two insulated conductors A and B are wound on the inner layer. An optional spacer layer not shown may be present between inner and outer layers. The winding directions of insulated outer conductors may be in the same direction of the insulated inner conductors as shown in Figure 16A or they may be in different direction as shown in Figure 16B. In some embodiments, the conductive layer and the biocompatible external layer may be separate layers as shown in Figure 16 A.
In an alternative embodiment, these layers may be combined into one layer. This would be the case when the medium conductive material also happens to be biologically compatible. In any event, the external layer is in contact with a body tissue or body fluids. The inside layer is designed to be adjustable so that the distances between electrodes and connected to the coiled wire termini can be adjusted during the implantation. Thus the extendable lead allows for deployment of electrodes at locations with variable distance as shown by the extension of original space A in Figure 17A to extended space B in Figure 17B.
The lead is composed of an outer layer and inner layer of conductors , each having one or more coiled insulated conductors. The inner layer contains a section which can be extended or contracted by means of an inner guide wire or sheath not shown that can be brought in through the central space Since locations of stimulation or sensing in an organ, for example, heart, is not accurately predictable and varies amongst individuals, usually multiple leads are inserted to reach two locations for sensing and or stimulation.
This configuration allows a single lead to fulfill this need, obviating the need for multiple leads and simplifying the procedure. When the inner lead is extended, it does change the overall length of the lead structure, but without changing the length at the other, non- extendable end. In one embodiment shown in Figures 17A and 17B, when the extension range is less than 7. In this embodiment, a medium conducting layer and a biocompatible insulating layer are also added over the conductors on the extended part. The insulated, coiled outer conducting wires are coated by a layer of medium conductive material mentioned before, which layer is biologically compatible and electrically insulating.
The distal part of the lead less than one eighth of the wavelength need not be coated with a medium conductive layer The conductive wires in the lead usually terminate at a bare, non-insulated terminus and an electrode or that are in contact with the tissue to be stimulated. The terminus is a bare, non-insulated portion of a conductor in the outer layer of conductors , which is adapted to be exposed to body tissue or body fluids upon implantation in an animal.
The length of the terminus is determined by a wavelength which is a function of the velocity v of the electromagnetic wave in the animal tissue divided by the frequency of the electromagnetic wave. In an exemplary embodiment where there are two coils in the inner conductive wires and two coils in the outer conductive wires, each of those conductive wires has a separate terminus and electrode. The winding directions of insulated outer conductors may be in the same direction of the insulated inner conductors or they may be in different direction.
Additionally, the transmission line characteristic impedance CI is also an important design parameter. This is the virtual impedance of any wire pair. For example, television coaxial cables have a typical characteristic impedance of 50, 60 or 75 Ohm; for phone lines, the CI is Ohm; and for ribbon cable with 1. In the present case, there may be two transmission lines, inner and outer, each with their own CI in the range of 10 to Ohm.
It does not contain any materials that have a magnetic moment, such as soft iron, nickel or cobalt, as their presence would cause image artifacts. If the unmodified section is less than any quarter wavelength in body fluid of the MRI scanner, there will not be any image quality IQ issues. In general the IQ issues increase with field strength, with most issues anticipated at the common 1.
In these cases, a 5 cm to 7 cm segment may be left partially unmodified, without causing significant adverse IQ affects. It should be noted that a pacing lead can also be left unmodified over the last 5 cm to 7 cm segment without causing adverse MR IQ effects in 1. The insulated, coiled conducting wires are coated by a layer of medium conductive material mentioned before.
Insight: implantable medical devices - Lab on a Chip (RSC Publishing)
The entire structure is covered by a biologically compatible insulating layer In an exemplary case of two electrode defibrillator, the insulated conductor comes out of the lead body without insulation and is wound on the lead body without touching each winding of the coil as electrodes and Similar series resonant circuits may be provided for other scanners as well.
The resonant circuits are housed in the ICD container The end termini are connected to the pacing electrodes not shown. If the inner insulated conductor for pacing is more than one-eighth of a wavelength of the MRI scanner in contact with the body fluid or tissue for pacing, then the medium conducting coating covers the surface of the inner conductor followed by an outer insulating layer.
The inside tube is present through out the lead and is terminated with an anchoring component which helps in the anchoring of the lead. The anchoring component is made up of an MRI compatible material described earlier. Second, the fatigue resistance is essential for many applications, for example, in a cardiac apex application, the lead end would flex with each heart beat. They are used as a follow-up to mastectomy due to breast cancer , for correcting some forms of disfigurement , and modifying aspects of the body as in buttock augmentation and chin augmentation.
Examples include the breast implant , nose prosthesis , ocular prosthesis , and injectable filler. Other types of organ dysfunction can occur in the systems of the body, including the gastrointestinal , respiratory , and urological systems. Implants are used in those and other locations to treat conditions such as gastroesophageal reflux disease , gastroparesis , respiratory failure , sleep apnea , urinary and fecal incontinence , and erectile dysfunction. Medical devices are classified by the US Food and Drug Administration FDA under three different classes depending on the risks the medical device may impose on the user.
