What is a catheter for hysterography

Friction of guide wires and catheter tubes

By Annabelle Schofer, Markus Niemann and Volker Bucher, Furtwangen University, Mechanical and Medical Engineering

Catheters and guidewires are used in various medical fields for both diagnostic and therapeutic purposes. Due to the many different possible applications, there is a very large number of variants in terms of shape and structure. The most important property of a medical device that must be present in principle is biocompatibility. Since catheters and guidewires are medical products whose area of ​​application lies within the body, catheters and guidewires also have to meet numerous application-specific requirements. This work is limited to the aspect of the friction that occurs during insertion and removal. If a catheter or guide wire is inserted into a hollow organ, friction occurs when the instrument and tissue come into contact. Too much friction essentially leads to two problems. On the one hand, the feeling for the exact movement of the instrument in the body is lost, which makes the treatment more difficult and injuries can occur, especially when treating in narrow vessels. On the other hand, too much friction is painful for the patient and can lead to additional damage to health, especially with longer catheterizations. To reduce friction, special coatings are applied to the catheters and guide wires. In most cases, hydrophobic coatings are used for the guide wires, whereas hydrophilic coatings are usually used for the catheters. Both types of coating achieve a reduction in friction despite different modes of operation, with a somewhat greater reduction being possible through hydrophilic coatings. Each of the two types of coating is ideally suited for a different area of ​​application, so that both types have their right to exist. In general, there are numerous different hydrophilic and hydrophobic coatings that are the same in their basic function, but differ in their special properties. The selection of the right coating should therefore always be made for the specific application.

Research on friction of guide wires and catheters - Part 1

Catheters and guide wires are used for diagnosis and therapy in different medical fields. Because of the different areas of application, there is a wide variety concerning the shape and structure of the device. The most important property of a medical device is the biocompatibility. Each medical device must be biocompatible. Because they are used inside the body, catheters and guide wires have to meet a lot of additional application-specific requirements. This paper only deals with the friction that is noticeable while the device is inserted or withdrew. If this friction is too high, there are two main problems. The first thing is that the treating person loses the exact feeling for the movement of the device. Especially by using in small vessels these difficult conditions could cause injuries. Secondly, a high friction is uncomfortable and painful for the patient and in the case that the catheter stays inside the body for a longer time, the friction can cause additional health impairments. To reduce the friction, special coatings containing polymers are applied on the device surface. Coatings for guide wires are mostly hydrophobic, coatings for catheters are mostly hydrophilic. Although the operating principle of a hydrophobic coating is different from a hydrophilic coating, both of them reduce the friction, but the reduction reached with a hydrophilic coating is a little bit higher. Because each of them shows optimal results in another application field, both coatings have the right for existence. In general, there are a lot of different possibilities to realize hydrophobic as well as hydrophilic coatings. The fundamental operating principle for friction reducing is equal, but they are different concerning the special properties. The selection of the most suitable coatings should always occur application-specific.

1.1 Objective

The aim of the present work is a detailed research on guide wires and catheter tubes with a focus on the surface treatment to reduce the friction that occurs during insertion and removal. First, a general overview of catheters and guide wires should be given. This includes the question of why and how catheters and guide wires are actually used and what materials and coatings they are made of. Furthermore, the exact origin of this friction is explained in more detail and in particular the currently existing or researched concepts for friction reduction are presented.

1.2 Motivation

At the beginning of the considerations, the question arises why it is necessary at all to process the surface of guide wires and catheters and to reduce the friction.

Every catheterization is a medical procedure in which a foreign body is introduced into the human organism. Such a measure always offers a certain risk of injury and complications. Too much friction damages the surrounding tissue. Especially in the urological area, where catheters often remain in the body for a long time, this can lead to considerable impairment and discomfort for the patient. At the same time, however, it also makes it more difficult to insert the catheter in a controlled and careful manner.

With many catheterizations it is very important to go to certain positions exactly in order to be able to carry out the desired treatment. During a cardiac catheter examination, for example, the catheters are inserted into very thin, fine coronary vessels. With such small vessels, even minimal deviations from the desired location can lead to treatment failure. If the friction is too great, it may only allow jerky movements of the catheter, so that an exact control and ultimately also hitting the desired point is almost impossible. However, due to insufficient friction, slipping occurs, as a result of which the user of the catheter no longer has a proper feeling for the control of the catheter.

