Note: Descriptions are shown in the official language in which they were submitted.
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METHODS AND APPARATUS FOR PHYSICALLY PATTERNING
NONOPERATIONAL STRUCTURES OF AN OPTICAL DISC
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optical disc
technology. Specifically, the invention relates to methods
and apparatus for the physical patterning of readable
nonoperational structures on internal or external surfaces
of optical discs.
2. Description of Related Art
Recent developments in optical disc design,
optical disc manufacture, and in the design and manufacture
of drives for reading these discs have now made it possible
to use optical disc drives to interrogate disc surfaces for
the presence of nonoperational structures.
These nonoperational structures produce sigrials
during trackable reading that are discriminably embedded
within the normal electrical responses; the embedded signals
report physical properties of the nonoperational structures.
In conjunction with physical synchronization approaches,
analysis software newly developed for this purpose anci
described more fully in "Methods And Apparatus For Analyzing
Nonoperational Data Acquired From Optical Discs", Worthington
et al., U.S. Patent No. 6,888,951, permits these signals to be
characterized, classified, mapped, and represented visually.
In essence, these developments permit the disc
drive to be used for scanning confocal laser microscopic
inspection of one or more disc surfaces.
The robustness of this approach makes possible the
optical inspection of structures having enormous variety in
shape, size, and chemical and optical
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properties. This flexibility in turn permits such
nonoperational structures to be used for a wide variety of
signaling chores. For example, as described more fully in
U.S. Patent No. 6,342,349, such nonoperational structures
can be used to signal the results of chemical and biological
assays, ranging from immunoassay, to enzymatic assays, to
nucleic acid hybridization assays, to direct detection, of
mammalian cells.
In each of the laser microscopic applications of
optical disc drives, though, the nonoperational signaling
structures must be disposed upon or near a surface of the
optical disc prior to reading. For some applications, it
may suffice to dispose the nonoperational structures
randomly upon the disc surface, as in certain simple
counting applications. For other applications, such as in
nucleic acid array analysis (see, e.g., WO 98/12559), it may
instead be preferable to dispose these nonoperational
signaling structures in one or more ordered arrays. There
thus exists a need in the art or methods, apparatus, and
compositions that facilitate the physical patterning of
readable nonoperational structures on an optical disc.
SUMMARY OF THE INVENTION
The present invention provides methods and
apparatus for physically patterning readable nonoperational
structures on an optical disc. In one embodiment, the
nonoperational structures are magnetic beads, and the method
involves applying external magnetic fields to the disc. In
another embodiment, the nonoperational structures have net
electric charge.
In one aspect, there is provided an optical disc
comprising: a disc body having a surface including magnetic
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structures arranged in a pattern on said surface, said.
magnetic structures having a capture agent that attaches to
a specific analyte; and encoded information associated. with
said disc body, said encoded information utilized by a disc
drive to provide step rotation of the disc body between
selected locations on said disc body to thereby perform an
assay including said capture agent and said specific
analyte.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects and advantages of the
invention will be apparent upon consideration of the
following detailed description, taken in conjunction with
the accompanying drawings, in which like reference
characters refer to like parts throughout, and in which:
FIG. 1 shows a light microscopic photograph of a
portion of an optical disc upon which a plurality of beads
of uniform composition and diameter have been manually
disposed;
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FIG. 2 shows a portion of the signal acquired by reading through a
single one of the tracks of the disc portion shown photographically in FIG. 1;
FIG. 3a is a light microscopic photograph of a portion of a trackable
optical disc showing an aggregation on the disc's metal surface of beads of
disparate size and composition;
FIG. 3b is a trace chart aligning, in X-axis registration, the electrical
response reported in the buffered HF signal along ten of the tracks that pass
through the region of the disc shown photographically in FIG. 3a;
FIG. 4a shows a side elevational view of an illustrative embodiment of
an apparatus for aligning magnetic particles on a surface of an optical disc
(disc not shown) according to the present invention.
FIG. 4b shows a top perspective view of the disc support member with
embedded magnets, also shown in FIG. 4a, according to the present invention.
FIG. 4c shows a top plan view of the disc support member with
embedded magnets, also shown in FIGS. 4a and 4b, according to the present
invention.
FIG. 4d shows another side elevational view of the apparatus shown in
FIGS. 4a-4c, with an optical disc positioned thereon, according to the present
invention.
FIG. 5 presents a light microscopic photograph of the surface of an
optical disc upon which paramagnetic beads have been aligned into several
chains using externally applied magnetic fields according to a method of the
present invention.
FIG. 6 illustrates physical patterning of bead chain on an optical disc in
accordance with the present invention.
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DETAILED DESCRIPTION OF THE INVENTION
In order that the invention herein described may be fully understood,
the following detailed description is set forth. In the description, the
following
terms are employed.
