Material Selection Process For Medical Devices

The constant endeavour towards making use of safer and high-quality products that are available at lower costs has led to the transformation of products used in the medical industry. Therefore, material selection is done for designing medical devices in such a way that they are bio-compatible, environment-friendly, safe, and meet the operational requirements for medical devices. With the condition to do the correct material selection for the medical devices and equipment, there are numerous constraints and threats as well.

Polymers have been constantly used in the medical industry since ages. Recently, there has been a shift towards thermoplastic materials and other compounds for the medical devices. Conventionally, the material selection done for orthopaedic and surgical medical devices has been metal. However, the latest innovative developments are largely incorporated in the form of plastic resins. The anti-static, wear-resistant, and radiopacity and elastomer materials are increasingly used in designing medical devices.

What the medical industry demands!

When we talk about the medical industry, the safety and well-being of human life comes into the picture. The need for clean habitation and various drug-coated implants is driven by the current trend of easy movability and cost-effectiveness. Recently, there has been widespread awareness about sanitation and tidiness and this has encouraged the use of top-quality and cost-effective medical devices. While reusability was a common trend seen in the past days, the practice of one-time use instrument has gained huge popularity. This calls for effective material selection considering the factors such as functionality, bio-compatibility, and sterilizability.

Material selection instructions:

Material selection, especially for creating medical devices is indeed one of the toughest jobs. Which material to choose from? What all aspects to consider? Should we choose polymers or plastics or additives? For answering all these queries, you should follow the below steps to select the best material for designing medical devices.

  • Objective

It is the foremost job to specify the design specifications and final objective. Some of the environmental considerations include –

  1. Bio-compatibility
  2. Single-use or multi-use medical device
  3. Contact with chemicals or solvents during cleaning
  4. Sterilization methods and frequency of use
  5. Humidity and temperature conditions and duration of exposure
  6. Contact with body organs and duration of contact
  7. The necessity of UV resistance
  8. Painting or electroplating requirements
  9. The necessity of flame retardant ability
  10. Dimensional stability and tolerances
  11. Outdoor uses, if any
  12. Considerations of colour aspects
  13. Use of the medical device in X-rays and fluoroscope processes

Some of the mechanical and operational requirements include –

  1. Dimensional features and stability requirements
  2. Toughness and impact resistance properties
  3. Any use as an electrical insulator
  4. Tensile or Flexural stress
  5. Micro-molding or insert-molding processes required, if any
  6. Amount of load and the duration of carrying the load
  7. The projected life of the medical device
  8. Manufacturing processes
  9. Manufacturing properties
  10. Bearing and abrasion requirements
  11. Intended cost of the medical device
  12. Conductivity or dielectric properties
  • Analysis

Properly analyze the conditions and stick to the mentioned instructions for an effective material selection according to your requirements.

  • Bio-compatibility

Checking the biocompatibility of the medical device which is exposed to its surroundings is pertinent. Since these medical devices are going to be in contact with the body parts and tissues, the material selection should be done based upon the bio-compatibility aspect. According to ISO 10993-1, while performing the biocompatibility tests during material selection several factors need to be considered that include wastes, pollutants, and degradability of the product.

The data shown in the table is taken from www.roechling-plastics.us

Therefore, you will find that a few numbers of polymers are utilized in designing medical devices that are of excellent medical grade. The degree of contact with the inner and outer tissues and internal fluids helps us determine the polymer to be chosen. The duration of contact of the medical device with the tissues or fluids is categorized into a short-term or limited type where the contact is less than one day, medium or continual type where the contact is from 1 to 30 days, and long-term or perpetual type where the contact with the body parts is for more than 30 days.

While considering the bio-compatibility factor for the medical devices it is crucial to determine the effect of the compound on its surrounding, i.e., the body organs and tissues. While polymers have a wide application in the orthopaedic department due to their similarity to the looks of natural polymeric tissue parts, they are also a huge drawback as they start degrading in the body conditions because of several biochemical and mechanical aspects. This leads to inflammation and other hazardous reactions due to the ionic attack.

  • Drug Flow Path

The drug flow path is referred to as the situation in which the material has direct contact with the drugs flowing via an instrument. Therefore, for successful material selection for ensuring its compatibility with the drug flow path, chemical resistance is important to consider. The material should be chemically resistant and shouldn’t degrade under the influence of drugs. It should also be considered that the efficiency of the drug shouldn’t be compromised with the use of thermoplastic substances. These situations arise during the reaction with additives or monomers or polymers.

  • Sterilization affiliation

It is important for the plastic medical devices used in the body fluid to be subjected to sterilization. The reaction of the plastics to the sterilization technique varies from material to material. Usually, there are two types of sterilization techniques such as radiation like gamma rays or e-beams, autoclave or steam exposure, and chemical or Ethylene Oxide (ETO) contact.

While most of the thermoplastic polymers can tolerate any exposure to harmful Ethylene oxide during sterilization with no remarkable modifications observed in their characteristics or color, those subjected to radiation sterilization will be highly afflicted. This may change the significant mechanical properties of the substance like the tensile strength, cross-linking, and impact strength.

The frequency and duration of radiation exposure also play an important role in sterilization processes. The styrenic thermoplastics like PARA, PES, ABS, TPU, PEEK, PEI, and PPSU are highly resistant to radiation. Although Polycarbon or PC is usually resistant to the radiations it will start discolouring with prolonged exposure.

