What is Temperature Moderated – Collision Mediated Coating (TM – CMC)?
HKPB Scientific has developed Temperature Moderated – Collision Mediated Coating (TM – CMC), (Haverty and Kennedy, 2008, PCT/EP2008/064005) a cost effective, single step process for the deposition of functional, therapeutic coatings on hard surfaces, most notably those of biomedical implants.
Coatings are typically formed by two processes, which may occur simultaneously – coating deposition and coating curing. These two processes are commonly encountered in everyday situations: when paint is sprayed or brushed (coating deposition) onto walls and doors etc. and then allowed to dry (coating curing) to form an adherent coating.
Traditional curing technologies are problematic for biomedical compatibility. Curing by heating destroys thermally sensitive therapeutic drugs and the use of chemical curing agents is often inadvisable due to associated toxicity problems. Curing by solvent evaporation can lead to poor coating adhesion and can result in a small residue of toxic solvent remaining in the coating.
TM – CMC circumvents these limitations and uniquely can form functional, drug-loaded coatings in a single step. In TM – CMC shot peening is used to convert an aerosol into a well adhered coating. The aerosol is formed by atomising a suspension of solvent (water or bio-compatible n-alkane), coating material and drug. In essence the action of the peen creates transient heat and pressure at the surface of the implant, which in effect cures the coating as it is deposited. The heat and pressure generated is moderated sufficiently by the presence of the liquid component of the aerosol so that thermally sensitive agents can be incorporated into the coating.
The process is visually illustrated in the animation below. Please consult the links below to read more about TM – CMC, its’ applications and history.
Shot Peening
J. Almen discovered shot peening while working at General Motors in the 1930’s. Shot peening involves the bombardment of metal surfaces with hard particles (referred to as shot) of micron to millimetre diameter, travelling at high velocity. The stream of shot forms multiple surface indentations on collision with the substrate, each individual particle acting like a tiny peening hammer. Compressive stress induced in the upper surface layers as a consequence of this peening action renders metal components less prone to cracking and reduces fatigue failure. It is well established that high temperatures can be induced in substrates during collision processes such as shot peening (Reissig et al., 2004; Shipway and Weston, 2008).
References
Reissig, L., Völkl, R., Mills, M. J. and Glatzel, U. “Investigation of near surface structure in order to determine process-temperatures during different machining processes of Ti6Al4V” Scripta Materialia, 2004, 50, 121-126
Shipway, P.H., and Weston, D.P., “Thermal effects in blasting and erosion of polymeric materials” Journal of Materials Processing Technology, 2008, doi:10.1016/j.jmatprotec.2009.03.006
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TM – CMC History
In the 1970’s and 1980’s a team of scientists working for the National Aeronautics and Space Administration (NASA) on the thermal, wear and corrosion properties of metals used in the aerospace industry, developed a process known as peen plating. The earliest peen-plating patent (Babecki and Haehner, 1973) describes a process where a stream of fine aluminium and or copper powder (the coating) is impacted onto a metal surface by the simultaneous peening action of a stream of glass bead shot. Subsequently the peen plating process was optimised and extended to coating with other fine metallic powders (Chu and Staugaitis, 1985) and non-metals (NASA Doc. ID: 19710000255).
The peen plating process was further developed for coating metallic substrates with lubricants such as molybdenum disulphide. SURFGUARD was successfully commercialised by Techniblast Co. under licence from NASA (NASA Doc. ID: 20020087616) and the equipment used in the SURFGUARD process was patented by R. Spears (Spears, 1988). Those interested in reading more about the work of NASA on the peen plating process may do so by accessing the technical reports on the NASA website.
Subsequent to the invention of the peen plating process many variants on the theme of using the action of particle collisions to adhere materials to the surface of metals have been developed for a wide range of applications including the addition of biocompatible ceramics to the surface of biomedical implants (Müller and Berger, 2005, 2008).
References
Babecki, A. J. and Haehner, C. L. “Peen Plating” 1973, United States of America Patent No. 3,754,976
Chu, H.-P. and Staugaitus, C. L. “Method of Coating a Substrate with a Rapidly Solidified Metal” 1985, United States of America Patent No. 4,552,784
Babecki, A. J. and Haehner, C. L. “Plating by Glass Bead Peening” 1971, NASA Document I. D.: 19710000255
Anonymous “Lubricant Coating Process” 1989, NASA Document I. D.: 20020087616
Spears, R. L. “Apparatus and Method of Powder-Metal Peen Coating Metallic Surfaces” 1986, United States of America Patent No. 4,753,094
Müller, W.-D. and Berger, G. “Surface Treated Metallic Implant and Blasting Material” 2005, European Patent No. 1,395,300
Müller, W.-D. and Berger, G. “Surface Treated Metallic Implant and Blasting Material” 2008, United States of America Patent No. 7,377,943
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TM – CMC
Whether used in the aerospace, biomedical or other industrial sectors previous shot peening based coating processes, such as developed by NASA, have been limited to the coating of metal substrates with fine ceramic or metallic powders primarily because these coating materials can withstand the heat generated during the collisions accompanying shot peening and because these materials are readily available in fine particulate form. The TM – CMC process circumvents these limitations enabling the coating of substrates with thermally sensitive materials, such as therapeutics. In addition the material being coated is not restricted to particulate, vastly extending the range of materials that may be incorporated into a coating.
