Medical Device Link.

Originally Published IVD Technology January/February 2003

Product Development

Using infrared imaging and atomic force microscopy enhances surface analysis of hydrophilic heat-seal films.

Herbert M. Hand Sr., Peter Gabriele, and David M. Schaefer

Figure 1. A conceptual view of the surface-wetting phenomenon. (click to enlarge)

Tapes with inherently good surface-wetting characteristics are needed to aid in the wicking of blood, serum, sputum, and other biological fluids in diagnostic test strips. These single- and double-faced hydrophilic adhesives and coated-tape products play an important role as components in in vitro diagnostic biosensors. For example, they are used as support components in lateral-flow devices by joining membrane and conjugate pads with a rigid, low-energy substrate.

The hydrophilicity of these tapes reduces the surface tension of biological fluids, allowing rapid transfer from an inlet area to a remote reagent area. In capillary designs, the tapes are often used as roof components, with the hydrophilic side facing into the channel. As double-faced constructions, hydrophilic tapes are used to create high-energy channels between cover and base components. Hydrophilic pressure-sensitive adhesives and heat-seal adhesives offer the added benefit of bonding to a variety of low-surface-energy substrates, thereby providing a seal to maintain a constant-volume sample area.

Figure 2. An atomic force micrograph of a hydrophilic coating.

To meet the growing needs of IVD manufacturers, proprietary hydrophilic adhesives are being made by matching polymer resin chemistries and surfactants, resulting in a dual bonding-and-wetting quality.¹ Unlike hydrogel coatings and other coatings with interpenetration polymer networks that exhibit bulk hydrophilic properties, these proprietary hydrophilic adhesives contain active surfaces that reduce tension in aqueous fluids at the surface. The result is ease of wetting, thus allowing for rapid transport of biological fluids across a surface (see Figure 1). This property overcomes the need for surface modifications, such as corona treatment, acid etch, or anodization, that are subject to time-related stability issues and higher costs due to complex production processes.

Figure 3. The prominent peak at 2958 cm–1 in the ATR spectrum is assigned to the C-H stretch of a CH3 group on the surfactant additive in the hydrophilic adhesive and is used to monitor the additive accumulation in FTIR images. (click to enlarge)

This article examines the surface behavior of hydrophilic adhesives formulated using polymer resins and surfactants. Several proprietary coatings have been developed using a variety of polymer and surfactant chemistries that induce the spontaneous spreading of biological fluid.¹ The introduction of surfactants into a liquid polymer matrix is believed to lead to saturation at the film-air interface, which is caused by surfactant migration during the evaporation process. The hydrophilicity of the adhesive surface is controlled through surfactant chemistry and concentration, as well as distribution of the surfactant on the surface of the dried coating.

This article also describes the use of atomic force microscopy (AFM) and infrared imaging techniques to provide submicron-level resolution of the surfaces of hydrophilic adhesive constructions. While wetting contact angle is used to characterize the surface energy of prepared films at varying degrees of hydrophilicity, and Fourier transform infrared spectroscopy via attenuated total reflection (FTIR-ATR) for measuring chemical differences, both of these techniques are limited to measuring only gross differences at the surface of the film. However, observation and analysis using AFM and infrared imaging provide investigators with a well-defined picture of the surface. AFM further identifies the effect of surfactant loading on the surface, while infrared imaging provides spatial resolution of surfactant distribution on the hydrophilic surfaces.

Figure 4. A plot of the absorbance of the C-H stretch as a function of concentration of the additive shows a curve flattening resulting from surface saturation of additive with increasing concentration. (click to enlarge)

Together, AFM and infrared imaging are effective techniques that support the physical information related to the surface-wetting behavior of hydrophilic films. Device designers, investigators, and quality control engineers can utilize these methods to ensure consistency of the prototype device during the preclinical stage, and the end product in the manufacturing environment. These methods can ascertain important information on the physical and chemical functionality of surfaces that results from material-handling issues as supplied by or as a function of stepwise converting processes. For hydrophilic coatings and adhesives, traditional techniques such as contact angle may also be used to measure surface changes. However, while contact angle provides a good first look at gross surface differences, AFM then refines the images of the physical surface in question to the micron and submicron levels.

