Bonding Dissimilar Substrates Using Novel Adhesive and Surface Treatment MethodsAuthors: Elkem Silicones USA Corp.

Phyl Gaither

Elizabeth Bales

Matthew Kihara

Silbione®

www.silicones.elkem.com

Elkem Silicones l www.silicones.elkem.com1

Silbione®

MATERIAL AND ADHESIVE SELECTIONS

Devices designed for neuromodulation applications use a combination ofmetallic, thermoplastic, and silicone materials. Neurostimulation leads (Figure1) demonstrate the need for combining materials, a metallic electrode isencased in an insulative silicone and/or thermoplastic sheath to deliver acontrolled electric signal to a targeted area. Advancements in cochlearimplants, the most common neurobionic prosthetic device currently on themarket, have deemed silicone coated titanium casings to be more water tightand stable than ceramic alternatives. As neuromodulation and neurobionicprosthetic devices are used in new treatment methods, it will be critical to havesmaller and more sensitive electrode arrays with increased flexibility. Theseneeds are driving developers to consider wider ranges of metals andelastomeric materials. The Elkem team has assessed the adhesion strength andideal surface preparation of these materials (Table 1) with Silbione® BiomedicalAHD1 M200, a monocomponent, tin free adhesive that cures in ambientconditions. This adhesive was selected based on the processing advantages itoffers in conjunction with a formulation that offers regulatory peace of mind(Table 2).

INTRODUCTION

The neuroengineering field is defined by the coupling of engineering and neuroscience; bringing togethertwo different, but complimentary entities. Similarly, to create a successful neuroengineering device, it isnecessary to couple the strengths of various, and sometimes distinctly different materials. Already,neurological devices have been successfully used to treat hearing loss, chronic pain, and epilepsy amongother disorders. As these devices advance, soon Parkinson’s disease, dementia, Alzheimer’s, obesity, andsleep apnea may be added to that list. The realm of possibility for applying neuroengineering to medicaldevices is vast, but options for pairing the materials to make these advancements is more limited.

Three hurdles that exist in selecting materials are creating a bond between substrates, developing asimple and feasible manufacturing process, and selecting materials that meet not only current regulatoryguidelines, but follow regulatory trends for the future. New adhesive technologies are an option to helpovercome these hurdles by widening the range of material options to pursue new therapies. ElkemSilicones seeks to create novel materials that can push the boundaries of existing devices and create newpossibilities for the future. This paper will explore the performance of a novel adhesive designed to meetevolving regulatory requirements, optimize production processes, and overcome the barriers of materialselection by bonding with a variety of substrates for use in the field of neuroengineering.

Versatile Innovative Adhesive for Medical Implants

Figure 1. Neurostimulation device can and lead

Elkem Silicones l www.silicones.elkem.com2

Versatile Innovative Adhesive for Medical ImplantsSilbione®

Figure 2. Cure mechanism for ADH1 M200 silicone adhesive at ambient conditions with catalyst

Table 1: Material information for substrates used in experimentMaterial Type Hardness Tensile Strength (psi)

Thermoplastic Polyurethane Substrates

Carbothane PC 3572 DAliphatic polycarbonate-

based69 shD 9,600

Pellethane 2363-75D Aromatic polyether-based 76 shD 5,810

Pellethane 2363-90A Aromatic polyether-based 90 shA 5,850

Tecothane 1075-75D Aromatic polyester-based 75 shD 8,300

Tecothane Soft AR62A Aromatic polyester-based 62 shA 1,500

Tecoflex RG-80A Aliphatic polyether-based 72 shA 5,800

Metallic SubstratesAluminum Bare smooth mill finish 15 HV >63,000

Stainless Steel Grade 304-BA 129 HV 98,000

Titanium Grade 2 145 HV 59,500

Silicone SubstratesSilbione HCRA 4150 Silicone rubber 50 shA 1,600

ADH1 M200 a biocompatible designed specifically for situations where a high strength elastic bond tosilicone, thermoplastic and metal substrates is required. This adhesive uses a non-tin based catalyst topromote a polycondensation reaction at ambient temperature (23°C) and humidity. The mechanism ofcure is demonstrated in Figure 2.

3

Versatile Innovative Adhesive for Medical ImplantsSilbione®

SAMPLE PREPARATION & ANALYSIS

Adhesion of ADH1 M200 to each of the substrates (Table 1) was determined by testing the failure modeof prepared samples. Each substrate was used to prepare four samples that differed in surfacepreparation- no preparation, application of Silbione M&P Primer, OpenAir™ Plasma treatment, and acombination of primer application and plasma treatment.

Samples of the substrates adhered with Silbione Biomedical AHD1 M200 were prepared as outlined inASTM D1005 and cured at ambient temperature (23°C) and humidity (50%) for 7 days before testing(Figure 3). Samples were then evaluated with a 180° peel test according to ASTM D903 (Figure 4).Peeled samples were observed an determined if the mode of failure was cohesive, the adhesive orsubstrate breaks before separation from the substrate (Figure 5), or adhesive, the adhesive separatesfrom the substrate without breaking (Figure 6). The goal of this experiment to determine whichcombination of surface preparations will lead to cohesive failure.

Figure 3. Aluminum AR-14 lap-shear sample preparation usingASTM D903 Standard TestMethod .

Figure 4. Lap-shear methodusing ASTM D903 StandardTest Method .

