Surface Characterization of Materials for Blood Contact Applications

2 Surface Characterization of Materials for Blood Contact Applications BUDDY D. RATNER

Downloaded by EAST CAROLINA UNIV on March 8, 2017 | http://pubs.acs.org Publication Date: July 27, 1982 | doi: 10.1021/ba-1982-0199.ch002

University of Washington, Department of Chemical Engineering, Seattle, WA 98195

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n recent years the surface characterization of biomaterials has been forcefully emphasized (1-3). Unfortunately, a clear understanding of how

surface characterization can be of value to biomaterials research, development, and production has, in many cases, not been realized. This chapter addresses the subject of surface characterization of biomaterials by considering three aspects of the problem: first how surfaces differ from the bulk of materials; second, how the important parameters of surfaces can be

measured and what new techniques might be developed; and finally, how surface characterization can help in understanding and predicting the bio

-compatibility (and in particular, the blood compatibility) of synthetic materials.

Unique Properties of Surfaces The surfaces of materials are almost always different in structure and chemistry from the bulk or interior of the materials. These differences result from surface contamination (a consequence of surface chemistry), molecular orientation, and surface reaction. The driving force for this surface/bulk differentiation can be explained (at least for the first two factors) by considering surface energetics from a thermodynamic standpoint—the interfacial energy of any system tends to be reduced. Examples will be presented to show how each of these three factors can alter the nature of a surface. Surface contamination is ubiquitous and almost unavoidable. Even in ultrahigh vacuum environments, the question is not if a surface will become contaminated, but when. For high-energy surfaces, such as metals and inorganics, the driving force for reducing the interfacial energy is extremely high. A "clean" metal surface will recontaminate with a monolayer of organic material in a vacuum environment at 10

-6

Torr in approximately 1 s. Con-

tamination at the monolayer or multilayer level will occur essentially instantaneously in a laboratory environment at atmospheric pressure. 0065-2393/82/0199-0009$06.00/0 1982 American Chemical Society Cooper et al.;© Biomaterials: Interfacial Phenomena and Applications Advances in Chemistry; American Chemical Society: Washington, DC, 1982.

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BIOMATERIALS: INTERFACIAL P H E N O M E N A A N D APPLICATIONS

Surface c o n t a m i n a t i o n can take a n u m b e r of forms. W e are s u r r o u n d e d by a c o m p l e x m i x t u r e of h y d r o c a r b o n molecules as a consequence of b o t h o u r industrial a n d natural e n v i r o n m e n t s . L o w surface energy silicones (e.g., v a c u u m greases a n d p u m p oils) are c o m m o n l y present i n the laboratory environment.

