Microstructures

Further preferred features and advantages of the present invention will appear from the following detailed description given by way of example of some preferred embodiments illustrated with reference to the accompanying drawings wherein:

FIG. 1 is a generally schematic perspective view of a first embodiment of a bio-mechanical structure of the invention;

FIG. 2 is a similar view of a second embodiment;

FIGS. 3 to 6 are plan views of some further embodiments;

FIGS. 7A to 7I illustrate principal stages in a process for the production of a substrate structure of the invention;

FIG. 8 is a plan view of a structure produced by the process of 7A to 7I;

FIGS. 9, 10 and 11 are vertical sections along the lines B-B', A-A', and C-C', in FIG. 8 respectively; and

FIG. 12 is a plan view of a spiral groove structure of the invention.

FIG. 1 shows a bio-mechanical micro-structure 1 of the invention comprising a fused silica substrate 2 in the form of a small plate. On the upper surface 3 are provided two spaced apart and diverging ridge structures 4 in the form of gold fingers each having a width of 6 .mu.m and a thickness of 0.4 .mu.m. The spacing between the fingers 4 ranges from 2 to 20 .mu.m and they are formed on the surface 3 by ion beam etching of gold. On either side of the gold fingers 4 the silica substrate is optionally coated with a layer of untreated polystyrene 100 having a thickness of about 0.1 .mu.m. The polystyrene coating 100 is applied to the silica surface 3 by evaporation from toluene solution.

A suspension of nerve cells cultured from chick embryo brain (2-10.times.10.sup.4 cells per ml of aqueous solution containing Ham's FlO saline with 1% foetal calf serum and additions of transferrin, insulin, and nerve growth factor) was then applied to the above-described substrate surface and maintained in contact therewith for 60 minutes at 37.degree. C. The excess solution was then drained off and the substrate washed with fresh culture medium.

Inspection of the substrate then showed that a number of nerve cells 5 had become selectively attached thereto at the gold wires and in particular across adjacent pairs of converging gold wires.

FIG. 2 shows another bio-mechanical microstructure comprisng a fused silica substrate 12 with a groove 13 etched in its upper surface 14 by means of argon iron beam etching through a photomicrographically defined photoresist layer.

Within the groove, which has a width of about in the range 0.5 to 10 .mu.m is provided a plurality of upstanding projections 15 formed of gold.

Each projection 15 has a height of some 0.1 to 0.5 .mu.m and a thickness of about 0.02 to 0.1 .mu.m.

The projections are desirably formed so as to have a substantially smooth surface so that they penetrate the cell wall readily and the cell wall seals around the projections. The projections may be formed by any suitable photolithographic technique. Conveniently the projections may be formed by depositing a thin layer (20-30 nm) of gold or other metal onto a hydrocarbonpolymer pillar formed by polymerisation effects of an electron beam in vacuo making use of traces of organic materials from the vacuum pump oil of the pump used to produce the vacuum (Broers (1)).

Nerve cells 16 are applied to the substrate surface in a similar manner to that described above and become attached to the groove 13. As shown in the drawing certain projections 15 become phagocytosed by the cell providing a direct mechanical and electrical connection with the cell interior.

If desired electrical conductor or semi-conductor material tracks or other structures may be provided within the grooves and/or extending outside the grooves a cell-adhesion inhibiting coating may be provided.

FIGS. 3 to 6 show various different arrangements of pluralities of grooves 17 and which can be used to define desired networks of cells 18.

FIGS. 7A to 7I illustrates schematically the principal stages in a suitable lighographic manufacturing process for the production of another substrate structure further illustrated in FIGS. 8 to 10, suitable for use in making direct electrical connections with the interior of a cell. Since the basic lithographic techniques employed in the process are known in the art they will only briefly be described.

Aluminium metal was evaporated over a glass substrate 19 to form a metal layer 20 of approximately 50 nm thickness. This was then overcoated with a layer of photoresist 21 of similar thickness and exposed through a mask (not shown) defining the pattern of leads to the probes (see below). The photoresist 22 was then developed to produce a protective pattern 22 as shown in FIG. 7A which after chemical etching of the unprotected metal 20 and subsequent removal of the resist pattern 22, left a metallic lead pattern 23 as shown in FIG. 7B. Photoresist was then coated onto the processed substrate surface and exposed to the negative of the first mask to leave, after development resist 24 everywhere but on the metal lead pattern 23 as shown in FIG. 7C. Needle-shaped columns 25 of polymerised contamination are then formed at desired spots on the conductor tracks 23 (See FIG. 7D) by focussing an electron beam on said spots. The columns 25 of material so formed have a diameter a few times the beam diameter and their height can be made many times the beam diameter (Broers (1)). Any residual hydrocarbons in the vacuum are polymerised by the electron beam.

