the role of zeta potential in manufacture of healthcare materials

7/29/2019 The Role of Zeta Potential in Manufacture of Healthcare Materials

1/8

www.ceram.com

The Role of Zeta Potential inManufacture of Healthcare Materials

Author: Matthew Cantelo

This work byCeramis licensed under aCreative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License
http://www.ceram.com/http://www.ceram.com/http://www.ceram.com/http://creativecommons.org/licenses/by-nc-sa/3.0/deed.en_GBhttp://creativecommons.org/licenses/by-nc-sa/3.0/deed.en_GBhttp://creativecommons.org/licenses/by-nc-sa/3.0/deed.en_GBhttp://creativecommons.org/licenses/by-nc-sa/3.0/deed.en_GBhttp://creativecommons.org/licenses/by-nc-sa/3.0/deed.en_GBhttp://creativecommons.org/licenses/by-nc-sa/3.0/deed.en_GBhttp://www.ceram.com/


2/8

2

by Ceram.

Introduction

In the manufacture of healthcare products and medical materials, especially ceramicmaterials like zirconia (used as an implant material) and hydroxyapatite (syntheticbone replacement material), there is often a stage in the manufacturing process

involving a powder suspension. The behaviour of this powder suspension willcorrelate strongly to (a) how well it is processed and so (b) final yields and productperformance. Surface chemistry dominates the particle-particle interactions insuspension, with different materials having different surface charges. Theseinteractions in turn dictate suspension rheology. Zeta potential is used to investigateand monitor the surface interactions in powder suspensions, and can also be used tooptimise the processing method. In a previous white paper, the in-depth theory ofzeta potential was presented and discussed, whereas this white paper will discuss itsapplications for the manufacture of some healthcare materials and how Ceram hasassisted some of these manufacturers.

Zeta Potential The Theory

All inorganic materials form a surface charge when dispersed in a liquid, usuallywater in industrial processes. The material will either form a positive or negativesurface charge in the liquid, and this charge will attract counter-ions from the liquid.This forms a double layerof tightly bound counter-ions close to the surface of theparticle, and a more diffuse layer containing both positive and negative ions thatoffset the surface charge on the particle.

Zeta potential is defined as the energy required to shear a particle and its associateddouble layer away from the bulk solution. It is measured in millivolts (mV) and can bepositive or negative. A large zeta potential ( 50 mV) is indicative of a stable system,where the particles are well dispersed and there is minimal flocculation. A low zetapotential ( 10 mV) indicates an unstable system, where there is lots of flocculationoccurring and the particles are not properly dispersed. A value of 25 mV isconsidered to be the minimum for a stable system.

Zeta potential is often measured as a function of pH. As this is varied, the surfacechemistry of the particles will be altered, and will produce a zeta vs pH graph that ischaracteristic of that particular material. The line on the graph should also passthrough a point where mV = 0, the isoelectric point.

Applications for Medical Materials

Zirconia, ZrO2, is one of the most widely studied ceramic materials and one of themost stable. It has found many applications in medical devices, from dental sub-frames and prostheses1 to hip replacement acetabular cups and femoral heads.When used as a bone replacement material, zirconia has many advantages overmetal implants; it is more bio-inert than most metals, and there is much less risk oftoxic ions leaching into the body. If these materials are going to be used in the body,then their surfaces are going to come into contact with a wide range of speciespresent in body fluid, so the surface chemistry (and therefore zeta potential) needs to

be investigated. Zirconia suspensions are more stable at acidic pH and are muchmore dispersed than at the alkaline end of the scale. Body fluids are generallyaround the pH 7 mark, and at this point there will be some agglomeration of theparticles, but this proves advantageous as it prevents the material becoming too


3/8

3

by Ceram.

brittle and cracking4. Other materials are often added to zirconias, such as smallamounts of yttria and alumina in hip arthroplasty, and these dopants can greatlyincrease the mechanical strength of the implant. Their behaviour is still similar to purezirconia as they are very stable at acidic pH, so for a good dispersion of the particles,the working pH of the process should be acidic (around 4 4.5)2. A typical zeta vs pHcurve for zirconia is shown in figure 1.