In many medical fields, catheterization is now a standard procedure that is very frequently used. In order to be able to carry out these interventions as accurately as possible and at the same time to keep the patient's discomfort and impairment as low as possible, it is necessary to process the surfaces of the catheters and guide wires in such a way that friction is reduced while at the same time controllability is maintained.

2 basics

For a better understanding of the challenges that arise in the context of the friction of a catheter or guide wire and methods for their avoidance, the necessary basics are first taught. Friction always requires two components. In this case these are the catheter or guidewire and the patient's body. In order to properly analyze the friction that occurs during catheterization, it is important to know the structure, nature and properties of both components. As part of some general basics about catheters and guidewires, it is helpful to examine the anatomy of the areas of the body where catheterization will be performed.

2.1 anatomy

The macroscopic anatomy, i.e. the structure of the organism that is visible to the naked eye, is important in order to understand the application and the paths that a catheter has to cover in the body. The microscopic anatomy plays a role in the interaction between the instrument and the tissue.

2.1.1 Macroscopic anatomy - kidney, urinary tract, blood vessels

As shown in Figure 1, the urinary tract includes the kidneys, ureters, bladder and urethra. The main function of the two kidneys is to produce urine. Figure 2 shows the structure of a kidney. It consists of several kidney lobes, each of which consists of the renal medulla (inner part) and kidney cortex (outer part). The renal medulla has a pyramidal shape. The tips of the pyramids each lead into a kidney calyx. The kidney calyxes pass into the renal pelvis, where the urine is collected. The renal pelvis tapers and merges at the renal hilus into the ureter, which is the connection between the kidney and the urinary bladder.

Fig. 1: Human urinary system [73]

 

Fig. 2: Structure of the kidney [2]

 

The two ureters are about 25 cm to 30 cm long hollow muscular organs and have a diameter of 4 mm to 7 mm. The urinary bladder is a hollow organ that stores urine and is divided into the bladder tip, bladder body (largest part of the bladder), bladder base (rear lower part of the bladder) and bladder neck. At the side, one of the two ureters opens into the urinary bladder. The neck of the bladder is tapered and merges into the urethra. The urethra is the last part of the lower urinary tract. The female urethra is about 2.5 cm to 4 cm long, the male has a length of about 17 cm to 20 cm.

Arteries are blood vessels that lead away from the heart. The central artery of the human body is the aorta. Veins are blood vessels that lead to the heart. The arteries branch out into smaller and smaller branches. The smallest branches are the capillaries. This is where the exchange of substances takes place. The capillaries are the transition from the arteries to the veins.

The human heart is divided into a left and a right chamber as well as a left and a right atrium (Fig. 3). The atrium and ventricle are each separated by a heart valve (leaflet valves); the vertical division into two halves is made by a septum. The aorta branches off the left ventricle and is separated from the heart by the aortic valve. The pulmonary artery branches off from the right ventricle and is separated from the heart by the pulmonary valve. The four pulmonary veins open into the left atrium, the upper and lower vena cava flow into the right atrium.

Fig. 3: Structure of the heart [74]

 

The great blood circulation, also called the body circulation, begins in the left ventricle of the heart. The oxygen-rich blood is pumped into the aorta by contraction of the heart muscle and distributed into the capillaries of the entire body via the branching arteries. There is an exchange of substances. The deoxygenated blood flows back into the right atrium via the superior and inferior vena cava.

The small blood circulation, also called the pulmonary circulation, starts in the right ventricle and connects to the large circulation. The oxygen-poor blood is pumped into the pulmonary capillaries via the pulmonary artery, where it is again enriched with oxygen. The blood, which is now oxygen-rich again, is passed through the pulmonary veins into the left atrium.

The heart is supplied with blood via the two coronary arteries shown in Figure 4. These are two vessels that arise from the aorta, surround the heart like a wreath and from which many small branches branch off. The arteria coronaria dextra (right coronary artery) arises from the sinus aortae dexter. The main trunk of the left coronary artery arises from the sinus aortae sinistra directly above the aortic valve. Immediately afterwards it divides into two branches. The anterior interventricular branch runs on the front of the heart caudally to the apex of the heart. The circumflex branch runs dorsally.