S i l i c o n e c o n t a m i n a t i o n is particularly troublesome

and is

f o u n d on m a n y surfaces because few materials have surface energies lower


than silicone c o m p o u n d s . P o l y m e r s have significantly l o w e r surface free energies than metals or inorganics. T h e r e f o r e , the d r i v i n g force to reduce interfacial energy is lower and, c o n s e q u e n t l y , c o n t a m i n a t i o n i n a laboratory e n v i r o n m e n t is slower. H o w e v e r , such c o n t a m i n a t i o n still exists for most systems. F o r materials such as Teflon, the surface free e n e r g y is v e r y l o w . A substance w i t h a surface energy l o w e r than that o f Teflon w o u l d have to adsorb to the surface to reduce the interfacial energy. Since few such substances are present i n the atmosphere, c o n t a m i n a t i o n for such fluoropolymers can be n e g l i g i b l e . Two i m p o r t a n t points m i g h t be m a d e about e n v i r o n m e n t a l l y propagated surface c o n t a m i n a t i o n w h i c h w e must, i n most cases, live w i t h . F i r s t , some level of c o n t a m i n a t i o n is u n a v o i d a b l e , but contamination b e y o n d this backg r o u n d l e v e l is unnecessary. F o r e x a m p l e , i n a study of the effectiveness of various techniques for c l e a n i n g glass, relatively stable glass surfaces c o u l d be p r e p a r e d i n an o r d i n a r y laboratory e n v i r o n m e n t w i t h surface carbon/silicon ratios r a n g i n g f r o m 0.19 to 2.2 (4). Glass w i t h a carbon/silicon ratio of 2 w o u l d be unnecessarily c o n t a m i n a t e d w h i l e glass w i t h a carbon/silicon ratio of 0.2 w o u l d be as clean as can be r e a d i l y achieved u n d e r reasonable w o r k i n g conditions. O b v i o u s l y , the l o w e r the ratio, the m o r e desirable the glass surface for s t u d y i n g b i o l o g i c a l interactions w i t h glass. T h e biomaterials scientist is responsible for r e d u c i n g c o n t a m i n a t i o n to the lowest possible levels and for i n s u r i n g that a l l specimens i n an e x p e r i m e n t (and i n later e x p e r i ments) have r e p r o d u c i b l e (low) levels of contamination. Second, even though glass, p l a t i n u m , a n d p o l y (ethylene terephthalate) m i g h t all be contaminated w i t h an a p p a r e n t l y s i m i l a r layer of h y d r o c a r b o n - l i k e material from the atmosphere, the essential p r o p e r t i e s indicative of these three substances still manifest themselves at t h e i r surfaces. T h u s " c l e a n " glass w h i c h has at least one monolayer of organic c a r b o n c o m p o u n d s at its surface is "glass-like" and not " h y d r o c a r b o n - l i k e " (e.g., " p o l y e t h y l e n e - l i k e " ) i n its interactions w i t h proteins and cells (4). T h e m e c h a n i s m s b y w h i c h the properties of a specific substance are propagated to the surface t h r o u g h contaminant films are not c o m p l e t e l y clear. S t i l l , the i n t r i n s i c properties are v i s i b l e at the surface and can, for m o d e r a t e l y clean materials, be e x p l o i t e d to study or influence b i o l o g ical systems. Molecular orientation creased attention i n r e c e n t d r i v i n g force for the surface t h e r m o d y n a m i c terms as a

at the surface of biomaterials has r e c e i v e d i n years. A g a i n , as for surface contamination, the r e o r i e n t a t i o n observed often can be e x p l a i n e d i n m e c h a n i s m for r e d u c i n g the interfacial energy.

Cooper et al.; Biomaterials: Interfacial Phenomena and Applications Advances in Chemistry; American Chemical Society: Washington, DC, 1982.

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Materials for Blood Contact Applications

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F o r p o l y m e r i c systems, c h a i n segments or p e n d a n t functional groups migrate to or f r o m the surface i n m a n y instances. T h u s , for block c o p o l y m e r s containi n g siloxane c h a i n segments, the t e n d e n c y is for these l o w - e n e r g y blocks to migrate to the surface (5). F o r o t h e r b l o c k c o p o l y m e r systems i n w h i c h the surface energies of the two blocks differ b y smaller amounts, p r e f e r r e d surface localization o f the l o w e r surface energy block can be i n f l u e n c e d by the casting solvent u s e d or b y the nature of the substrate against w h i c h they are cast (6, 7). F o r some p o l y m e r i c systems i n w h i c h a degree of chain

flexibility

exists, r a p i d c h a i n c o n f i g u r a t i o n a l alterations m i g h t occur as polar side groups Downloaded by EAST CAROLINA UNIV on March 8, 2017 | http://pubs.acs.org Publication Date: July 27, 1982 | doi: 10.1021/ba-1982-0199.ch002

