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Polymers for Implantable Medical Devices

 
Traditionally, degradation resistant polymers such as PTFE, PEEK, PMMA, polyethylene and silicones have long been the preferred materials for use in implantable medical devices. In fact, the success of implantable devices from pacemaker leads to joint replacement prostheses has for many years depended on the use of non-toxic degradation resistant materials that have the ability to maximise fatigue strength and wear resistance. Whilst these polymers have been successful as implantable platforms (in urinary, cardiac, ocular, orthopaedic, respiratory medical devices), their use is frequently associated with clinical problems including microbial biofilm formation and medical device related infection, encrustation, poor biocompatility and low lubricity. To address these problems, polymers with advanced physicochemical properties have been developed, including:
• Bioresorbable polymer implants, employed in tissue engineering, absorbable sutures, arterial stent coatings and implants for controlled drug delivery
• Stimuli responsive polymers, employed as drug delivery systems/implants, for tissue engineering, drug delivery and to modulate the biocompatibility and resistance to microbial biofilm   formation
• Bioactive biomaterials for controlled delivery of bioactive agents, designed for the prevention of microbial biofilm formation
• Novel polymeric and non-polymeric single, layer and multilayered coatings to enhance biocompatibility, lubricity and resistance to biofilm/encrustation formation
• Polymeric (electronic based) biomaterials as diagnostic systems
• Processing/sterilisation challenges of polymeric implantable systems
Recognising the fact that R&D is being invested in this fast growing area, iSmithers and sister company Smithers Rapra have organised an international conference on polymeric materials for implantable medical devices to illustrate many of the key developments in this rapidly changing area of biomaterial science. Presentations will include:
Biomaterials for implants in ophthalmology: Polymers for implantable medical devices have to meet different specific challenges. Especially implants used in ophthalmology have to meet particular requirements in respect to physical, chemical and biological properties. In addition, polymers are modified on the surface to mimic the function of the substituted tissue. Therefore, active polymers are bonded to the surface to initiate cell adhesion and proliferation to provide integration in the surrounding tissue. Also, the intrinsic properties influence the interaction with tissue and can thus be used for intelligent implant design. Special focus is given on synthetic polymers, varying from hydrophobic materials and their modification by nano technological methods to hydrogels.
Anti-infective biomaterial surfaces based on surface-localised sensitisers: Bacterial adherence to indwelling medical devices is recognised as a key initial stage in the development of infection. New methods to give ultrathin surface-localised sensitiser layers on polymers suitable for a range of medical device applications, including ophthalmic, respiratory and urinary devices will be discussed. Reductions in numbers of adherent organisms by more than 99.99% (4 log cycles) are possible using the technology, and the catalytic process allows for persistence of activity for >1 month, with none of the issues of resistance associated with conventional antibiotics.
Practical challenges of material selection for implantable bladder catheters: The material and engineering challenges the ‘Foley’ catheter faces in use are extremely varied and complex. The catheter needs to be smooth and soft for urethral insertion and the integral retention balloon must stay inflated for up to 90 days in the bladder. In use, the catheter should minimise encrustation and biofilm formation, and at the same time have good tensile strength and the ability to resist blockages. Removal of the catheter provides the biggest challenge as the balloon must have excellent elastic recovery properties to facilitate easy pain free removal of the catheter.
Material selection process for vascular access catheters: Vascular access catheters allow access to their major vessels to provide drug therapy, blood transfusion/sampling, parenteral nutrition, CT contrast, or dialysis treatment for short or long term needs. Patients with vascular catheters may develop blood vessel damage, catheter related thrombosis or infection, and bleeding/bruising at insertion site. So, it is critical to design and develop these catheters to minimize the associated risks. This presentation will provide an overview on design criteria and material selection process used for catheter development, and highlight major design and manufacturing limitations and role of effective material selection.
A mock circulatory system for the evaluation of intravascular implantable polymers: An easily adaptable artificial circulation that is able to mimic pressure and flow patterns of the human arterial, venous and ventricular system to test intravascular applications of polymers. The artificial circulation consists of four elements: a pump system, a circulatory system, a test compartment module, and an acquisition and analysis monitoring system. The system can be filled with water, a blood analogue (a glycerol/water mixture or Krebs Henseleit buffer) or blood. By using this pulse duplicator a large number of polymers can be tested in a reproducible setup to gain information about their stability in an intravascular environment.
