Diamond-like Carbon Thin Film with Controlled Zeta Potential for Medical Application

[Nitta et. al., Diamond & Related Materials 17 (2008) 1972-1976]

MSE 576 Thin Films & Analysis Presentation
Dec 4th 2008
Deepak Rajput
Graduate Research Assistant
Center for Laser Applications
University of Tennessee Space Institute
Tullahoma, Tennessee 37388-9700
Email: drajput@utsi.edu Web: http://drajput.com

Outline

* Diamond-like carbon thin films

* Zeta potential

* Discuss paper by Nitta et. al.
Diamond-like carbon
* Amorphous thin films with both graphite and diamond bonds.

* Interesting properties:

* Low coefficient of friction
* Wear resistance
* Wide band gap

* Applications: Dies and automobile parts.

Diamond-like carbon
* DLC thin films are potential medical materials because:
* biocompatibility
* antithrombogenicity

* Reason: Medical devices that are in contact with the blood, e.g., artificial hearts and blood pumps.

* Present problem: blood clotting, performance.

Diamond-like carbon
* Present material: Polymers, but they have problems:

* Blood compatibility is not outstanding.
* Adhesion is not great to metallic substrate.

Diamond-like carbon
* DLC thin film:

* Chemically stable amorphous hydrocarbon thin film.
* Smooth with atomic flatness.
* Superior compatibility with tissue and blood.

* Problem: Not effective in all the situations.

* Account must be taken of the interactions between the cell and the DLC thin film surface.

* Important parameter: Zeta potential !!

What?s zeta potential ?
* It?s an abbreviation for electrokinetic potential in colloidal systems (says Wiki).

* Theoretically, it is the electric potential in the interfacial double layer (DL) at the location of the slipping plane versus a point in the bulk fluid away from the interface.

* In simple terms, it is the potential difference between the dispersion medium and the stationary layer of fluid attached to the dispersed particle.
What?s zeta potential ?

Source: http://www.malvern.co.uk/LabEng/technology/zeta_potential/zeta_potential_LDE.htm
What?s zeta potential ?

Source: http://www.geocities.com/CapeCanaveral/Hangar/5555/zeta.htm
The significance of zeta potential
* Its value can be related to the stability of colloidal dispersions.

* It indicates the degree of repulsion between adjacent, similarly charged particles (the vitamins) in a dispersion.

* A high zeta potential: Stability ! (+ or -)

* A low zeta potential: Flocculation !
Because attraction exceeds repulsion, and the dispersion breaks.
The significance of zeta potential
Zeta potential in biological environ
* Cells: Negatively charged, and their surface potential varies depending on the individual cell.

* Stimulation to the cells can be reduced by controlling the zeta potential.

* Method by Nitta et. al.: Introduce functional groups such as amino (-NH2) and carboxyl groups (-COOH).

* How: Plasma surface treatment.
Zeta potential in biological environ
* Carboxyl groups: high negative charge.

* Amino groups: high positive charge.

* If the quantities of these functional groups can be controlled at the DLC thin film surface, it will be possible to control the zeta potential.
Experimental
* Plasma surface treatment in a chamber (5 Pa)

* Process chamber connected to a RF power supply with an excitation frequency 13.56 MHz at power of 300W.

* RF power of 30 W was injected to generate plasmas.

* Capacitatively Couple Plasmas (CCP) was generated by means of two parallel plate electrodes.

* Gases used: O2, Ar, NH3 and C2H2 (15 seconds).
Experimental

* DLC thin films used were prepared by ionization-assisted deposition using benzene.

* DLC thin film thickness: 40 nm.

* After plasma surface treatment:

* XPS: Composition ratios of the DLC samples.
* Contact angle meter: Static contact angle.
* Zeta potentiometer: Zeta potential of the samples.

Results: C2H2 followed by O2 treatment

* Binding amounts in an untreated DLC sample were:
82.7 (C-C), 11.7 (C-O), 3.8 (C=O), and 1.7 (O=C-O) (atomic %)

* Comparing them with the XPS results of the DLC samples show that C-C bonds or C-H bonds were cleaved by radicals, electrons, and ions in the plasma.

* Thereby oxidation reactions such as C-O, C=O and O=C-O were promoted.

* O2 or O radicals in plasma mainly drew H from C-H bonds. Amount of C-C bonds or C-H bonds in DLC thin films were dependent on functional groups introduced to DLC surface.

* Thus, it is considered that amount of functional groups introduced to DLC thin films surface can be controlled by controlling amount of C-C bonds or C-H bonds in DLC thin films.

* The O=C-O peaks stem from the carboxyl groups and were three times more numerous than that of untreated DLC sample.

* Carboxyl groups can be introduced efficiently onto the surface of DLC thin films by plasma surface treatment.

* C-C bonds were larger than those of C2H2+O2 plasma treatment.

* N1s peak was remarkable compared to that of C2H2+O2 plasma treatment.

* C-H bonds or C-C bonds were cleaved by radicals, electrons, and ions in the NH3 plasma, and nitrogen was introduced into the DLC thin films surface.

* C-NH2 peak dominated

* It is possible to generate amino groups on DLC thin films surface.

* It is possible to control the zeta potential of DLC thin films by controlling the amounts of the carboxyl groups and amino groups.

* A new method discovered to develop a biocompatible material.