In essence, AFM provides a microscopic snapshot of surface features, supplementing the gross findings offered by the contact angle method, and may be useful in situations in which surface deterioration results from transport handling, process handling, storage issues, etc. Infrared imaging, meanwhile, takes measurement a step further by providing information on chemical functional groups related to hydrophilic-hydrophobic properties and other properties of the material in question. This technique could be particularly useful when measuring chemical changes related to temperature, atmospheric, or other environmental effects resulting in gross and microscopic surface contamination. Consequently, AFM and infrared imaging are useful for not only basic research, but also for general materials evaluation.

Developments in Surface Analysis

Figure 5. A comparison of AFM peak height as a function of additive concentration and wetting angle. (click to enlarge)

The technology described in this article attempted to link previous work done in this area by other researchers.^1,2 One researcher described the wetting behavior of biological fluids on hydroactive surfaces of thin-film coatings and combined several classical theoretical models to relate capillary flow to surface-wetting behavior. Spontaneous spreading could be achieved by film formulations containing surfactant chemistries within a polymer matrix that effectively reduces the surface tension of aqueous liquids, and therefore could be done by formulating proper polymer resin and surfactant chemistries.

In addition to using contact angle and FTIR-ATR to describe gross physical and chemical differences on the surfaces of several hydrophilic films, this researcher used AFM to describe microstructural changes to surface morphology as a result of surfactant loading (see Figure 2). Alteration of the film surface at the film-air interface was observed with an increasing amount of surfactant in the adhesive formula. While this transformation was observed at low surfactant loading, during which raised surface features were noticed on the film surface, increased surface topography was observed as well at higher surfactant loading. Another investigator also looked at surfactant exudation in polybutylmethacrylate films by using AFM to visualize topographical changes in which the exudates were described as "hilly islets, and, at times, mountains" on the film surface.³

While previous studies have looked at the migration of surfactant molecules in latex films, recent studies have used FTIR-ATR techniques to measure spectral vibrations related to fugitive chemistry on the surfaces of films. One investigator purported enrichment of surfactant at the air-surface interface in coalesced latex films as a result of the evaporative process.⁴ In this study, introduction of a large amount of surfactant resulted in saturation coverage of the surface. This investigator also used x-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS) to examine the enrichment of anionic surfactants on the latex surface.⁴

Figure 6. A comparison of typical surfaces of the hydrophilic adhesive. FTIR image of the surface (a) and AFM surface (b) for the polymer adhesive base without the additive. Note the low intensity (blue color) representation for the C-H stretch (a), and the smooth look to the AFM surface (b). FTIR image (c) and AFM (d) for typical surfaces at the 5% additive level. The raised-surface feature suggests saturation of the surface. (click to enlarge)

Another researcher used FTIR to measure surfactant exudation on the surfaces of latex films at the air-film and film-substrate interface.^5–7 This researcher envisioned surfactant molecules oriented at the film-air interface, thereby altering properties such as wettability and adhesion. Other studies used FTIR-ATR to investigate the distribution and mobility of surfactant in hydrophilic adhesives containing sodium didecylsulfo succinate. Absorption peaks for sodium didecylsulfosuccinate were observed in the fingerprint region of the spectrum at wavelengths that correspond to the methyl stretch at 2958 cm^–1 and the sulfur-oxygen vibration at 1049 cm^–1. The disappearance of both the methyl stretch and sulfur-oxygen vibration was observed during washing.

Other recent studies combined infrared imaging with AFM to describe aggregate accumulations of functionally active material at the surface of polyisobutylene-resin ester blends. The loss of tack was related to the ratio of resin to tackifier, and described by the appearance of submicron structures on the film surface.² Spatial resolution of the surface chemistry was achieved in close proximity to AFM to link functional-group chemistry with physical appearance.