Polymeric embolisation devices for intra-arterial occlusion and drug delivery: Therapeutic embolisation of blood vessels is a procedure commonly performed to treat a variety of conditions including bleeding problems, arterio-venous malformations, benign and malignant hypervascular tumours. A range of degradable or permanent embolisation devices composed of natural or synthetic polymers may be used for such applications. Microspheres composed of a sulfonate-modified polyvinyl alcohol hydrogel (DC Bead®) are finding wide-spread use for the occlusion and subsequent local, controlled and sustained delivery of chemotherapeutic agents directly within the vasculature of liver tumours. These so-called Drug Eluting Beads (DEB) are available in a series of calibrated size ranges for matching vessel size; have appropriate physic mechanical properties to enable them to be delivered with ease through narrow gauge micro catheters; and have the ability to actively sequester and release positively charged anticancer drugs by an ion-exchange mechanism.
Medical device coatings- from alchemy to commercialization: Biocompatible surface coatings are finding increasing applications in medical devices to reduce surface friction and to avoid undesirable conditions such as bacterial infection, blood clots and blood vessel or tissue damage. The US market for medical device coatings is estimate to reach US$3.4 bln by 2012 at a CAGR of 5.7%. Amidst such fast paced market growth and increasing raw material and energy costs, the medical device coating industry also wrestles with every stricter environmental regulations on volatile organic compounds (VOCs) and hazardous air pollutants (HAPs). Long-term biostability of permanent implantable polydimethyl siloxane : Biocompatibility for implantable medical devices is verified and validated based on ISO 10993 with a risk-based approach where the toxic or harmful effects are assessed through a combination of biological testing and chemical characterisation techniques. This study investigated potential changes in permanent implantable grade PDMS (polydimethylsiloxane) polymers following long-term in vivo aging. The investigation included changes in surface morphology, changes in surface nano-mechanical properties and changes in the chemical composition on the surface.
Nanocomposite biomaterials- driving the future of organ development: Prof Seifalian’s team has developed and patented a family of nanoparticles and nanocomposite materials in the development of organs using stem cells to enhance organ function. Presentation will include development of cardiovascular implants, as well as other organs and tissues and also address experiences in the translation of this technology from the laboratory to the patient, and commercialisation of new surgical implants, along with application of nanoparticles including fluorescent nanoparticles or “quantum dots” in the development of organ replacements. Bioactive collagen-based scaffolds for tissue regeneration: Tissue engineering uses a combination of scaffolds, cells and signalling mechanisms to restore the function of damaged or degenerated tissue. Research carried out investigates each of these three areas with target applications in bone, cartilage, cardiovascular and other tissues. We have developed a series of porous collagen-based scaffolds with the optimal composition, pore structure and stiffness to promote tissue regeneration. In the cellular area, the regenerative potential of bone marrow and amniotic fluid derived stem cells in combination with the scaffolds is being researched, focusing on the potential of these scaffolds as a bioactive delivery platform for growth factors and genes for tissue repair.
Where extrusion and microstructure match: In the scaffolds industry, beside the needs of polymer compatibility with the final function, there is also the need to preserve the molecular weight and to condition properly the polymer during processing for maintaining the initial desired properties. Micro extrusion is so called not because of the size of product, because of the size of machineries used for obtaining it or because of the amount of processed polymer, but rather because of the scale of size of the conditioning parameters. Microextrusion technology is able to preserve the molecular weight and to condition the cristallisation. The molecular weight is conditioning for example the time that the body will take for the degradation of a scaffold or a biodegradable suture, as well as degradation and release kinetics of a drug-eluting device.
Other topics include Morphological and mechanical assessment of porous biodegradable scaffolds processed by gas foaming, Advances in medical grade polyurethanes, Reliability testing of electroactive hydrogel polymers for intravascular occlusion, Poly(aspartic acid) hydrogels with reversible disulfide cross-links.
 
 
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  Aug 12, 2011
 
 
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