Fig. 4: Coronary arteries with vascular occlusion and infarct area [2]

 

If atherosclerotic changes in the coronary vessels occur, this leads to so-called coronary stenoses (narrowing of the coronary vessels). The presence of such narrowing of the coronary arteries is called coronary artery disease (CHD). Circulatory disorders of the heart muscle occur, which can manifest clinically as angina pectoris or myocardial infarction.

2.1.2 Microscopic anatomy - endothelium, vascular structures

The vessel wall of arteries and veins consists of three layers: tunica intima, tunica media, tunica externa (Fig. 5). The tunica externa is a layer of connective tissue and anchors the blood vessels in their surroundings, whereas the tunica media is a layer of muscles. The tunica intima includes the endothelium, the subendothelial stratum, a subendothelial layer of connective tissue, and the internal elastic membrane. The endothelium lines the inside of the vessels, i.e. the part of the vessel that comes into contact with catheters and guide wires. It is a single-layer squamous epithelium (Fig. 6), i.e. a thin layer of flat epithelial cells. The individual endothelial cells are attached to one another via tight junctions. The endothelial cells sit on a basement membrane (stratum subendotheliale). On the apical side there is a glycocalyx that prevents blood cells from attaching.

Fig. 5: Structure of the vessel wall of arteries and veins [2]

Fig. 6: Single-layer squamous epithelium [67]

Fig. 7: Structure of the ureter and urethra wall [2]

 

The ureter and urethra have the typical three-layer structure of a hollow muscular organ (Fig. 7). The tunica adventitia is the outermost layer and anchors the hollow organ in its environment. The tunica muscularis is a muscle layer consisting of a longitudinal and a circular muscle layer. The tunica mucosa is the innermost layer and consists of an epithelium and the lamina propria below. The tela submucosa is a layer of loose connective tissue between the tunica mucosa and the tunica muscularis.

In the ureter and in the initial area of ​​the urethra, the epithelium is the so-called urothelium. This is also found in the renal pelvis and in the urinary bladder. The urothelium is a transitional epithelium (Fig. 8). The transitional epithelium is a multilayered epithelium, which varies in height depending on the filling state of the hollow organ. It consists of the basal cells that lie on the basement membrane, the intermediate cells and the cover cells, which cover the epithelium as the top layer. The ureter and urethra have a star-shaped lumen when the muscles are contracted (Fig. 9).

Fig. 8: Urothelium [67]

Fig. 9: Ureter with star-shaped lumen [67]

 

Blood consists of a cellular part (about 45%) and the liquid blood plasma (about 55%). The main component of the cellular part are the erythrocytes, the red blood cells. The cellular portion also includes leukocytes (white blood cells) and thrombocytes (blood platelets), but these occur in significantly lower numbers. The platelets are involved in blood clotting and thus contribute to the formation of a blood clot. The cellular proportion of the total volume is known as the hematocrit value. Plasma is an aqueous electrolyte solution that contains numerous proteins, including coagulation factors.

Urine consists essentially of water, urea, uric acid and electrolytes. The electrolytes are essentially salts. In contrast to blood, urine contains no cellular components and no coagulation factors.

2.2 Catheter tubes

Basically, a catheter is understood to be a rigid or flexible tube that can be inserted into hollow organs and body cavities for both diagnostic and therapeutic purposes. Through the lumen, the inner cavity of the catheter, liquids and instruments can be brought to a specific body part to be treated or examined, or body fluids can be drained out of the body. Catheters are used in very many different medical fields and therefore exist in a very large number of variants. The most important areas of application of catheters, their exact structure and the materials used are shown below.

2.2.1 Applications

Basically, a distinction is made between two types of application: Single-use catheters are inserted into the body for a one-off examination or treatment and disposed of immediately after removal from the body, whereas indwelling catheters are intended to remain in the patient's body for a longer period of time.

Urology is an essential medical field in which catheters are used. In the therapeutic area, urological catheters are mainly used to drain urine from the body, but are also used for diagnostic purposes. There are two fundamentally different types of urological catheters.