The processed substrate is then overcoated with metal so that probe is now metallised 26 and electrically connected to the conductor track pattern 23 (see FIG. 7E). The photoresist 24 is then dissolved in a strong solvent thus removing the resist together with the unwanted metal 27 on top of it (see FIG. 7F). The resulting processed substrate is then overcoated with polyimide and baked at 350.degree. C. to form an insulating layer 28 having a thickness greater than the contained height of the conductor track 23 and metallised probe 26 thereon. The polymide is used in two roles viz to form the ridge to which the cells adhere and to provide an insulator for the probe leads. The polyimide layer 28 is then overcoated first with Aluminium 29 and then with photoresist 30 (see FIG. 7G) and exposed via a mask defining a polyimide ridge pattern. After development of the photoresist and chemical etching of the aluminium through the photoresist pattern, the processed substrate is placed in a reactive oxygen plasma and the polyimide not protected by aluminium etched away to leave a polyimide ridge pattern 3 with substantially vertical walls (See FIG. 7H). The protective aluminium layer 29 having served its purpose as an oxygen plasma etch mask, it is removed by chemical etching and the processed substrate then returned to the oxygen plasma etcher and more polyimide removed until the probes 26 stand proud of the polyimide ridges 31 (see FIG. 7I).

The processed substrate is then overcoated with resist and exposed to a further mask defining an electrical screen pattern. After development of the resist overcoating by another metal layer and lift-off of resist pattern and unwanted metal thereon, the metal screen layer 32 will cover the whole specimen except for the cell-supporting polyimide ridges and the contact pads (see FIGS. 10 and 11 in particular). Finally a selection of glass tube 33 is glued on to the processed substrate to separate the central biological area containing cell support medium 34 and which will therefore be wet--from the surrounding electrical area. By this means the probe leads 36, which are connected to the remote ends of the conductor tracks 23, are electrically isolated from the saline solution cell support medium 34 by a ground plane everywhere except for a few micrometers close to the metallized probe 26. Even here only the tip 36 of the probe will not be insulated and this will be isolated from the medium 34 once it is phagocytosed by a cell 37 attaching to the cell-support polyimide ridge 31 (see FIG. 9). FIG. 12 shows schematically a further substrate structure of the invention produced using photolithographic procedures with ion beam milling through suitable masks to form a series of logarithmic spiral grooves 40 in the surface 41 of a glass plate 42. The grooves 40 were ten micrometers wide and half a micrometer deep. The smallest (innermost end 43) radius of curvature of the spiral was 20 micrometers and the largest (outermost end 44) two hundred and fifty micrometers. BHK fibroblast cells 45 were applied on to this substrate in a serum containing culture medium (Eagle's MEM medium with 10 percent tryptose-phosphate broth and ten percent calf serum) and cultured at 37.degree. C. for two days. During this period the areas containing the spirals 40 were filmed by time-lapse video recording using phase contrast microscopy and a very low light level video camera, the latter being used in order to minimise any possible adverse effect of light or heat radiation from the microscope lamp. The cells were observed locate at the groove within as little as 15 to 30 minutes and take up a spread shape on the groove edges 46 related to the radius R of the spiral 40 at that region. Cells 47 on the small radius end 43 of the groove 40 did not spread properly and remained immobile. Cells 48 settling on wider radii sections elongated to an extent related to the radius of curvature at that region and migrated outwardly along the groove edges 46 to the end of greatest radius 44.

Further possible uses of bio-mechanical structures of the invention, particularly those including cell-adhesion enhanced zones, include the separation of cells from other biological material and/or the fractionation of different types of cells on the basis of differences in morphology and cyto-skeleton lability, the cells and/or particular types of cells being preferentially retained by the substrate to which the mixture of interest has been applied whilst other components of the mixture are eluted out of the system.

With reference to the embodiment of FIGS. 7 to 11 and in particular the shielding of the conductor tracks 23 as illustrated in FIG. 11 it should be noted that the polyimide (or other suitable dielectric) sheath 31 should be dimensioned relative to the dimensions of the conductor tracks 23 and outer metal shielding 32 and the dielectric properties of the sheath so as to obtain a suitable characgteristic impedance for this electrical connection. Desirably the connection should have a characteristic impedance in the range from 25 to 200 ohms. In the specific example shown (note: FIGS. 7 to 11 are not drawn to scale) the conductor track 23 conveniently has a thickness of about 0.2 .mu.m, the polyimide insulation sheath 31 a thickness in the region of 1 to 2 .mu.m, and the outer conductor shield 32 a thickness of about 0.3 .mu.m.

Literature Reference (1):

App. Phys. Lets. 29 296-298 (1976)

A. N. Brouers et al.