This notion has been expanded to translate physical topography into surface functional-group chemistry in hydrophilic adhesive systems. As micron and submicron accumulation of aggregate structures relates to resolution by AFM, surface functional-group vibrations are related to chemical topology as seen through infrared imaging. Unlike a traditional FTIR spectrum, contrast in infrared imaging is a spatially defined image of functional-group intensity displayed as a single wave number, or frequency, plane in a false-color scheme.² For example, the color blue indicates low functional-group absorbance intensity, green an intermediate absorbance intensity, and red the highest absorbance intensity. However, FTIR images must not be interpreted as being representative of physical topography, but rather conceptually as chemical topology.

Experimental Process

The purpose of this article is to describe the combination of AFM and FTIR imaging and how it complements surface analysis of hydrophilic adhesive systems and to provide insight into the relationship between structure and wetting behavior. This article characterizes topography and surface chemistry spatially as a result of aggregate accumulation on the film surface. Hydrophilic coatings were investigated using FTIR imaging and AFM to characterize the surfaces by observing microstructural changes as a function of surfactant concentration. While AFM was used to spatially resolve hydrophilic surfaces in physical terms by relating surfactant loading to aqueous wetting, infrared imaging was used to spatially resolve chemical microstructural changes on the surface related to surfactant chemistry.

Hydrophilic Adhesives Preparation. The hydrophilic adhesives were prepared in a laboratory with sodium dioctylsulfo succinate (SDOSS) surfactant loading concentrations of 0%, 1%, 5%, and 10%. Dissolution of a polymer resin in organic solvents was followed by measuring solution solids and viscosity during several hours of mixing and introducing SDOSS into the liquid polymer mixture. Gentle agitation for several minutes was sufficient to achieve homogeneity in the polymer mixture.

Hydrophilic Film Preparation. The hydrophilic films were prepared in a laboratory using a coating apparatus that evenly spreads the liquid adhesive on a film backing. The liquid adhesive was deposited onto a film backing that was drawn through two stainless-steel bars until the adhesive solution spread across the film to produce a coating of even thickness. The cast films were then dried for 5–10 minutes in a convection oven set at 105°C. The dried coatings had an approximate thickness of 12.7– 25.4 µm and were protected with a low-surface-energy film substrate, or release liner.

Infrared Analysis. The initial chemical surface analysis of the hydrophilic coatings was conducted using infrared spectroscopy via ATR. The spectra were recorded using a single-bounce sample port unit with a zinc selenide (ZnSe) crystal. Film samples were compressed onto the crystal using a compression arm at full contact pressure. While infrared spectra were collected using an FTIR bench with a mercury-cadmium-telluride (MCT) nitrogen-cooled detector, absorbance spectra were collected from 30 scans per sample from 4000 cm^–1 to 600 cm^–1 at 2-cm^–1 resolution.

Contact-Angle Testing. The hydrophilic coatings were tested for surface wetting using deionized water. A ramé-hart contact-angle goniometer was employed to measure the contact angle of water on the surface of the hydrophilic film. A 2-µl drop of deionized water was suspended on the tip and lowered toward the film until the water drop dispersed and spread across the surface. Once equilibrium was established in 30 seconds, the contact angle was read directly from the goniometer scope reticle at the six o'clock position and recorded to the nearest degree on both sides of the spread water drop.

Examining Surface Topography with AFM. The surface topography of the hydrophilic coatings was observed using AFM. The hydrophilic tapes were mounted onto 1-cm-diameter magnetic stubs and imaged in the tapping mode. Using this mode, the AFM cantilever oscillated at its resonant frequency, with contact between the oscillating tip and the tape surface causing a decrease in the measured amplitude of oscillation. Since contact is made at the largest displacement from the cantilever equilibrium position, little energy is transferred to the sample, and deformation of the sample is minimal. Images were obtained by raster scanning the sample surface under the tip and recording the z motion of the sample necessary to maintain constant amplitude during the scan.