TM – CMC involves atomising a liquid based precursor coating composition to form an aerosol, which is directed to the surface of the substrate in conjunction with a stream of shot particles as shown in the animation. The collision energy released by the impacting shot mediates the transformation of the precursor composition into a well-adhered coating. While the energy released is necessary to transform the precursor composition into a coating, unchecked the consequential temperature rise is detrimental to the inclusion of thermally sensitive components. The key to incorporating thermally sensitive components is moderating this heat. This is achieved through the liquid element of the aerosol, which absorbs part of the heat generated, protecting thermally sensitive components. and where necessary, the underlying substrate. Precise control of the atomisation and composition of the liquid based precursor is critical to the process; insufficient liquid and the temperature moderating effect is absent, while excessive liquid prevents the formation of a coating.
A nano-porous polymer layer coated on a titanium surface by TM - CMC
Using a liquid medium also extends the range of materials that can be coated beyond particles. Many potentially advantageous coating compositions are not available in particulate form, for example nano-particles are generally supplied as colloidal suspensions to prevent agglomeration and or to protect functionality with which they may be augmented. In addition the process readily extends to the precursor gels, sols and resins of a wide range of polymers and ceramics without the requirement for complex chemical coupling or curing treatments.
TM – CMC is easily implemented in a manufacturing environment, combining two widely used and readily automated equipment platforms, atomisation and shot peening, in a new way.
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Biomedical Applications
TM – CMC has many advantages. As a coating technology it is a cost effective, single-step process that is highly manfacturable and applicable to an extensive range of coating materials. The most functional advantage of TM – CMC is without doubt that it uniquely allows for the deposition of coatings containing thermally sensitive components. The process is flexible as a wide choice of solvents, including water, are readily employed. Current methods of applying ceramic biocompatible coatings to medical implants are characterized by high process temperatures precluding the inclusion of antibiotics and other therapeutic agents in the coating. In addition polymeric coatings deposited at implant surfaces often involve thermal curing steps or the use of chemical coupling agents precluding the simultaneous inclusion of therapeutic agents and affecting the biocompatibility of the coating respectively. TM – CMC has the potential to resolve such problems and consequently the company is targeting three key applications in the drug delivery space.
Hard Tissue Implants
TM – CMC has the potential to solve the two major problems encountered in the arena of hard-tissue implants:
Reduction of Aseptic Implant Loosening - The purity of the hydroxyapatite coating achievable with TM -CMC allows for enhanced bone in-growth at hard-tissue implants. Current high-temperature processes for applying hydroxyapatite coatings ablate final surfaces and reduce the biocompatibility of the coating. TM -- CMC results in high fidelity surfaces with dramatically improved biocompatibility to promote bone in-growth and reduce implant loosening.
Reduction of Infection at Hard-Tissue Implants - Implant infections are problematic as they typically involve biofilms that complicate treatment procedures. While cemented implants have the capacity for incorporating antibiotics into the material, the more desirable press-fit implants do not have this capability as the high-temperatures that characterize processes currently used to apply coatings of hydroxyapatite destroy antibiotics. Using TM -- CMC for forming press-fit implant coatings does not destroy such thermally sensitive compounds, opening up new opportunities for device manufacturers and reducing healthcare complications for patients and providers.
Drug Eluting Stents
TM – CMC has the potential to offer improvements in the area of cardiovascular drug delivery:
Improving the Biocompatibility of Drug Delivery Matrices - Cardiovascular stents have been used for over 15 years to reopen arteries clogged by cholesterol deposits, with the common side-effect of inflammatory restenosis being recently overcome by the advent of coating the stent with a slow releasing immuno-suppressant drug. Polymers are typically used as a carrier matrix for the slow release of the drug, but clinical evidence shows that the presence of currently used polymer coating and associated issues may be a contributing factor to late stent thrombosis. TM -- CMC allows for the use of both polymer and ceramic-based biocompatible carrier matrices without the requirement for problematic initiators, catalysts or solvents (O’Brien and Carroll, 2008). This provides clear benefits for the patient and provider in terms of reduced rates of post-operative complications.
Improving the Loading and Distribution of Therapeutics within the Delivery Platform – As a consequence of the complex chemistry and multi-step processing aspects of currently used drug delivery platforms, achieving optimum elution profiles in vivo remains problematic. TM – CMC offers an effective means to readily control the loading of the therapeutic agent within a given carrier matrix through the composition of the precursor. Additionally the single-step nature of the process (drug and carrier matrix co-deposition) provides for the homogenous distribution of the drug within the coating.
Pacemakers and Defibrillators
Based on Centre for Disease Control (CDC) research (Klevens et al., 2007), hospital acquired infections (HAI) are responsible for 5% of surgical failures of pacemakers and defibrillators. Pacemakers have been identified as a medical device that would benefit from Antimicrobial coatings but currently no effective methodology exists to coat pacemakers with suitable anti-microbial formulations. TM -- CMC allows for the low temperature deposition of coatings comprising a range of biocompatible materials, which may be augmented with a variety of biocides including antibiotics.
References
O’Brien B, Carroll W, “The evolution of cardiovascular stent materials and surfaces in response to clinical drivers: A review” Acta Biomaterialia, 2008, doi:10.1016/j.actbio.2008.11.012
Klevens, R., Morrison, M. and Nadle, J. “Invasive Methicillin Resistant Staphylococcus Aureus Infections in the United States” JMMA, 2007, 298
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