Material fact: Improvements in catheter material
Pyam Ramnes examines the improvement of the design of catheter material.
The existing cerebral spinal fluid (CSF) shunt systems typically contain three main components:
1. Inflow catheter (ventricular catheter)
2. Valve
3. Outflow catheter (peritoneal catheter)
According to the FDA, infection, shunt malfunction, and improper drainage are some of the most common risks of CSF shunts. The primary objective of this design concept is to address the risks associated with shunt malfunction. Shunt malfunction is commonly due to a blockage or some obstruction within the shunt system. This concept is based on speculating the possibility of improving the design of the material from which the catheters of the CSF shunt is made. This improvement will be focused on reducing the risk of blockage or obstruction in these catheters. Subsequently, the major considerations in material design need to be identified, the material of the existing catheters and properties of this blockage need to be studied, and the desired state should be evaluated.
Consideration 1: Infection
Shunt colonisation is one of the major categories of shunt-related infections. The commonly used material from which the shunt catheters are made is silicone. This material is a surface where colonisation occurs. The traumatised tissue in the immediate vicinity of the implant, in the absence of a proper host defence mechanism, provides ideal conditions for colonisation by organisms. Therefore, the surface of the material used plays an important role in the process of colonisation.
Existing solution
Currently, antibiotics such as rifampicin and clindamycin are used for the impregnation of these catheters. There are other technologies as well to prolong the antimicrobial properties by incorporating silver and copper in the carrier.
Desired state
Skin flora is identified as the main source of bacteria that causes shunt colonisation. Thus, an agent that specifically reduces the accumulation of infectious organisms in the vicinity of the implanted shunt resulted from skin flora would be desirable. Also, the catheter material can be impregnated in such a way that the infection is more efficiently and in a longer duration prevented from growth. Regarding the fact that currently, the antibiotic-impregnated catheters are used only in those patients who are at risk of non-curable infections; this solution should not present any new risk and should be cost efficient.
Consideration 2: Radiopacity
According to ISO 7197:2006, Neurosurgical implants – sterile, single-use hydrocephalus shunts and components, all parts of the shunt shall be identifiable via X-ray examination.
Existing solution
Currently, the silicone elastomer from which the shunt catheter is made is barium impregnated to allow visualisation via X-ray. However, barium impregnation could be factored in the development of calcification which leads to the progressive deterioration of implanted catheter longer term (more than five years), and eventually promotes breakage.
Desired state
The compound developed for manufacturing the catheter should be visible with X-ray. However, this compound should not present the risk of breakage or could improve the current solution with a higher durability.
Consideration 3: Biocompatibility
ISO 7197:2006 requires that the biocompatibility of hydrocephalus shunts and components shall be assessed based on the guidance given in ISO 10993-1, Biological evaluation of medical devices.
Existing solution
Evidently, the products and material currently used for the shunt system shall meet the biocompatibility requirements.
Desired state
The compound developed shall pass the tests and meet the biocompatibility requirements.
Considerations 4-7: Resistance to leakage, ability to withstand overpressure, dynamic breaking strength, and bursting pressure.
Consideration 8: Geometry
The first step in ventriculoperitoneal shunting surgery is shaving the area; the second step is making cuts behind the ear and belly, then, a small hole is drilled in the skull and a thin ventricular catheter passed into a brain ventricle. Next, the peritoneal catheter is placed under the skin behind the ear and sent down the neck, chest, and into the belly area. Lastly, a valve connected to both catheters is placed underneath the skin behind the ear. As it appears in the procedure of surgery, the holes through which the catheters are passed are relatively small (approximately 2.5mm in diameter). Thus, the ventricular catheter outside diameter (OD) shall be equal or less than 2.5mm. Similarly, the peritoneal catheter OD needs to be small in order to be placed in the body and sent down to the belly. The inside diameter (ID) of the catheter is a function of the flow and design of tubing and usually about 1 – 1.5mm. The length of ventricular catheter is 3 – 15 mm and the length of peritoneal catheter is 120cm. Both ventricular and peritoneal catheters can be geometrically variable but generally have a cylindrical shape. The ventricular catheter usually has some rows of holes along the proximal tip with a slit tip. The peritoneal catheter usually has some slit openings along the distal end.
Existing solution
The below figures show some of the existing catheters:
Desired state
The optimal geometry of the catheter made from the developed compound, shall be accomplished.
Consideration 9: Occlusion
One major malfunction of the existing CSF shunts is occlusion, which impedes the flow of cerebrospinal fluid in the catheter and results in excess accumulation of the fluid and causes brain damage. The occluded catheter requires an immediate replacement through a surgical procedure. One solution to reduce the opportunity to this malfunction could be the enhancement of the flow through friction reduction or boost lubricity in the catheter. Another solution could be a using chemical agent in the structure of catheter that prevents the occlusion.
Existing Solution
An approach to reduce friction or add lubricity to the catheters is coating. However, before exploiting this approach, the consistency of the coating with medical device usability should be evaluated. Thus, the coating biocompatibility, internes (coating must not contaminate the substrate), cure temperature (must be within the performance range of the substrate), cure forces (must not degrade or distort the substrate), conformability, finished thickness, mechanical loading, resistance to flaking, sterilisability, etc should be studied with respect to the application. There are several coatings currently developed which is used to enhance lubricity. For example, PTFE (Polytetrafluoroethylene) coating reduces the coefficient of friction by as much as 50% for certain applications.
Another coating developed for lubricity enhancement is lubricious coating which is based on nonreactive hydrophilic/hydrophobic polymer matrices. The hydrophilic type of coating absorbs and retains the moisture in contact with aqueous body fluids using the hydrophilic characteristic. The absorbed moisture creates a slippery surface. Then, the hydrophobic components hold the matrix together and firmly anchor it the surface. Another approach to reduce friction in catheters is slip additives. This approach is based on compounding the base material with a monomer to improve the lubricity characteristic of the material surface. As a result, the coefficient of friction will be reduced.
Heparin is an anticoagulant which is used for coating the medical devices to reduce the effect of introducing the foreign material into the patient’s body. The natural reflex of patient’s tissue upon contacting the medical device may cause protein disposition and platelet activation leading to coagulation.
Desired state
The catheter with slippery lumen could be more appropriate for the cerebral shunt use as it could potentially mitigate the obstruction risk. Possibly, the correlation between friction and shunt performance can be identified through an in vitro test. In so doing, the coefficient of friction of the developed catheter can be studied with regards to the obstruction time, pressure, and flow. In case the friction reduction plays a significant role in mitigating the obstruction risk, catheter with slippery lumen will be desirable.
Heparin is mainly used as blood thinner and heparin coating commonly used for reducing blood clotting caused by introducing the medical devices into the patient’s body. However, hypothetically, the anticoagulant property of heparin can reduce the obstruction risk of the shunt’s catheter which can be studied further. Also, hypothetically, studying the chemical properties of cerebrospinal fluid and its obstruction may lead to the development of a new biocompatible and durable coating that counteracts to the agglomeration inside catheter.