This mode of imaging has several advantages over direct-contact-mode imaging. Lateral forces that are prevalent during contact-mode scans are eliminated. Additionally, tapping mode provides a nondestructive method for imaging soft samples. Once all samples were initially scanned in the air, the hydrophilic coating was rinsed with deionized water for 10 seconds, and wiped dry with a paper tissue. The sample then dried overnight and was imaged the next morning.

Infrared Imaging. FTIR imaging of the hydrophilic surfaces was performed on an imaging spectrometer system that was coupled to a 64 x 64-pixel MCT focal-plane-array detector. The spectrometer collection setup had 16-cm^–1 resolution, mirror velocity 5 Hz, 513 steps, and 20 images per step. A gold mirror was used for background collection, and the infrared imaging data were recorded over a 400-µm-square area. Spatial specificity followed the absorption of the methyl group vibration at 2958 cm^–1 associated with the surfactant chemistry, and was presented in the false-color scheme described above.

Experimental Results

Figure 7. A comparison of hydrophilic adhesive model false-color FTIR images and AFM. AFM (a) and FTIR image (b) are expansions of the 10% surface to show the apparent raised-surface features and craterlike topography captured by each technique (see arrows). (click to enlarge)

The FTIR-ATR data from these experiments demonstrated that the hydrophilic surface is altered as surfactant concentration increases (see Figure 3). Based on this data, a plot of methyl absorption at 2958 cm^–1 versus the concentration of surfactant in the polymer resin shows that as SDOSS content increases, the methyl absorption peak associated with the surfactant levels off (see Figure 4). This is a gross, average surface phenomenon that occurs at the immediate surface. The actual ATR infrared beam penetration is approximately 3–5 µm deep for a ZnSe crystal at 2958 cm^–1. The flattening response is the result of aggregate accumulations of SDOSS at the surface as SDOSS percentages are increased, which is consistent with decreases in wetting contact angle.

With each progressive accumulation of surfactant at the air-surface interface, cross-sectional views were developed and analyzed for each SDOSS-loaded hydrophilic film, and measurements of these films were taken at different surfactant-concentration levels. A plot comparing the average peak height of the film terrain and wetting angle as a function of SDOSS concentration showed an inverse effect (see Figure 5). This inverse effect is supported by the FTIR-ATR study that suggests surface saturation with increasing concentration of SDOSS, and by the functional-group chemistry presented in the false-color schemes of the infrared imaging.

Wetting is ameliorated with increasing surface topography caused by increased loading of SDOSS. When no surfactant is present, the surface of the coating appears to be smooth with little topography. With increasing surfactant in the adhesive formulation, the surface becomes rougher.³ Using AFM, the height of the cross-sectional features demonstrated an increase from 0 nm at 0% SDOSS to approximately 5 nm at 1% SDOSS. Increasing amounts of SDOSS also resulted in a buildup of the surface features. In addition, the corresponding effect of surfactant concentration on the water contact angle of the film surface shows a relatively hydrophobic surface at 0% additive.

These experiments presented chemical information supported by infrared imaging and the corresponding topology changes by AFM. The transformations are captured at the micron level in the respective FTIR images and at the submicron level in the AFM images collected from identical surfaces (see Figure 6). These images indicate an increasing methyl group functionality associated with SDOSS accumulation on the surface. These changes are described by false-color images defining the spatial resolution of the methyl C-H stretch at 2958 cm^–1, with the color change from blue to yellow indicating higher absorbance values for the spatially resolved C-H stretch. These chemical topology features are supported by the respective physical topography observed by AFM, showing a similar pattern of surfactant aggregate accumulation.

The complementary quality of infrared imaging is reinforced by enlarging the images of the saturated hydrophilic surface containing 10% SDOSS (see Figure 7). The apparent raised-surface features and craterlike topography in AFM is captured as well in the infrared false-color image. This is also supported by cross-sectional measurements showing increasing peak height of the microrough surface as more surfactant is added to the system. Such phenomena may be a result of the aggregate accumulation during film formation and may be exacerbated by flux conditions during solvent removal. Nevertheless, the perfect geometric shapes of the stalagmite-like structures may be of interest in future studies.