There have also been numerous studies into titania (TiO2) based systems as medicaldevices. Titanias mechanical strength and good biocompatibility is very well known,and it has also been investigated as a thin-film coating for biological applications.When combined with carbon, silicon, zirconium, calcium and niobium in varyingquantities, these bio-active films can help promote integration between the implantand the existing bone and release these useful ions. TiO and TiN films in particularhave shown excellent biocompatibility. When the surface chemistry of these filmswas investigated, it was found that they all possessed a negative surface charge.This negative charge can attract the calcium ions present in body fluid, and as aresult, hydroxyapatite phases can start to form and integrate with the existing bone,creating a very strong bond between implant and bone4.

Multi-substituted hydroxyapatites (Mx-HA) are used for a variety of healthcareapplications, including orthopaedic implant coatings. The crystal structure of

hydroxyapatite (HA) can be modified in many different ways, various cations (Mg

2+

,Sr2+, K+) can be substituted in place of calcium, and various anions can replace thephosphate groups (SiO4

4-, CO32-, SO4

2-) or the hydroxyl groups (F-, Cl-). As mentionedpreviously, if the surface of the hydroxyapatite is negatively charged, it can attract thefree calcium ions present in the body and start to form new bone material, creating astrong bond with the implant. One study reports using HA substituted with fluorine toachieve a negative charge on the surface that was greater than normal HA. Thiswould allow for greater attraction of calcium ions to the surface due to the increasedelectronegativity of fluorine, and would give an even stronger bond to the existingbone than normal HA. Another study reports using a deflocculant to bind to the HAparticles in solution to change the surface charge from positive to negative. Thedeflocculant used was a type of Dispex, a well known polyacrylate dispersant that will

bind to positively charged particles through its free carboxylic acid groups5

. A keyaspect of HA surfaces is the role surface charge plays in attracting growth factors(zwitterions, peptide species) that in turn attract ions and osteoblast cells for newbone growth.

Figure 1 A Typical Zirconia Zeta vs pH Curve3


4/8

4

by Ceram.

It is also worth noting that zeta potential is very useful in the processing of emulsionsas well as powder suspensions. Emulsions are essentially one liquid phasedispersed in another (e.g. oil in water) where charged organic surfactant species thataggregate as micelles in solution. For an oil-in-water emulsion, droplets of the oil aresurrounded by inward-facing hydrophobic chains on surfactant molecules thataggregate into micelles. On the outside of these micelles, there are charged groups(positive or negative) that interact in a much more hydrophilic way with the water. Infigure 2 below, these arrangements of different parts of the molecule are shown.

The surface charges on the hydrophilic heads associated with micelles will give azeta potential signal in the same way that solid particles do. This can be used tocontrol the stability of these micelles in many different applications, from shampoosand sun-creams to anaesthetics. The organic components of an anaesthetic must beadministered into the body in an aqueous medium, so surfactants are employed tokeep them as stable as possible within this suspension, and zeta potential analysis

can track and help improve this. It can also be used for investigating the shelf-life ofvarious products, by investigating how the micelles surface chemistry might alterover time. As this surface chemistry also affects the rheological properties of asuspension, zeta analysis of emulsions can be used to investigate, for example, howeasily a sun-cream formulation can be squeezed out of a bottle while still retaining itscolloidal properties.

OILHYDROPHILICHEAD GROUP

HYDROPHOBICTAIL GROUP

Figure 2 A Typical Oil In Water Emulsion


5/8

5

by Ceram.

Our Work

Ceram has undertaken many projects in the past for various clients, performing zetapotential measurements on a range of different materials.

Client 1:

Requested zeta potential testing as a quality control method for zirconia

powders.

Dispersing the powders in water and measuring the pH over time showed a drift

towards acidic pH, due to the dissolution of acid present in the sample from the

manufacturing process.

After an hour the pH indicated that the system had reached equilibrium, but

when the zeta potential was measured over the same time, this showed it had

not equilibrated.

The starting zeta potential values proved to be quite different for each of the 13

samples, and this could be due to a number of factors: residual acetic acid from

the manufacturing process, the presence of polyacrylate (increases negative

charge on the surface) from the milling process or polyacrylamide (increases

positive charge on the surface) that was used as a dispersant.