The so-called ureter rails (urether catheters) are inserted directly into the renal pelvis. Inner ureter rails are located with one end in the renal pelvis and the other end in the urinary bladder and thus direct the urine from the renal pelvis into the urinary bladder (Fig. 10). Outer ureter rails also have one end in the renal pelvis, but either transurethrally or transcutaneously direct the urine out of the body into a urine bag. Inner ureteral splints are more suitable for longer-term catheterization.

Fig. 10: Inner ureteral stent [75]

 

Urether catheters are used therapeutically for urinary obstruction in the kidney or ureter area. In diagnostics, they are used for retrograde pyelography, the pictorial representation of the kidney.

The second group is made up of urinary catheters, which drain urine from the urinary bladder and are also used in therapy and diagnostics. Transurethral catheters are inserted into the bladder through the urethra (Fig. 11). Suprapubic urinary catheters are inserted into the bladder through the abdominal wall (Fig. 12). Suprapubic catheters are used almost exclusively for permanent catheterization. Transurethral catheters are used for both permanent and one-time catheterizations.

Fig. 11: Transurethral urinary catheter [76]

Fig. 12: Suprapubic urinary catheter [77]

 

In therapy, urinary catheters are most often used for voiding disorders. The cause of a bladder emptying disorder can lie in the area of ​​the bladder, the urethra, but also in the nervous system. Also belonging to the therapeutic area is the drainage of urine in bedridden patients, during longer operations and the temporary drainage after injuries.

In the diagnostic area, transurethral urinary catheters are used to monitor kidney function. With the help of the catheter, urine is taken from the patient over a certain period of time and examined for quantity and concentration. Further diagnostic purposes are the creation of a cystogram, the measurement of the bladder pressure, the determination of the urethral width and the monitoring of residual urine. A cystogram is an x-ray of the urinary bladder that is used to examine the location, size, and shape of the urinary bladder. A contrast agent is injected into the urinary bladder via the catheter so that it is visible on the X-ray image.

Another important application of catheters is cardiac catheterization in cardiology. The cardiac catheter is advanced through an artery or vein under X-ray control to the desired location in the heart or in the coronary vessels. This procedure is mainly used for examining and diagnosing abnormal changes in the heart. However, the modern catheters can be used to treat the disease at the same time.

A basic distinction is made between right and left heart catheter examinations. With right heart catheterization, the catheter is inserted into a vein in the area of ​​the elbow or groin and then carried with the bloodstream into the right atrium, the right ventricle and finally into the pulmonary arteries (Fig. 13). The respective pressure is measured in each section. In addition, blood can be taken from the catheter in order to determine the oxygen content and to measure the cardiac output. With the help of this examination, heart valve defects, circulatory disorders of the heart muscle as well as atrial and ventricular septal defects can be diagnosed.

Fig. 13: Insertion of a right heart catheter [78]

Fig. 14: Insertion of a left heart catheter [79]

 

The left heart catheter examination enables the examination of the left heart chamber and the measurement of the prevailing pressure. The catheter is inserted into an artery in the area of ​​the groin and advanced against the bloodstream via the main artery into the left heart chamber (Fig. 14).

The most common application of the left heart catheter examination is coronary angiography, the pictorial representation of the coronary vessels to detect constrictions of these vessels. The presence of such constrictions is known as coronary artery disease (CHD). Such a disease can be treated with the help of special catheters directly after the cardiac catheter examination. As can be seen in Figure 15, the catheter is advanced over the main artery to the junction of the coronary vessels. A contrast agent is injected into the coronary arteries through this catheter so that they can be seen on an X-ray.

Fig. 15: Insertion of a guide wire into a coronary vessel [2]

 

If a pathological narrowing is diagnosed, it can be treated with a balloon catheter immediately afterwards. This catheter, which has a small, inflatable balloon at its tip, is then advanced to the narrowed point in the coronary arteries. Inflating the balloon will widen the constriction. If this measure is not sufficient to keep the vessel open for a long time, a stent is threaded over the balloon, which anchors itself in the vessel wall after the balloon is inflated and thus keeps the vessel open for a longer period of time.

For many treatments, but especially in emergency medicine, it is necessary to administer medication or infusions intravenously, that is, to introduce them into the bloodstream through a vein. Venous catheters (central and peripheral) are used for this.

