Technology

POLYMERIC PRESSURE SENSITIVE ADHESIVES FOR MEDICAL APPLICATIONS

-By Mr. P A Mahanwar & Ms. Farhana Yasmeen, University Department of Chemical Technology, Matunga, Mumbai 

[The last issue, had covered the preliminary aspects introducing pressure sensitive adhesives and some typical formaulations ] ....Click to read

2. ACRYLIC POLYMERS

Acrylic polymers currently dominate the pressure sensitive medical market, mainly because of their low level of allergenicity. 

Pressure sensitive acrylic adhesives may be prepared by either solutionor emulsion polymerization.  It is generally thought that solution polymers yield somewhat better adhesives4.  A majority of the skin contact applications have listed acrylics as the most frequently used adhesive type.  Acrylic PSAs designed for skin clean removal, low odour, low rates of mechanical and chemical irritation and acceptable resistance to cobalt and ethylene oxide sterilizations.

3. SILICONE ADHESIVES

Silicone pressure sensitive adhesives used in industrial applications are generally not suitable for use in medical applications, and vice versa.  Also, because very high cohesive stength is not required for most applications, these adhesives are cured simply by solvent removal and not further cross-linked with catalysts like benzoyl peroxide.  In the medical field, silicone pressure sensitive adhesives are used primarily in skin applications and particularly, in transdermal drug delivery systems (TDDS).  TDDS requires the use of release liner to protect the adhesive layer prior to applications to the skin.  Medical-grade silicone pressure sensitive adhesives can be release from liners much more easily than their industrial counterparts, as they are formulated to be less aggressive.  Typical adhesive properties and release value for a medical and are industrial pressure sensitive adhesive are shown in Table 3.

One of the most important properties of an adhesive to be used in medical applications is biocompatibility.  Medical grade silicone pressure sensitive adhesives are essentially free of catalysts, organic plasticizers and tackifiers.  Although the permeability of the patch blocking plays an important role, silicone pressure sensitive adhesives are formulated to adhere to the skin for extended periods of time and can be easily removed without discomfort.5.

CHARACTERISATION OF PRESSURE SENSITIVE ADHESIVES  

There are two ways to look at pressure sensitive adhesive characterization :

i) Application view point :

Most of the information needed for product formulation can be obtained though product specifications, mechanical testing and end use of properties.  Measurements such as viscosities, stress\strain and peel tests are commonly used to great advantage.  ASTM procedures have been put forward for many of the standardized test.  Highly sophisticated methods, such as rheology and mathematical modeling, have been employed with considerable success.

ii)Coposition view point :

It consists of chemicals, each of which possesses a unique structure that can be related to some specific properties.  Thus, structural characterization can be carried out on the entire adhesive or individually on the components of the adhesive.

In the past few years, analytical chemistry has under gone major changes.  Developments in electronics and computer technology have transformed analytical thinking and introduced new techniques.

The trend is towards highly sophisticated instrumentation that provides very specific and detailed information.

NMR : As an illustration of these techniques, and adhesive system consisting of an acrylic resin and rosin ester is analysed.  The 13 C NMR spectrum of the entire adhesive, dissolved in d - CHCI 3 , interpretation of the spectrum requires a prior knowledge of the spectral feature of, adhesove cpmponents.

FRIR : An FTIR spectrum of this same adhesive corroborates the finding.  Thus ester functionality are shown by the C=0 scattering frequency 1470 cm -1 and the C-0 bending modes at 1200 cm -4 .  The rest of the spectrum consist of the C-H strech (at 2900 cm -1) and various C-H modes (1400 - 1500 cm -1 ).

UV : The UV spectrum of the same adhesive, dissolved in methylene chloride, shows a distinctive pattern again is characteristic of Foral esters.  I general, UV is an excellent method to quantify the levels of rosin derivatives.  In addition, UV can be used to detect and quantify antioxidants stabilizers.

MICROSCOPY: The some adhesive can also be examined for its physical structure Microscopy is the most direct and the most often used technique.  A transmission electron microscopy graph of the adhesive.  The adhesive had been stained with ruthenum oxide and thin sectioned.  The micrograph definitely indicated two phases, suggesting incompatibility of the two components.  

X-RAY DIFFRACTION: An X-ray diffraction pattern of the adhesive surprisingly showed some broad features at 20 values of 60 and 180 .

Although the adhesive is nominally amorphousm some structural order is apparently found in the sample to give rise to these maxima in the diffraction pattern.  Such molecular order would have and effect on the mechanical strength of the adhesive.

The sample has also been characterized by ESCA (electron spectroscopy for chemical analysis).  In this technique only the topmost 50-100 angstroms of the surface are studied.  The carbon spectral region indicates two bonds, one centered at 288e Vand corresponding to the C=0 (ester).  The rest of the carbons are found in the broad bond centered at 285 eV.  The result agrees with those of equal intensities.  This feature is characteristic of esters containing two different oxygen : -C (0) -0.  The result agrees with those of NMR and IR, although we are looking only at the topmost surface in this case.  The ability to examine the chemistry of the surface (where adhesive occurs) is a powerful feature of surface analysis techniques).6

FACTORS INFLUCING THE PERFORMANCE OF PSAs

The major biological factor to be considered while formulating PSAS for medical applications such as tapes, patches etc is skin irritation.

There are several causes of skin irritation :

1. The tape construction
2. Backing
3. Adhesive

Mechanical irritation :

Mechanical irritation may be caused by traping stigg tape over the skin which is, frequently flexed.  The skin is irritated by the continuous pull of the adhesive.  Usually, however, the mechanical irritation is caused mostly by the tape removal.

Occlusion is an environmental alteration which overhydration dascerates, and softens the outer layers of the skin as a result of the accumulation of fluid.

Chemical irritation :

It is caused by the diffusion of the chemica lirritants present in either the adhesive or the backing, into the skin tissue.

Allergic irritation :

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