o r the n o n p o l a r c h a i n b a c k b o n e s r e s p o n d to the n a t u r e of the e n v i r o n m e n t . F o r e x a m p l e , f o r p o l y ( h y d r o x y e t h y l m e t h a c r y l a t e ) , h y d r o x y l g r o u p s are a p p a r e n t l y e x p o s e d at the surface i n a n a q u e o u s m e d i u m ( l i q u i d - s o l i d i n t e r face) w h i l e the c h a i n b a c k b o n e o r i e n t s itself t o w a r d s the surface i n the d e h y d r a t e d ( g a s - s o l i d i n t e r f a c e ) s i t u a t i o n (8-10). O t h e r levels of i n d u c e d m o l e c u l a r surface organization for p o l y m e r i c systems have b e e n o b s e r v e d . F o r e x a m p l e , casting a p o l y m e r system onto a given surface p e r m i t s that surface to act as a m o l e c u l a r template w h i c h can orient the p o l y m e r chains (7, 11).

S u c h " t e m p l a t e - i n d u c e d " surface struc-

tures may relax s l o w l y w h e n the t e m p l a t e is r e m o v e d . T h e casting solvent may significantly affect the o u t e r m o s t surface of even s i m p l e polar p o l y m e r s such as p o l y ( m e t h y l methacrylate) ( P M M A ) , as experiments that u t i l i z e d inverse gas c h r o m a t o g r a p h y suggest (12). I n these studies, P M M A cast f r o m five solvents onto gas c h r o m a t o g r a p h i c supports shows vastly different specific r e t e n t i o n v o l u m e s . F i n a l l y , anisotropics i n the m o l e c u l a r chain o r i e n tation may be i n d u c e d b y casting

(13).

T h e s e a n d o t h e r r e l a t e d observations lead to a n u m b e r of conclusions and considerations c o n c e r n i n g p o l y m e r chain surface orientation. F i r s t , the nature of the surface differs, i n most cases, f r o m that of the b u l k . Second, the surface can change i n response to the e n v i r o n m e n t such that a probe of surface structure (e.g., a d r o p of l i q u i d used for m e a s u r i n g contact angles) may alter that w h i c h it was i n t e n d e d to measure. T h i r d (and related to the second item), the e n v i r o n m e n t i n w h i c h a b i o m a t e r i a l is s t u d i e d may direct the surface s t r u c t u r e — a n a p p r o p r i a t e e n v i r o n m e n t for biomaterials study is an aqueous m e d i u m . F o u r t h , surface structures may relax (and change) i n response to the e n v i r o n m e n t ; d e p e n d i n g on the kinetics of this process, i r r e p r o d u c i b l e surface m e a s u r e m e n t s c o u l d occur. F i n a l l y , the l e v e l of surface characterization (depth of penetration) suitable for b i o m a t e r i a l characterization m u s t be c o n s i d e r e d . E x a m p l e s g i v e n i n this section have d e s c r i b e d surface/bulk differentiations r a n g i n g f r o m m i c r o n s to angstroms. C o n s i d e r i n g the a b i l i t y of b u l k m a t e r i a l p r o p e r t i e s to propagate themselves t h r o u g h t h i n surface layers, to w h a t d e p t h m u s t w e analyze a surface? T h i s question may be answerable w h e n w e l e a r n m o r e about the sensitivity of proteins and cells to small c h e m i c a l p e r t u r b a t i o n s . Surface c h e m i c a l reactions are also i m p o r t a n t considerations i n t r y i n g to


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understand h o w the surface of materials differs f r o m the b u l k . C e r t a i n l y the most c o m m o n surface reaction is oxidation. A n o x i d i z e d layer exists at the surface of m a n y p o l y m e r s a n d metals. P o l y e t h y l e n e , w h i c h m i g h t be v i e w e d as the simplest p o l y m e r f r o m a c h e m i c a l standpoint, has a rather complex surface structure d u e to such oxidation. T h e carbon/oxygen ratios at the surfaces of a n u m b e r of n o m i n a l l y p u r e p o l y e t h y l e n e specimens stored i n air range f r o m 300 to 9 (14). T h e s e surfaces contain several different types of c a r b o n - o x y g e n bonds. T h e s i m p l e ( C H - C H ) „ structure that is often assigned to p o l y e t h y l e n e seems meaningless, c o n s i d e r i n g the variety of rather polar structures w i t h w h i c h cells and proteins m i g h t interact at the p o l y ethylene surface. S i m i l a r surface c o m p l e x i t y as a result of oxidation w i l l be expected for m a n y o t h e r p o l y m e r s . T h i s oxidation has b e e n e x p l o r e d i n a systematic way on o n l y a few systems.