Figure 8. A comparison of unwashed and washed surfaces of the hydrophilic adhesive. FTIR image (a) and AFM (b) show the 10% surface unwashed. FTIR image (c) and AFM (d) show the same sample surface following an aqueous rinsing. (click to enlarge)

AFM and infrared imaging are also useful in applications in which aggregate accumulations on the surface are the result of a secondary phase comprised of fugitive chemistries not chemically bound to a continuous matrix, such as an adhesive or polymer coating. Both methods provide unique snapshots of surface and chemical architecture as it relates to the coating and drying process, where aggregation or migration is affected by parameters such as thickness, or temperature changes affecting the bonding interface.

For example, in the case of polymer films containing surface-active agents, it appears that the SDOSS surfactant is loosely bound to the matrix. Disappearance of surfactant chemistry and different surface topography is evident after thorough washing with deionized water. This experiment presented a comparison of FTIR imaging and AFM of unwashed and washed resin-surfactant hydrophilic surfaces (see Figure 8). The disappearance of the methyl functionality as shown in the false-color image is evident after aqueous washing, and the AFM shows the loss of surface features that were clearly evident before washing.

Conclusion

When combined, infrared imaging and AFM analytical techniques complement surface analysis of hydrophilic adhesive systems and provide insights into the relationship between structure and wetting behavior. These techniques provide physical and chemical information not available through contact angle analysis and traditional infrared techniques to characterize microchemical and microstructural surface changes. In the hydrophilic films examined in this study, increased aqueous surface wetting correlated to gross chemical differences on the surfaces of the dried films. AFM observations detected microstructural changes on the film surfaces with increasing surfactant loading. Likewise, functional-group spatial resolution to surfactant aggregate accumulation chemistry was observed using infrared imaging, targeting the methyl stretching vibration at 2958 cm^–1.

This study showed how effective combining infrared imaging and AFM techniques is for revealing the surface phenomena of homogeneous and heterogeneous films to assign chemical functionality to physical topography. With this knowledge, surface properties can be characterized and customized to meet the bonding and surface-energy needs of IVD manufacturers.

References

1. WG Meathrel, HM Hand Sr., and LH Su, "The effects of hydrophilic adhesives on sample flow," IVD Technology July/August (2001): 56–65.

2. P Gabriele et al., "Infrared Imaging: A Complementary Tool to AFM for Adhesive Surface Analysis," Adhesives Age June (2001): 20–30.

3. D Juhué et al., "Surfactant Exudation in the Presence of a Coalescing Aid in Latex Films Studied By Atomic Force Microscopy," Journal of Polymer Science: Part B: Polymer Physics 33 (1995): 1123–1133.

4. CL Zhao et al., "Surface Composition of Coalesced Acrylic Latex Films Studied by XPS and SIMS," Journal of Colloidal and Interfacial Science 128, no. 2 (1989): 437–449.

5. KW Evanson and MW Urban, "Surface and Interfacial FTIR Spectroscopic Studies of Latexes. I. Surfactant—Copolymer Interactions," Journal of Applied Polymer Science 42 (1991): 2287–2296.

6. KW Evanson, TA Thorstenson, and MW Urban, "Surface and Interfacial FTIR Spectroscopic Studies of Latexes. II. Surfactant—Copolymer Compatibility and Mobility of Surfactants," Journal of Applied Polymer Science 42 (1991): 2297–2307.

7. KW Evanson and MW Urban, "Surface and Interfacial FTIR Spectroscopic Studies of Latexes. III. The Effects of Substrate Surface Tension and Elongation on Exudation of Surfactants," Journal of Applied Polymer Science 42 (1991): 2309–2320.

Herbert M. Hand Sr. is a scientist in the R&D department of the medical business unit, and Peter Gabriele is an R&D group leader at Adhesives Research Inc. (Glen Rock, PA). They can be reached via hhand@arglobal.com and pgabriele@arglobal.com, respectively. David M. Schaefer, PhD, is an associate professor in the department of physics, astronomy, and geosciences at Towson University (Towson, MD). He can be reached via schaefer@towson.edu.

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