Peripheral venous catheters, also known as indwelling venous cannulas, are very small and short catheters that are inserted into a peripheral vein in the area of ​​the elbow, the back of the hand or the forearm. Peripheral venous catheters can remain in the vein for a longer period of time and are therefore used for the multiple administration of medication or infusions and are mostly used during hospital stays.

Central venous catheters, on the other hand, are inserted into a larger body vein and are intended for long-term use. They are used in patients who have to receive regular venous medication or infusions over a much longer period of time in order to avoid constant re-puncture of the peripheral veins. A special type of central venous catheter is the port catheter. This is firmly implanted in the area of ​​the rib cage under the collarbone in an outpatient operation (Fig. 16).

Fig. 16: Position of a port catheter [80]

 

2.2.2 Form and structure

Catheters for the numerous different possible applications in different areas of the body have different anatomical structures. Thus, a special shape is necessary for each catheter according to its application, which makes a general description difficult.

Basically, catheters consist of a mostly flexible, thin tube that has a cavity inside. The outside diameter of this hose depends on the application. The size of this diameter is given in the unit Charriere (Ch.) (1 Ch. = 0.33 mm). The inner lumen of a catheter is called a lumen and has two basic types. The monolumen catheter (Fig. 17) is the simplest type and has only a single lumen, i.e. only one cavity inside. With the multi-lumen catheter (Fig. 18), several channels run parallel inside the catheter tube. This means that different liquids or instruments can be pushed into the same place at the same time.

Fig. 17: Monolumen central venous catheter [81]

Fig. 18: Multi-lumen catheter [82]

 

The end of the catheter that remains outside the body is usually open and has a type of coupling piece so that syringes for introducing fluids or other instruments can be connected to the catheter.

The front part of the catheter, the so-called tip, is advanced to the appropriate part of the body. Different types of tips are available for the different areas of application (Fig. 19). A distinction is made between catheters with straight and curved tips, as well as the so-called pigtail catheters. With this design, one or both ends are slightly rolled up in a spiral to prevent slipping.

Fig. 19: Catheter with straight and curved tip, pigtail catheter [83]

 

A special type of catheter often used in various fields of application is the balloon catheter. On the tip of the catheter there is a small balloon that is inflated through its own channel in the catheter and can perform various tasks.

The transurethral single-use catheters (Fig. 20) belonging to the urethral catheters are usually monolumen catheters. They are only used for the one-time drainage of the urine, so that one lumen is sufficient for the outflowing urine. Transurethral disposable catheters are available in different lengths: 30 cm to 45 cm for men, 15 cm to 30 cm for women and 20 cm to 30 cm for children and adolescents. At the distal end there is a connector to which a urine bag can be connected if necessary.

Indwelling transurethral catheters (Fig. 21) are usually balloon catheters. Once it has entered the bladder, the balloon is inflated to prevent the catheter from slipping. These are available as 2-way or 3-way catheters, so-called flushing catheters. The 2-way catheters have a lumen for draining the urine and a lumen for inflating the balloon. At the distal end there is a connector for connecting the urine bag and a valve for inflating the balloon. The 3-way catheter has an additional lumen that is used to introduce irrigation solutions.

Suprapubic indwelling catheters are available as 1-way catheters that are fixed to the abdominal wall with a suture and are more suitable for short-term use around an operation. For longer missions, 2-way balloon catheters or pigtail catheters are used, which do not need to be attached with a suture.

Each of the catheters just described is available with different tips. The tip shapes most frequently used in urinary catheters are shown in Figure 22.

So-called double-J catheters are used for the inner ureteral rails, which have a pigtail tip at both ends, i.e. both in the renal pelvis and in the urinary bladder. In adults, they are about 26 cm to 34 cm long with diameters between 6 Ch and 8 Ch. Some models have a retrieval thread at the distal end. So-called MJ catheters, which can only be rolled up at the end that is in the renal pelvis, are used for outer ureteral splints. To prevent them from slipping out, they are either sutured or attached to a urethral catheter for transurethral use.