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O t h e r surface reactions also can be anticipated. Surface acidic or basic structures can exist i n an i o n i z e d and/or neutral f o r m . T h e ratio of the i o n i z e d to neutral f o r m w i l l d e p e n d strongly on a m a t e r i a l s e n v i r o n m e n t a n d p r e vious history. A l s o , any c o m p o u n d w i t h i o n i c charges can complex or couple w i t h many ions a n d some n e u t r a l m o l e c u l e s ; even neutral p o l y m e r s can have strong, rather specific interactions w i t h ions (15). F i n a l l y , molecules such as S 0 and N H c o m m o n l y are f o u n d i n the u r b a n and/or laboratory e n v i r o n ment. S u c h reactive m o l e c u l e s can, again, c o m p l e t e l y alter the nature of a surface. O f course, some reactions are p u r p o s e l y p e r f o r m e d on surfaces to alter their p r o p e r t i e s . A large n u m b e r of grafting and c h e m i c a l reactions come u n d e r this category. S u c h reactions are d e s c r i b e d elsewhere (3). 2

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T h r e e major m e c h a n i s m s w e r e d e s c r i b e d i n this section b y w h i c h surfaces alter t h e i r c h e m i c a l p r o p e r t i e s c o m p a r e d to the b u l k : contamination, m o l e c u l a r o r i e n t a t i o n , a n d reaction. Since, a p r i o r i , the nature of a surface cannot be p r e d i c t e d because of these factors, tools are n e e d e d to study surfaces. S u c h techniques for a n a l y z i n g surface structure are discussed i n the next section.

Techniques for Surface Characterization T h e d e v e l o p m e n t of n e w methods for s t u d y i n g surfaces is progressing rapidly, p r e c i p i t a t e d b y the p h e n o m e n a l g r o w t h and interest i n surface physics and c h e m i s t r y w h i c h was s t i m u l a t e d , i n part, b y the n e e d for clean, w e l l - c h a r a c t e r i z e d surfaces for m i c r o e l e c t r o n i c and other high-technology applications. T h e biomaterials f i e l d s h o u l d be able to capitalize u p o n this plethora of n e w methods w h i c h have appeared p r i m a r i l y i n the past 15 years. In particular, m a n y of the n e w techniques measure surface c h e m i s t r y d i rectly, i n contrast to o l d e r m e t h o d s w h i c h often r e q u i r e d i n d i r e c t or t h e r m o d y n a m i c data. A t the present stage of d e v e l o p m e n t i n the f i e l d of surface analysis, a " p i c t u r e " of a surface must be b u i l t u p b y using a variety of methods. C o m b i n a t i o n s of the "classic" surface analysis methods (e.g., con-


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tact angle d e t e r m i n a t i o n ) a n d the n e w e r methods have the potential to d e scribe t h e nature o f a surface adequately f o r biomaterials investigation (1,16). Surface analysis t e c h n i q u e s are categorized i n Table I. F i g u r e 1, often called a P r o p s t d i a g r a m , illustrates the " p r o b e s " a n d e m i t t e d species that can be m e a s u r e d i n m a n y o f the n e w techniques for surface analysis. M o r e than 40 of the possible c o m b i n a t i o n s have b e e n e x p l o r e d ; many others have not yet b e e n t r i e d . F i g u r e 2 presents an approximate comparison of the various techniques w i t h respect to t h e i r d e p t h of analysis. O n l y the techniques that Downloaded by EAST CAROLINA UNIV on March 8, 2017 | http://pubs.acs.org Publication Date: July 27, 1982 | doi: 10.1021/ba-1982-0199.ch002

have shown success o r show p r o m i s e for use i n biomaterials characterization w i l l b e c o n s i d e r e d i n m o r e d e t a i l i n this r e v i e w . Table I. Surface Analysis Methods • — —