There was no correlation between the inital pH of the sample and the time it took

for the sample to reach pH 7. This is also an indication of residual acid from the

manufacturing process being present, and it was established that this was due to

the low-temperature calcining of the zirconia powders that would not have

eliminated all the acid present6

.

Client 2:

Requested zeta potential analysis to investigate coating of titania particles

destined for use in sunscreen. The coating can provide an anchor for organic

materials that would otherwise degrade due to reaction with the free radicals

generated at the titania surface.

When coated with different materials, the response of the material to pH

changes became apparent. When an electrolyte was added as well, these

changes were more amplified, and the zeta potential range decreased. This is

due to the electrolyte molecules suppressing the double layer and causing

agglomeration of the particles.

The level of coating was also investigated using zeta potential, and it was found

that as the layer of coating increases in size, the zeta potential curve has a wider

range.

This analysis technique therefore can allow precise control of the surface

chemistry of these titania particles, as any changes to the formulation can easily

be tracked and compared7.


6/8

6

by Ceram.

Ceram has also done a wide variety of zeta potential analyses for various otherclients, including the polymer suspensions of shampoos, flour in water suspensions

in dough-making, and foamed suspensions.

-60

-40

-20

0

20

40

60

3 4 5 6 7 8 9 10 11 12 13

pH

ZetaPotential/mV

Uncoated TiO2

Alumina Coated TiO2

Silica Coated TiO2

Figure 3 How Adding A Coating to Titania Particles Affect Zeta Potential7

Unco

ated

Control

pow

der

Silica

Coa

ted

Silica+Lin

ker

Silic

a+Lin

ker+

Fatty

Acid

ChangeinColourspaceM

easurement

Photogreying

Yellowing

Figure 4 How Coatings of Titania Particles Affect Physical Properties


7/8

7

by Ceram.

Conclusions

Zeta potential is a very useful technique that can be used in the manufacture ofmedical materials and medical devices, at many key stages of the process. Surfacechemistry is a very important consideration for medical materials, and with zeta

analysis, this can be altered and improved in order to increase the bioactivity andintegration of the material into the body. Ceram has helped many clients solvesurface chemistry-related problems using this method of analysis, and it will helpshape our current research into new biomaterials.

References

1. D. Gutknecht, J. Chevalier, V. Garnier, G. Fantozzi, Journal of the EuropeanCeramic Society 27 (2007) 15471552

2. D.V. Shtansky, N.A. Gloushankova, A.N. Sheveiko, M.A. Kharitonova, T.G.Moizhess, E.A. Levashov, F. Rossi, Biomaterials 26 (2005) 29092924

3. Work undertaken at Ceram in December 2012 for a client, investigating the zetapotential of zirconia samples.

4. X. Ge, Y. Leng, F. Ren, X. Lu, Journal of the Mechanical Behaviour ofBiomedical Materials 4 (2011) 1046-1056

5. J.C. Knowles, S. Callcut, G. Georgiou, Biomaterials 21 (2000) 1387}1392

6. Work undertaken at Ceram in February 2009 for a client, investigating how zetapotential can be used for QC of zirconia powders.

7. Work undertaken at Ceram in 2011 for a client, investigating how zeta potentialcan be used to investigate titania coatings in sunscreen. This research waspresented at the PSA 2011 conference.


8/8

www.ceram.com

by Ceram

About Ceram

Ceram is an independent expert in innovation, sustainability and quality assurance of

materials.

With a long history in the ceramics industry, Ceram has diversified into othermaterials and other markets including aerospace and defence, medical andhealthcare, minerals, electronics and energy and environment.

Partnership is central to how we do business; we work with our clients to understandtheir needs so that we can help them overcome materials challenges, develop newproducts, processes and technologies and gain real, tangible results.

Headquartered in Staffordshire, UK, Ceram has approved laboratories around theworld.

About the Author

Matthew CanteloLaboratory ChemistMatthew Cantelo has a MChem (Hons) chemistry degree from Cardiff University. Hisfinal year research project was largely focused on inorganic synthesis withinhealthcare applications. This is an area that Matthew is continuing to concentrate onat Ceram, particularly with regards to hydroxyapatite synthesis.

the role of zeta potential in manufacture of healthcare materials

Documents