The basic model of the cardiac catheter for right heart catheter examination is the Swan-Ganz catheter, which was developed in 1970 by Wiliam Ganz and Harold Jeremy Swan. Modified, further developed models of this catheter are now available. If the cardiac output is to be measured during the examination and this is done using the thermodilution method, a four-lumen balloon catheter is usually used (Figs. 23 and 24). A lumen is used to inflate the balloon to prevent the catheter from slipping out. At the distal end there is a thermistor at a distance of 4 cm, which has an additional lumen with its own connection. The catheter has a distal and a proximal outlet opening about 30 cm from the distal end, each with its own connection. The distal opening is used to register and extract pressure, the proximal opening is used to inject an indicator solution necessary for thermodilution. With the help of the thermistor and the indicator solution, a temperature dilution curve can be recorded and the cardiac output can be determined.

If the cardiac output is determined using another method (Fick), a two-lumen balloon catheter is sufficient. This has only one distal opening for taking blood and measuring the pressure.

A peripheral venous catheter (Fig. 25) consists of a steel cannula, also called a stylet, and a catheter surrounding this cannula. The cannula can be moved within the catheter. After the vein has been punctured by the steel cannula, it is withdrawn slightly and the catheter is inserted further into the vessel via this cannula. Finally, the cannula is completely removed from the catheter. There is an injection valve at the end of the catheter.

A special type of central venous catheter (Fig. 26), the port catheter, consists of two parts, the port and the actual catheter. The port is a small plastic chamber that is closed with a silicone membrane. The actual catheter is connected to the port. The entire system is completely implanted in the body. The catheter tube is placed in the corresponding vein and the port is sewn into a skin pocket in the area of ​​the chest.

2.2.3 Material and coatings

Biocompatibility is required of all materials used for medical products. Biocompatibility is understood to mean the compatibility between a technical and a biological system [70]. As a result, the material must not damage the tissue with which it comes into contact and generally not have any negative impact on the organism. The biocompatibility is divided into several levels [70]:

  • Incompatible: Release of substances in toxic concentrations, which leads to harmful reactions in the body
  • Biotolerant: Products can stay in the body for several months and up to several years, there is no change or decomposition of the material, there may be minor undesirable tissue reactions, but the material does not have any harmful toxic effects on the tissue
  • Bioinert: There is no chemical or biological interaction between material and tissue, no toxic substances are released, so there is no tissue damage. The body forms a connective tissue capsule around bio-inert implants
  • Bioactive: Material enters into active, positive interactions with the surrounding tissue.

The level of biocompatibility required for a material is determined by the respective application. In addition, other application-specific requirements must be taken into account when choosing the right material. In general, such biocompatible materials can consist of various materials such as metal, ceramic, glass or polymers.

With regard to the mechanical properties, the catheter material must be flexible and pliable in order to be able to be guided through narrow and not always straight body cavities, such as the urethra or blood vessels. At the same time, however, it must be mechanically stable enough to be able to withstand the tensile and compressive loads during insertion and removal.

In order to meet the mechanical properties, biocompatible polymers are generally used as the base material for the manufacture of catheter tubes. Polymers are long-chain or cross-linked macromolecules that are built up from individual basic building blocks, the monomers [66]. There are various possible polymers to choose from, which differ in their basic components and thus in their properties. But even polymers from the same basic building blocks can differ in their mechanical properties. Some factors that contribute to these differences are, for example, the degree of polymerization, which influences the molecular mass, or the strength of the branching of the polymer chains. The more branched a polymer is, the lower its density (high-density and low-density plastics). The density in turn influences the degree of crystallization of the polymer.

The most important properties of the various plastics for the considerations made here are summarized below:

