Pressure-Sensitive Adhesives (PSA) & Coatings :
Medical Device Applications

II. Capillary Flow Device

The electrochemical sensor as shown in fig -11 shows why hydrophilic and hydrophobic is crucial. In this sensor, there is a capillary device that transports blood from one side to the sensor. And this is a typical sensor where there is a prick that smears on the finger and then the blood enters into the bio-sensor site. The central layer is a pressure sensitive adhesive that is holding the bottom and the top layer. Adhesives Research Inc. supplies the central layer which forms the capillary. The top layer is hydrophilic form. The company also supplies this hydrophilic coating part which is very important part of this device.

Fig. 12: Capillary Flow Device
 

III. Microfluidic Device

The above fig shows the blow out view of microfluidic device. There is a bottom layer, spacer, adhesive layer and the top hydrophilic tape. As shown in fig. 12, there is a capillary channel which is made using hydrophilic tape and a hydrophilic spacer. As you place a drop of liquid on the tape and if it is made of hydrophobic layer it will not move and if it is made of hydrophilic then the drop wicks in within a matter of fraction of second from one point to another.

Fig. 12: Microfluidic device

V. Electrically Conductive Adhesives & Films

Electrically conductive tapes are very important as they can provide a means to make a fully integrated circuit and these tapes acts to complete electrical circuit. These tapes are highly conductive and conductivity is in the terms of milliohms with low volume resistance and surface resistance. These films can be constructed to be isotropic which means that it can conduct equally in x, y and z direction or they can be made isotropic where there is no conductivity in x and y direction but only in z direction. These films are available in single- sided , double-sided and transfer films.

Technology

a) Carbon filled Adhesive :

The technology generally is filling the adhesive with special kind of carbon particles using a variety of base polymers or they can be cross-linked or uncross- linked.

b) Carbon and Metal filled Adhesives

They can be filled with a combination of carbon and metal.

c) Conductive film coated with conductive Inks e.g., Ag/AgCl

• Specialized formulations with compatible inks

• Un-cross linked

Porous Adhesives: Introduction
 

Generally adhesive would be a solid layer of a film. In porous adhesive there are pores in a solid layer of film done with the help of drilling. Fig -14, shows cross-section view of Porous Adhesive where as a indicate pores and b indicates wall of the pores. Fig-13 shows the Porous Adhesive which indicates the pores within the adhesive.

Fig. 13 : Porous Adhesives

Importance of Porous Adhesives

a) Vertical flow

When there is a requirement of movement of liquid from one layer to another layer , then the solid layer will not allow any of the liquids to flow through.

Fig. 14: Porous adhesive (Top View)

So the porous adhesive is not only use to bond different layers but also to allow different materials to flow through.

b) Membrane bonding

• Filter bonding

• Customizable based on filter material

c) Advanced wound care

Porous Adhesives are used in Advanced Wound Care where people want to manage moisture in wound, to remove the exudated and also to deliver therapeutics, proteins, and growth factors into the wound. So here it does two way communication from out of the wound and also the material inside the wound.

d) PCR and culture plates

Because of the pores there is very high MVTR and hence it allows very easy exchange of oxygen and carbon-dioxide so porous adhesives are used in PCR and culture plates

e) Biphasic structure: hydrogel filled pores

In this one can fill the pores with the hydrogels and have it conductive in one direction.

Microplate Sealing

Microplates Sealing are used for high through put drug discovery applications, cell culture plates, micro arrays where contamination from well to the equipment is not desirable. At the same time most of these techniques is used in optical detection method, generally U. V. Fluoroscence type of method or optical visible fluorescence type of method and the layer on top has to be an low fluorescing layer in these applications. Graph - shows the low fluorescence of some of the material.

Dissolvable Film Technology

The dissolvable film technology is a water soluble matrix which incorporates an active agent. And this agent can easily be release by dissolution of water or aqueous medium or some other solubilising fluid. And this release can be either immediate or controlled in terms of sec, minutes, hours or a day. This technology is finding lot of application in industry.

Applications:

a) The film can be incorporated into the device design. The dose can be premeasured into the film because it can be die cut; it can be handled like any free standing film so depending upon what size one cuts the dose is premeasured.

The dissolvable film allows for precise containment of a reagent so one have to not mess with the liquid or other forms of materials. And it also provides stability to the reagent because it is in dry form instead of being in liquid form.

b) It can also be used as isolation barriers so if there is a dissolvable film that’s separates two chambers of liquid depending upon what dissolution time is designed within that period the layer in between can get dissolved and the two reagents can get mix and the chemistry can be initiated at that time.

c) Dissolvable films can be used to make a layered configuration whether in a horizontal or vertical flow with separation of chambers.

 

Summary of Different Technologies
No.	Adhesive Technology	Applications
1.	Optically transparent adhesive - Low birefringence - Low autofluorescent adhesive - High light transmission	Biotech Applications (Microplate, Microfluidics)
2.	Breathable tapes, non-functional adhesive systems - High gas (O2/CO2) exchange - High/Low moisture barrier (MVTR)	Woundcare, Cell Culturing
3.	Conductive Adhesives & Films (electrical)	Biomedical Electrodes
4.	Dissolvable Coatings & Films	Novel Storage & Delivery Platform
5.	Clean Adhesive systems, Tight thickness tolerance	Skin contact, Biosensors
6.	Hydrophobic/hydrophilic adhesives with controllable surface energy - functionality	Lateral Flow
7.	Hybrid PSA with polar & non-polar characteristics for different substrates. This is very high bond material & bonds very well to the skin.	Compound Storage
8.	UV & thermal cure, PSA & non-PSA (hard systems), it sturts at as PSA & then after by either applying heat or U.V. to it, it turns more hard & it tends to be more on structural adlesive side.	Assembly of Lateral Flow
9.	Heatseals - permanent/removable bond his is not a PSA to start with but it is available in tape form. By applying heat to it melts and can be press into a bond.	Non-PSA (tack-free)
10.	Porous Adhesives	Vertical Flow, Woundcare
11.	Hydrogels, Dissolvable PSA’s	Woundcare, Diagnostics

Important Aspects for selection of right Pressure Sensitive Adhesive (PSA)

• Functionality requirement

• Materials and environment

• Physical properties (adhesion, shear, tack)

• Added functional properties (conductivity, permeability, etc.)

• Safety Testing

• Toxicity, sensitization, irritation

• Stability and Compatibility

• With reagents, drugs, biomaterials and tissues

• Sterilization processes

• Compliance issues : cGMPs, FDA & ISO

Summary :

Adhesive Tapes and Coated Films offer advantages to the design of diagnostic devices through:

• Customizable formulations that exhibit specific performance characteristics

• Variety of formats allowing for flexibility in design of device architecture and functionality

• Ease of handling/use in manufacturing and assembly processes

(This article is based on presentation given during the conference on, “Indian Medical Devices & Plastic Disposables Industry 2008”, at AMA, Ahmedabad , Feb 24- 25, 2008)

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