Designing the surface of medical devices

Designing the surface of

medical devices

Tullio Monetta, Annalisa Acquesta

Department of Chemical Engineering, Materials and Industrial Production

University of Napoli Federico II

GOAL study an ad hoc surface treatment on

dental implants to obtain medical devices showing improved performances.

Titanium

•Low density (4.5 g/cm3 against 7.9 g/cm3 for steel, 8.3 g/cm3 for VitalliumR and 9.2 g/cm3 for Co/Ni/Mo/Cr alloys );

•Excellent mechanical properties;

•Poor toxicity;

•Good biocompatibility;

•Non-magnetic;

•Good resistance to acids and alkalis;

•Good heat transmission;

•Great resistance to erosion, cavitation and impact attacks.

The rate of spontaneous oxide formation is very high.

Titanium-Titanium oxides

There are different types of oxides on the surface, including: Ti3O, Ti2O, Ti3O2, TiO, Ti2O3, Ti3O5 e TiO2.

The aim is to form TiO2, which is the most stable oxide.

Cells growth

by Steve Gschmeissner

SEM view of TiUnite surface when osteoblasts

have filled pores (Current Concepts in Dental Implantology, book Ilser Turkyilmaz,)

Osteoblast «fill in the hole»

• Micro/macro roughness is required to increase the implant osseointegration rate

• Surface showing “more valleys than peaks” for

cells adhesion and spreading is necessary • A reservoir on implant surface is needed for drug

delivery

Needs

Designing the surface

A «production process» has to be settled up allowing to obtain a surface with:

Large valleys

Small valleys

Small tanks to store chemicals

Surface

treatments

Chemical

Electrochemical

(anodization)

Acid etching

Physical

Thermal

Co-deposition

Nanostructured

oxide (Titania

Nanotubes)

Porous oxide

(Anodic Spark

Oxidation)

Compact oxide

Chemical Vapor

Deposition Cold Plasma

Sol-gel

H2O2

Thermal

Spraying

Plasma

Spraying

Physical Vapor

Deposition

Ion / Laser

beam Sandblasting

Surface Engineering

Sandblasting-acid etching

SEM IMAGES (1000X) OF Ti CP2 SANDBLASTED 1 MIN (A), Ti CP2 SANDBLASTED 1 MIN AND ACID ETCHED (B), Ti CP2 SANDBLASTED 8 MIN (C), TiI CP2 SANDBLASTED 8 MIN AND ACID ETCHED (D).

A

C

B

D

Sa(a) =Sa(b) =Sa(c)

Sa = average roughness;

Sq = root mean squared roughness;

Sku = describes the “peakedness” of the surface

topography (Kurtosis);

Ssk= describes the asymmetry of the height

distribution histogram (Skewness).

Roughness

Roughness

The anodizing titanium is connected to the positive pole of the DC power supply, where an oxidation process occurs. At the negative pole there is a reduction process of species present in the electrolyte. As a result of the anodic polarization, the oxide film increases and the immediate effect, resulting from the thickening of the layer, is the titanium staining. The growth of anodic oxide does not occur by expanding the porous layer outward, but by continuous oxidation and metal dissolution within the layer.

Titanium anodizing

Example of surface obtained by anodizing

Compact oxide Porous oxide

(Anodic Spark Oxidation)

Nanostructured oxide

(Titania Nanotubes)

Porous oxide

Titanium (oxide) Nanotubes

«Inorganic nanotubes» «Organic nanotubes»

Oxide behaviour Titanium not treated «Inorganic» nanotubes in Hank’s solution «Organic» nanotubes in Hank’s solution 10

2

103

104

105

106

10-2

10-1

100

101

102

103

104

0

24h

29h

96h

216h

360h

Imp

ed

an

ce

mo

du

lus |

Z|,

cm

2

Frequency, Hz

102

103

104

105

106

10-2

10-1

100

101

102

103

104

0

24h

29h

96h

216h

360h

Imp

ed

an

ce

mo

du

lus

|Z

|,

cm

2

Frequency, Hz

102

103

104

105

106

10-2

10-1

100

101

102

103

104

0

24h

29h

96h

216h

360h

Imp

ed

an

ce

mo

du

lus

|Z

|, o

hm

*cm

2

Frequency, Hz

Anodized screw

Anodized screw

Rotation speed: 10 rpm Screw: nanotubes diameter 50-60 nm, lenght 350 nm. Flat samples: nanotubes diameter 100 nm, lenght 800 nm. The nanotubes morphology could be due to the complex geometry of the screw and could be attributed to the different distribution of the electric field that occurs when using a screw instead of a flat sample. Unexpectedly, the average diameter of nanotubes does not differ if measured on the crest, on the side or on the bottom of the threads. In the assumption that the electric field assumes significantly different values between the points mentioned, it would be expected to get dissimilar structures, but this event has not been verified. The increase in anodizing time, from 90 min to 120 min, has little influence on diameter, wall thickness and nanotube length.

Anodized screw

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

1,1

5 10 15 20 25 30

Rodamina 6G

talqualenanotubi

Fra

zio

ne d

i m

ole

cola

elu

ita [ug/c

m^2

]

ore^0.5 [h^0.5]

Dru

g F

ract

ion

Rel

ease

d,

µg/c

m2

Time, h2

Titanium flat sample

Nanotubes sample

Rhodamine

Drug delivery

The untreated titanium sample restrained

7μg/cm2 of the absorbed drug, while the

nanotubes-covered sample restrained 12

μg/cm2.

After 5 days, the untreated titanium

sample released 80% of the absorbed

drug, while the nanotubes covered

sample released 60%.

After 13 days, the untreated titanium sample

released all the absorbed drug.

Nanotubes covered sample released the

overall amount of drug after 26 days.

0

2

4

6

8

10

12

14

0 5 10 15 20 25 30 35 40

Tal quale Nanotubi

y = 1,348 + 0,18174x R= 0,97414

y = 2,1818 + 0,30808x R= 0,9694

Fra

zion

e d

i fa

rmaco

elu

ita [

ug

/cm

^2]

Ore^0.5 [h^0.5]

Dru

g F

ract

ion

Rel

ease

d,

µg/c

m2

Titanium flat sample

Nanotubes sample

Time, h2

Doxorubicin Hydrochloride

Item performances can be improved by a

proper design of the surface.

Needs, aims, cost, added value, product

shelf-life, should be considered when

choosing the production process to modify

the surface.

A multidisciplinary approach is required.

Conclusions