  • Polyvinyl chloride (PVC) is a chain-like polymer consisting of the basic building block vinyl chloride, which is produced by polymerisation from the monomer vinyl chloride (Fig. 27). In its pure form, the thermoplastic polymer is hard and strong with a tensile strength of 50 N / mm2 to 60 N / mm2; it is mechanically very stable and very stiff. Soft PVC with a tensile strength of 10 N / mm2 to 15 N / mm2 is used in medicine because it has the flexibility required for a catheter. Due to the use of plasticizers (especially dioctyl phthalate), PVC is less suitable for indwelling catheters in order to prevent excessive release of the plasticizers into the bloodstream. PVC is quite cheap and easy to process. It is therefore mainly used in the manufacture of single-use catheters. Due to the plasticizers, PVC has a hydrophobic surface [70].
  • Polyurethane is a thermoplastic polymer whose basic building blocks are called the urethane group. This is created by combining a polyalcohol (also known as polyether or polyester) and a polyisocyanate (Fig. 28). The properties of the polyurethane are heavily dependent on the raw materials used. Polyurethanes made from polyester have high tensile strength, but are not suitable for medical applications because they are very sensitive to hydrolysis. The polyurethanes used in medical technology are all based on a polyether. You use diphenylmethane diisocyanate (Fig. 29) as the isocyanate component and polytetramethylene ether glycol (Fig. 30) as the polyether. A special manufacturing method can reduce the usual susceptibility to stress corrosion cracking. At around 30 N / mm2 to 45 N / mm2, their tear strength is significantly higher than that of PVC. Another advantage is the fact that they can be manufactured without plasticizers [70].
  • Silicones (polysiloxanes) are synthetic polymers with siloxanes as basic building blocks. The individual siloxane units of a polysiloxane are each connected by linking silicon and oxygen atoms and thus result in the corresponding polymer polysiloxane.Polysiloxanes have a very high biocompatibility and hardly show any undesirable body reactions in humans. They are insensitive to enzymatic degradation and are therefore very durable. Silicone catheters allow a larger inner diameter and are less prone to incrustations due to their very smooth surface. They are used in urology for indwelling catheters. Silicones are thermosets, which means that they cannot be deformed again after they have hardened and can therefore be sterilized even at high temperatures. However, their tensile strength of 3.8 N / mm2 to 9.5 N / mm2 is significantly lower than that of the other plastics and the manufacturing costs are significantly higher than those of the other plastics listed. An example of a siloxane is dimethylsiloxane with the polydimethylsiloxane made from it [70] (Fig. 31). Due to the non-polar methyl groups that surround the polar silicon-oxygen chain, the polydimethylsiloxane has hydrophobic properties. For medical applications, three-dimensional networks are created from several such polydimethylsiloxane molecules by creating connections between adjacent chains with the aid of catalysts. In addition, amorphous silicas are inseparably incorporated into the networks. These are intended to reduce stickiness and increase mechanical strength.
  • Latex is a naturally occurring raw material. Latex catheters are very soft and therefore offer patients a high level of comfort. However, due to the proteins they contain, some people are allergic to latex catheters. Furthermore, they have a rather rough surface and are therefore very prone to incrustations. They are used in urology for short-term urinary diversion with a maximum lying time of five days. Siliconized latex catheters allow a stay of up to seven days.
  • A material that is very popular for catheters is the polyether block amide available under the brand name PEBAX®. It is a straight, chain-shaped block copolymer that contains polyamide segments and polyether segments as components and belongs to the thermoplastic elastomers. It is flexible and at the same time resistant to tearing and bending. The exact hardness of the material depends on the ratio between polyamide and polyether. Therefore, by varying the composition, different degrees of hardness or flexibility can be achieved, which makes the material very well suited for catheters.

Since further application-specific requirements are placed on the surface of medical products in particular, corresponding surface modifications are used in the materials used. Suitable coatings are applied to the surface, with one coating fulfilling the various requirements to different degrees.

A catheter is always a foreign body for the organism and therefore offers the risk of infection, especially in the case of long-term catheterization in the urological area. Antibiotic coatings are said to reduce the risk of infection. Active ingredients used for this are, for example, chlorhexidine and silver sulfadiazine.

With urological catheters, deposits of urine components can lead to so-called incrustation. This is the deposition of crystals on tissue or solids. With urological catheters there is a risk that constituents of the urine will be deposited on the catheter. Incrustations on the catheter can clog it and cause discomfort for the patient.

Especially with cardiac and venous catheters that are in constant contact with blood, it is important that they have a hemocompatible (blood-compatible), antithrombogenic surface. An antithrombogenic surface prevents the adhesion of platelets and thus reduces the risk of blood clots forming on the catheter.