Autoclaving or sterilization with the use of steam makes use of both heat and moisture in continuous reps of 3-15 minutes to get rid of germs and contaminants. The medical devices subjected to high levels of residual stress are accompanied by Annealing that leads to dimensional instability and warping in temperature conditions of 121⁰C – 140⁰C.

In such situations, amorphous grade materials come in handy. Although PP, PC, PSU, and PA can be used in such cases, they require extra precaution of the number of reps of autoclaving they are subjected to. PPSU and PEEK are excellent polymers that can easily withstand autoclaving. Styrenes such as ABS and PS and Polyesters such as PBT and PET are not good materials for autoclaving.

In the case of multi-use medical devices, the parts are exposed to numerous sterilizations prior to being withdrawn. Thus, materials such as PSU, PC, PEEK, PPSU, and PEI plastics with remarkable strength and lesser vulnerability to discolouration should be used.

The data shown in the table above is taken from www.roechling-
plastics.us
  • Resistance to cleaning materials and chemicals

With continuous use in the medical industry and rigorous cleaning afterwards, there might be degradation of the plastics leading to poor performance. Stress cracking and breakage can be seen in the thermoplastic materials due to the deposition of bleaches, peroxides, and isopropyl alcohol on its surface.

The polymer characteristics solely determine the extent of chemical resistance. Semi-crystalline polymers such as PE and PP and several polyamides have significant chemical resistance as compared to amorphous polymers such as PC and ABS

  • Mechanical properties

During material selection, deciding upon the impact strength, tensile strength, wear resistance, stiffness, and other mechanical properties is really important. Thermoplastics such as PEEK, Polyacetal, PC, Nylon, PPSU, and POM exhibit outstanding mechanical characteristics in lower and higher temperature conditions. This property comes in handy during the transportation of medical devices to different parts of the world.

The material selection is also done on the basis of miniaturization. In order to decrease the effect on patient and security during surgical processes, various medical devices are customized to make use of reduced invasive surgery. Wear materials, conditions, and configurations also decide the material to be selected for designing medical devices.

Mechanical, electrical and thermal data shown in the tables
below are taken from www.curbellplastics.com.

  • Electrical and thermal characteristics

Medical devices should be instilled with properties such as increased heat resistance, high dielectric strength, and more. PC, Polystyrene blends, PC blends, and Polyphenylene have outstanding electrical properties regardless of the temperature condition

  • Dimensional Stability and Durability

Selecting materials with higher dimensional stability and durability is quite challenging because of the changing conditions such as chemical exposure, and increased heat and temperature. Amorphous plastics have excellent dimensional stability and minimal warpage thanks to their reduced and homogenous shrinkage properties. PC, PSU, PEI, PPSU, and PES plastics are dimensionally stable. While semi-crystalline plastics have increased shrinkage with reduced uniformity, liquid crystal polymer has highly reduced shrinkage thus offering increased dimensional stability.

  • Radiopacity and Conductivity

Radiopacity is defined as the property of the material to be visible in X-rays and fluoroscope processes. Various additives can be utilized in order to make the material radiopaque, the most common one being Barium Sulfate. This method is used in surgical implants and catheters during surgical processes.

In case of dry powder and various aerosol-based drug delivery methods, there is some static deposition on the thermoplastic components. This, in turn, induces the drugs and leads to anomalies in the dosage. Acrylic, clear ABS, ABS, and PolyCarbon blends, and Polypropylene are some of the antistatic compounds used for these purposes.

  • Manufacturing Feasibility

Lastly, even if you choose the perfect material for designing the medical devices, considering the technical aspects and cost-effectiveness is really crucial. Upon choosing a particular material, it’s important to decide if the medical device to be created is possible with this material. Also, deciding the cheapest yet effective way to make use of the material to create the medical device is also necessary.

Manufacturing data shown in the table below is taken from
www.mikrotech.com

Materials & Their Manufacturing Feasibility

Materials & Their Manufacturing Feasibility
  • Grading the materials

Now that we have analyzed the material, grading the materials for the ultimate performance of the medical device is mandatory. There might be numerous parameters or residue of materials after the analysis. So, you should follow the below steps for the best material selection for designing medical devices.

  1. Firstly, you should be able to determine the functional requirements, constraints, and variables.
  2. Secondly, decide upon the mixture of several material properties to produce increased performance.
  3. Make use of the Performance Equation and the selected materials to obtain the ranking of the materials considered.

P=[F*G*p], Where P = Performance, F=Functional Requirements, G=Geometric Parameters, p=Material Properties.

Since we are talking about the material selection for medical devices it goes without saying that pre-selection of material will not only reduce the time required to create the medical device but also decrease the overall costs required.

Material Selection Flow Chart

Material Selection Flow Chart

By making use of a minimal level of product testing techniques and assessing the overall performance, the durability and operational performance of the whole medical device can be determined. It also helps you in proper evaluation and the opportunity to make use of an alternate material for getting the job done.

Basically, pre-selection decreases the design costs associated and makes the material data easily available. This is what is required in the medical industry. There is a need for quick and cost-effective methods of designing medical devices with efficient materials.

Conclusion

Thus, it can be concluded that various factors determine the material selection for various medical devices.

According to the current trend, portability, increased performance, miniaturization, aesthetics, and bio-friendly aspects play an important role in designing medical devices.

For maximum product utility, durability, and increased performance and looks, considering accurate and efficient materials is indeed crucial. It is important to be informed that correct material selection can be done with the help of effective product testing.

Regardless of the information or data collected during material selection processes, performing real product testing will give you outstanding results.

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