Table 2: Material Properties of Silbione® Biomedical ADH1 M200Property Unit Value ASTM

Appearance -- Translucent D2090

Tack Free Time minutes 8 C679

Hardness shA 26 D2240

Tensile Strength psi 570 D412

Elongation % 66800% D412

Tear Strength ppi 64 D624

Specific Gravity -- 1.06 D792*Tested slabs prepared according to ASTM D3182

Elkem Silicones l www.silicones.elkem.com

4

Versatile Innovative Adhesive for Medical ImplantsSilbione®

Figure 5. Peel test results for un-treated metallic andsilicone substrates. From right to left: Adhesive failureon aluminum, adhesive failure on stainless steel,adhesive failure on titanium, cohesive failure onsilicone.

Figure 6. Peel test results for primed thermoplasticsubstrates. From right to left: Cohesive failure onCarbothane PC3572, cohesive failure on Pellethane2363-75D, cohesive failure on Techothane SoftAR62A..

RESULTS

Each of the substrates were successfully bonded and a surface preparation method to yield cohesivefailure was determined (Table 3). No surface preparation was required to yield cohesive failure withCarbothane PC 3572D, Pellethane 2363-75D, and Silbione HCRA 4150 substrates. Tecothane Soft AR62A,Tecoflex RG-80A, and Pellethane 2363-90A required application of Silbione M&P Primer to the surface ofthe substrate for a cohesive failure mode. Tecothane 1075-75D, and each of the metallic substratesdemonstrated cohesive failure with either OpenAir plasma treatment, or application of the primer. Whenboth plasma and primer surface treatments were applied, all samples failed cohesively.

Table 3: Mode of failure based on substrate surface preparation

Substrate No Surface Treatment OpenAir Plasma Silbione M&P Primer Plasma + Primer

Thermoplastic Polyurethane Substrates

Carbothane PC 3572 D Cohesive Cohesive Cohesive Cohesive

Pellethane 2363-75D Cohesive Cohesive Cohesive Cohesive

Pellethane 2363-90A Adhesive Adhesive Cohesive Cohesive

Tecothane 1075-75D Adhesive Cohesive Cohesive Cohesive

Tecothane Soft AR62A Adhesive Adhesive Cohesive Cohesive

Tecoflex RG-80A Adhesive Adhesive Cohesive Cohesive

Metallic Substrates

Aluminum Adhesive Cohesive Cohesive Cohesive

Stainless Steel Adhesive Cohesive Cohesive Cohesive

Titanium Adhesive Cohesive Cohesive Cohesive

Silicone Substrates

Silbione HCRA 4150 Cohesive Cohesive Cohesive Cohesive

Elkem Silicones l www.silicones.elkem.com

Elkem Silicones l www.silicones.elkem.com5

Versatile Innovative Adhesive for Medical ImplantsSilbione®

CONCLUSION

The three hurdles to advancements in the development of new neurological medical devices; bondinga wide range of substrates, optimized manufacturing processes, and following regulatory guidelinesand trends are significantly lowered with the introduction of Silbione Biomedical ADH1 M200. In thepeel tests conducted, each substrate was prepared in a manner that lead to cohesive failure; showingthe bond between adhesive and substrate is stronger than the adhesive itself. Results were achievedby curing the adhesive at room temperature and ambient humidity compared to other technologieswhich require costly ovens an humidity chambers to cure.

Manufactures now have an option for a fast and relatively simple processing set up that requires lessenergy than using silicone adhesives currently on the market. The experiment results and productproperties have demonstrated substituting a new catalyst for commonly used tin catalysts causes nonegative impact. In fact, as tin comes under increased scrutiny from regulatory institutions, a tin freeformulation is desirable. With the hurdles to development lowered, increased innovation in theneuroengineering field is imminent. The time where neurological devices can be used to treatAlzheimer’s, obesity, sleep apnea, and so many more diseases is closer than ever.

International Neuromodulation Society. (2018). Retrieved from https://www.neuromodulation.com/about-neuromodulationMedical Device and Diagnostic Industry. (2018). Retrieved from https://www.mddionline.com/neuromodulation-20-enabling-next-generation-device-based-therapies-treating-chronic-diseases

Elkem SiliconesElkem Silicones is one of the world’s leading fullyintegrated silicone manufacturers with applicationsand research laboratories, production sites and salesoffices located around the globe.At Elkem Silicones, we’re more than just high qualitysilicone products and associated services. We are a teamof professionals located around the globe ready to provideyou with the service and performance you deserve witha personal touch.From technical support to customized formulations andregulatory support, Elkem Silicones has the people inplace when and where you need them, committed to helpyou deliver your potential. Because we care!

The information contained in this document is given in good faith based on our current knowledge.It is only an indication and is in no way binding, particularly as regards infringement of or prejudiceto third party rights through the use of our products. Elkem Silicones guarantees that its productscomply with its sales specifications. This information must on no account be used as a substitute fornecessary prior tests which alone can ensure that a product is suitable for given use. Determinationof the suitability of product for the uses and applications contemplated by users and others shall bethe sole responsibility of users. Users are responsible for ensuring compliance with local legislationand for obtaining the necessary certifications and authorizations. Users are requested to check thatthey are in possession of the latest version of this document and Elkem Silicones is at their disposalto supply any additional information.

For detailed commercial contacts please visit our website : www.silicones.elkem.com