- to be continued

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[78] https://upload.wikimedia.org/wikipedia/commons/thumb/2/22/Blausen_0196_Catheter_Right Heart_
Body.png / 450px-Blausen_0196_Catheter_RightHeart_Body.png (27.9.2017)

[79] https://upload.wikimedia.org/wikipedia/commons/thumb/d/d4/Left_Heart_Catheter.png/450p x-Left_Heart_Catheter.png (27.9.2017)

[80] http://deti.zp.ua/images/big4/portkater_3.jpg (27.9.2017)

[81] https://www.vygon.de/images/G_008128.jpg (27.9.2017)

[82] https://www.vygon.de/images/000170-w320-h413.jpg (27.9.2017)

[83] https://meritemea.com/wp-content/uploads/2014/05/resolvelock1.jpg (27.9.2017)

[84] http://www.cronauer-handel.de/img/400/w640.png (27.9.2017)

[85] http://www.cronauer-handel.de/img/180105/w640.png (27.9.2017)

[86] https://www.urologielehrbuch.de/01/katheterspitzen.jpg (27.9.2017)

[87] http://edwardsprod.blob.core.windows.net/media/Default/devices/catheters/swan/2015-10- 01_5-22-51.jpg (27.9.2017)

[88] http://www.n-medica.de/media/image/25/30/b7/VasofixSafety2_1280x1280.jpg (27.9.2017)

[89] https://www.vygon.de/images/G_00221167.jpg

[90] http://www.seilnacht.com/Lexikon/k_pvc2.gif (27.9.2017)

[91] http://archiv.aktuelle-wochenschau.de/2007/images/woche24/abb3.gif (27.9.2017)

[92] https://www.google.de/imgres?imgurl=https://upload.wikimedia.org/wikipedia/commons/thumb/
f / f9 / 2% 252C4% 2527-Diphenylmethanediisocyante.svg / 160px-2% 252C4% 2527-Diphenylmethanediisocyante.svg.png & imgrefurl = https: //de.wikipedia.org/wiki/Methylendiphenid= .qpnid= .qp4nid= 62 & tbnw = 122 & usg = M8YVt2yig4Q_Mq0k5L33OE9ga1g% 3D & vet = 10ahUKEwiX5drdvdLXAhXG1xoKHZ5EDgEQ_B0IiwEwCg..i & docid = 6R6CxYQWItvMFM & itg = 1 & client = Safari & sa = X & ved = 0ahUKEwiX5drdvdLXAhXG1xoKHZ5EDgEQ_B0IiwEwCg (10.11.2017)

[93] https://de.wikipedia.org/wiki/Polytetrahydrofuran#/media/File:Poly-thf.svg (10.11.2017)

[94] https://patentimages.storage.googleapis.com/EP1604644A1/00080001.png (27.9.2017)

Fig. 1: Human urinary system [73]

Fig. 4: Coronary arteries with vascular occlusion and infarct area [2]

Fig. 5: Structure of the vessel wall of arteries and veins [2]

Fig. 6: Single-layer squamous epithelium [67]

Fig. 7: Structure of the ureter and urethra wall [2]

Fig. 9: Ureter with star-shaped lumen [67]

Fig. 10: Inner ureteral stent [75]

Fig. 11: Transurethral urinary catheter [76]

Fig. 12: Suprapubic urinary catheter [77]

Fig. 13: Insertion of a right heart catheter [78]

Fig. 14: Insertion of a left heart catheter [79]

Fig. 15: Insertion of a guide wire into a coronary vessel [2]

Fig. 16: Position of a port catheter [80]

Fig. 17: Monolumen central venous catheter [81]

Fig. 18: Multi-lumen catheter [82]

Fig. 19: Catheter with straight and curved tip, pigtail catheter [83]

Fig. 20: Transurethral disposable catheter [84]

Fig. 21: Transurethral indwelling catheter, 2-way balloon catheter [85]

Fig. 22: Catheter tips in urinary catheters [86]

Fig. 23: Structure of a 4-lumen Swan-Ganz catheter [32]

Fig. 24: 4-lumen Swan-Ganz catheter [87]

Fig. 25: Peripheral venous catheter [88]

Fig. 26: Central venous catheter (port catheter) [89]

Fig. 27: Formation of vinyl chloride [90]

Fig. 28: Formation of urethane [91]

Fig. 29: Diphenylmethane diisocyanate [92]

Fig. 30: Polytetramethylene ether glycol [93]

Fig. 31: Polydimethylsiloxane [94]

Video (s) on the subject