Thermodynamic Analysis C o n t a c t angle—surface energetics (17,18) B E T — s u r f a c e area

• — — — — — —

Surface Electrical Properties (1) Z e t a p o t e n t i a l (streaming potential) Faraday c u p Surface p o t e n t i a l difference Rest p o t e n t i a l Vibrating electrode Cascade d e v i c e for m e a s u r i n g t r i b o e l e c t r i c c h a r g i n g (19)

• — — — — — —

Surface Chemistry Analysis E S C A (electron spectroscopy for c h e m i c a l analysis) A E S ( A u g e r e l e c t r o n spectroscopy) S I M S (secondary i o n mass spectroscopy) I S S (ion scattering spectroscopy) E L S (electron loss spectroscopy) A T R - I R (attenuated total reflectance i n f r a r e d analysis)

• — — —

Spatially (Laterally) Resolved Surface Chemistry Analysis S A M (scanning A u g e r m i c r o p r o b e ) S I M S (also c a l l e d i o n m i c r o p r o b e ) E D X A (energy d i s p e r s i v e x-ray analysis)

• — — — —

Surface Topography Light microscopy S E M (scanning e l e c t r o n microscope) O p t i c a l h e t e r o d y n e p r o f i l o m e t r y (20) P r o f i l o m e t r y (stylus technique)

• — — —

Surface Crystallinity and Atomic Organization L E E D (low e n e r g y e l e c t r o n diffraction) S E X A F S (surface e x t e n d e d x-ray absorption fine structure) F E M (field i o n microscopy)



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Figure 1. Propst diagram as a representation of the possible spectroscopies that might he used to study surfaces. Each spectroscopy is represented by an arrow in and an arrow out.

Electron Spectroscopy for Chemical Analysis (ESCA). E S C A is the most valuable t e c h n i q u e p r e s e n t l y available for the study of b i o m a t e r i a l surfaces, a n d p a r t i c u l a r l y for materials i n t e n d e d for b l o o d contact a p p l i c a tions, for the f o l l o w i n g reasons: 1. T h e s a m p l i n g d e p t h for E S C A covers a relevant surface r e g i o n for biomaterials (~ 1 0 - 1 0 0 A d e p e n d i n g o n the m e a n free path of the e m i t t e d photoelectrons a n d the angle of the sample w i t h respect to the analyzer). 2. M a n y levels o f i n f o r m a t i o n can be o b t a i n e d f r o m a single E S C A e x p e r i m e n t (see Table II). 3. Samples can be s t u d i e d i n a h y d r a t e d (frozen) c o n d i t i o n (8). 4. S a m p l e p r e p a r a t i o n is s i m p l e . 5. T h e t e c h n i q u e , i f u s e d w i t h some care, is nondestructive. 6. H i g h s e n s i t i v i t y can be o b t a i n e d . 7. T h e t h e o r e t i c a l basis for E S C A , p a r t i c u l a r l y as it applies to the use of E S C A as an analytical t e c h n i q u e , is w e l l established. 8. E S C A is the o n l y t e c h n i q u e that (to date) allows a direct correlation to be m a d e b e t w e e n surface c h e m i s t r y and i n vivo b l o o d i n t e r a c t i o n (21-23).