According to 21CFR Class I devices include simple devices such as arm slings and hand-held surgical instruments. Class II devices are considered to need more regulation than Class I devices and are required to undergo specific requirements before FDA approval. Class II devices include X-ray systems and physiological monitors. Class III devices require the most regulatory controls since the device supports or sustains human life or may not be well tested.
Class III devices include replacement heart valves and implanted cerebellar stimulators. A variety of minimally bioreactive metals are implanted. The most commonly implanted form of stainless steel is L, and for long-term implants, cobalt-chromium and titanium-based implant alloys are also used. All of these are made passive by a thin layer of oxide on their surface. Stainless steel remains subject to corrosion, and is therefore only used for temporary implants, while the titanium and cobalt-chrome alloys can be implanted indefinitely.
A consideration, however, is that metal ions do diffuse outward through the oxide, and end up in the surrounding tissue. Bioreaction to metal implants includes the formation of a small envelope of fibrous tissue. The thickness of this layer is determined by the products being dissolved, and the extent to which the implant moves around within the enclosing tissue.
Pure titanium, a preferred implant material, may have only a minimal fibrous encapsulation. Stainless steel, on the other hand, may elicit encapsulation of as much as 2 mm. Under ideal conditions, implants should initiate the desired host response. Ideally, the implant should not cause any undesired reaction from neighboring or distant tissues. However, the interaction between the implant and the tissue surrounding the implant can lead to complications. Common complications include infection , inflammation , and pain.
Other complications that can occur include risk of rejection from implant-induced coagulation and allergic foreign body response. Depending on the type of implant, the complications may vary. When the site of an implant becomes infected during or after surgery, the surrounding tissue becomes infected by microorganisms. Three main categories of infection can occur after operation. Superficial immediate infections are caused by organisms that commonly grow near or on skin.
The infection usually occurs at the surgical opening. Deep immediate infection, the second type, occurs immediately after surgery at the site of the implant. Skin-dwelling and airborne bacteria cause deep immediate infection. These bacteria enter the body by attaching to the implant's surface prior to implantation. Though not common, deep immediate infections can also occur from dormant bacteria from previous infections of the tissue at the implantation site that have been activated from being disturbed during the surgery.
The last type, late infection, occurs months to years after the implantation of the implant. Late infections are caused by dormant blood-borne bacteria attached to the implant prior to implantation. The blood-borne bacteria colonize on the implant and eventually get released from it. Depending on the type of material used to make the implant, it may be infused with antibiotics to lower the risk of infections during surgery.
However, only certain types of materials can be infused with antibiotics, the use of antibiotic-infused implants runs the risk of rejection by the patient since the patient may develop a sensitivity to the antibiotic, and the antibiotic may not work on the bacteria.
Inflammation, a common occurrence after any surgical procedure, is the body's response to tissue damage as a result of trauma, infection, intrusion of foreign materials, or local cell death , or as a part of an immune response. Inflammation starts with the rapid dilation of local capillaries to supply the local tissue with blood. The inflow of blood causes the tissue to become swollen and may cause cell death.
The excess blood, or edema, can activate pain receptors at the tissue. The site of the inflammation becomes warm from local disturbances of fluid flow and the increased cellular activity to repair the tissue or remove debris from the site. Implant-induced coagulation is similar to the coagulation process done within the body to prevent blood loss from damaged blood vessels. However, the coagulation process is triggered from proteins that become attached to the implant surface and lose their shapes.
When this occurs, the protein changes conformation and different activation sites become exposed, which may trigger an immune system response where the body attempts to attack the implant to remove the foreign material. The trigger of the immune system response can be accompanied by inflammation. The immune system response may lead to chronic inflammation where the implant is rejected and has to be removed from the body.
The immune system may encapsulate the implant as an attempt to remove the foreign material from the site of the tissue by encapsulating the implant in fibrinogen and platelets. The encapsulation of the implant can lead to further complications, since the thick layers of fibrous encapsulation may prevent the implant from performing the desired functions. Bacteria may attack the fibrous encapsulation and become embedded into the fibers. Since the layers of fibers are thick, antibiotics may not be able to reach the bacteria and the bacteria may grow and infect the surrounding tissue.
In order to remove the bacteria, the implant would have to be removed. Lastly, the immune system may accept the presence of the implant and repair and remodel the surrounding tissue. Similar responses occur when the body initiates an allergic foreign body response. In the case of an allergic foreign body response, the implant would have to be removed.
The many examples of implant failure include rupture of silicone breast implants , hip replacement joints, and artificial heart valves , such as the Bjork—Shiley valve , all of which have caused FDA intervention. The consequences of implant failure depend on the nature of the implant and its position in the body.
Thus, heart valve failure is likely to threaten the life of the individual, while breast implant or hip joint failure is less likely to be life-threatening. Devices implanted directly in the grey matter of the brain produce the highest quality signals, but are prone to scar-tissue build-up, causing the signal to become weaker, or even non-existent, as the body reacts to a foreign object in the brain. In , Implant files , an investigation made by ICIJ revealed that medical devices that are unsafe and have not been adequately tested were implanted in patients' bodies.