Figure 2. Comparison of some surface analytical methods with respect to depth of analysis. Key: *, static SIMS; **, dynamic SIMS (destructive); ion; O , electron; and i , x-ray.


to

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T h e E S C A e x p e r i m e n t has its basis i n the photoelectron effect—matter b o m b a r d e d w i t h x-rays (electromagnetic radiation) w i l l e m i t photoelectrons w i t h an e n e r g y : ^photoelectron = hv - b i n d i n g energy - w h e r e h v is the e n e r g y o f the b o m b a r d i n g x-ray, is a w o r k f u n c t i o n that is established for each s p e c t r o m e t e r , a n d £ h toeiectron is the k i n e t i c energy of the p

0

p h o t o e l e c t r o n w h i c h is m e a s u r e d b y the E S C A i n s t r u m e n t . T h u s , the b i n d ing energy of the e j e c t e d e l e c t r o n can b e d e t e r m i n e d . This b i n d i n g energy Downloaded by EAST CAROLINA UNIV on March 8, 2017 | http://pubs.acs.org Publication Date: July 27, 1982 | doi: 10.1021/ba-1982-0199.ch002

is a sensitive f u n c t i o n of the atomic e n v i r o n m e n t , w h e r e the e n v i r o n m e n t is d e f i n e d b y the nature of atom w i t h w h i c h the ejected electron was associated and b y the atoms b o u n d to the atom that has suffered the p h o t o e m i s s i o n . B y m e a s u r i n g the i n t e n s i t y a n d e n e r g y d i s t r i b u t i o n of the p h o t o e m i s s i o n , the information l i s t e d i n Table I I can b e o b t a i n e d . A diagram i l l u s t r a t i n g the components of an E S C A s p e c t r o m e t e r is p r e s e n t e d i n F i g u r e 3. A n u m b e r of good reviews d e s c r i b i n g the E S C A t e c h n i q u e exist (24,25). I n a d d i t i o n , reviews d i r e c t e d p a r t i c u l a r l y toward the u n i q u e aspects of E S C A for s t u d y i n g p o l y m e r i c systems also have b e e n w r i t t e n (26). T h e significance of E S C A for biomaterials investigations is discussed i n the next section. T h e a b i l i t y to w o r k w i t h h y d r a t e d (frozen) samples is an i m p o r t a n t advantage of the E S C A t e c h n i q u e for biological studies. T h e importance o f experiments o n h y d r a t e d biomaterials is s u p p o r t e d by studies i n d i c a t i n g that certain surfaces r a d i c a l l y change t h e i r character (probably due to alterations i n side g r o u p a n d b a c k b o n e orientation) u p o n d e h y d r a t i o n (8,-10). A t l i q u i d nitrogen t e m p e r a t u r e s a l l significant c h a i n backbone and side g r o u p m o t i o n i n p o l y m e r s (and proteins) is i n h i b i t e d . T h e r e f o r e , b y r a p i d freezing, a chain conformation s i m i l a r to that at the l i q u i d - s o l i d interface s h o u l d be frozen i n place. D i s t o r t i o n s i n this h y d r a t e d interface configuration may, i n fact, b e

Table II. Information Derived from an E S C A Experiment • • • • • •

• • •

A l l e l e m e n t s present (except h y d r o g e n and h e l i u m ) A p p r o x i m a t e surface concentrations of elements ( ± 10%) B o n d i n g state (molecular e n v i r o n m e n t ) and/or oxidation l e v e l of most atoms I n f o r m a t i o n o n aromatic or unsaturated structures f r o m shake-up (77 —> IT*) transitions I n f o r m a t i o n o n surface electrical properties f r o m charging studies N o n d e s t r u c t i v e d e p t h p r o f i l i n g a n d surface heterogeneity assessment using (1) photoelectrons w i t h d i f f e r i n g escape depths and (2) a n g u l a r - d e p e n d e n t E S C A studies D e s t r u c t i v e d e p t h p r o f i l e u s i n g argon e t c h i n g (for inorganics) Positive i d e n t i f i c a t i o n of f u n c t i o n a l groups u s i n g derivatization reactions " F i n g e r p r i n t i n g " materials u s i n g valence b a n d spectra


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Photoelectron Counts X-roy Source

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Chort Recorder

X- roys Sample


Multichannel Analyzer pnnn

Kinetic Energy * * Binding Energy

Photoelectrons-»>

Surface Characterization of Materials for Blood Contact Applications

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