As used herein, the term "radial denotes, in the plane of one or more of
a disc's data-encoding surfaces, the direction forward or backward along a
tracking spiral. A disc surface, according to this invention, can be an
internal
or external surface.
As used herein, the term "tangential" denotes, in the plane of one or
more of a disc's data-encoding surfaces, the direction inward or outward along
a line drawn from the disc's physical center to its outer circumference.
As used herein, the phrase "radial plane" refers to the plane in which a
disc's tracking (e.g., spiral traclQng) features are disposed, and is the
plane of
one or more of the disc's data-encoding surfaces.
As used herein, the term "nonoperational structure means any
structure on or within an optical disc that is capable of producing a signal
when the disc is read by an optical disc reader, the signal of which, however,
is
not required (although possibly useful) for drive operation during reading.
Nonoperational structures include, for example, analyte-specific signal
elements, as described immediately below.
As used herein, the term "analyte-specific signal element" refers to any
nonoperational structure that can be used to signal the presence of a specific
analyte in a sample applied to an optical disc. The term thus includes, inter
alia, such signal elements as are exemplified herein - including beads - as
well as those that are described in U.S. Patent No. 6,342,349. The term
includes both those structures that are alone detectable by an optical disc
reader and those that require additional components to be rendered
detectable.
As used herein, the term "magnetic bead' includes magnetic,
paramegnetic, and superparamagnetic particles, both spherical and
nonspherical, the diameter of which can vary from about 1 nm to about 1 cm.
FIG. 1 shows a light microscopic photograph of a portion of an optical
disc, with a plurality of
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beads uniformly 2.8 p.m in diameter (also termed "spheresA) disposed on the
disc's reflective surface. The traclflng features (e.g., a wobbled groove), is
readily observable; magnification precludes the continuity of the groove from
being observed.
FIG. 2 shows a trace chart displayed the electrical response reported in
the HF signal during trackable reading of the disc along a single one of the
wobble features that passes through the area of the disc shown in FIG. 1. The
trace chart reconstructs and plots the electrical signal from digitized data
earlier written to a bitstream file, as more fully described in
"Methods And Apparatus For Analyzing Nonoperational Data Acquired From
Optical Discs", Worthington et al., U.S. Patent No. 6,888,951. The X axis
shows radial distance, that is, distance along the track; the Y axis shows
voltage.
HF signals displayed by digital oscilloscope in real time look essentially
the same.
Data features 20 and 22 are correlated with the presence of the 2.8 m
beads on the surface of the disc. Features 22 have a characteristic biphasic
shape that has previously been correlated with the optical response from the
disc surface as the laser traverses first the leading edge, then the center,
and
finally the lagging edge of the bead. Feature 20, lacking the characteristic
biphasic shape, has been correlated with the optical response as the laser
grazes the edge of a bead just touching the wobbled feature being tracked.
Each of the features 20 and 22 in FIG. 2 is preceded and followed in the
electrical trace by a baseline signal feature 24, a feature resulting from the
nonadjacent spacing of discrete beads along the track. FIG. 2 thus
demonstrates that electrical signals generated by nonoperational structures
disposed upon a disc surface may individually be identified and characterized
when the structures are sufficiently well spaced as to permit the entirety of
the
characteristic signal to be observed.
FIG. 3a is a light microscopic photograph of a separate trackable optical
disc of similar design upon which have been disposed beads of disparate size
and composition. An aliquot of 4 m diameter blue polystyrene beads
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(available from Spherotech, Inc., of Libertyville, IL), 6.8 m diameter blue
polystyrene beads (available from Spherotech), and silica beads (available
from
Sigris Research, Inc., of Brea, CA) was mixed in water, spotted onto the metal
surface of the disc, and the disc then air dried. FIG. 3a is a light
microscope
photograph of the disc surfaceshowing an aggregation (e.g., a cluster) of
beads
that include two silica beads 30, a single 4 rn blue polystyrene bead 32, and
three 6.8 },sn blue polystyrene beads 34.
FIG. 3b is a trace chart aligning the electrical responses reported in the
buffered HF signal along ten of the tracks that pass through the region of the
disc shown photographically in FIG. 3a. With reference to trace 36 of FIG. 3b,
it can be seen that baseline signal features 37 precede and 'follow feature
38.
In contrast to features 20 and 22 of FIG. 2, however, each of which represents
the signal produced by a single, well-spaced, nonoperational structure,
feature
38 represents a complex response caused by the aggregation of several beads
on a disc surface.
Although software-based algorithmic approaches, more fully described
in "Methods And Apparatus For Analyzing Nonoperational Data Acquired From
Optical Discs," Worthington et al., U.S. Patent No. 6,888,951,
can be used to deconvolute such a complex signal once acquired, there
nonetheless exists a need in the art for complementary approaches that
facilitate the return of individually discriminable and identifiable
electrical
signals from nonoperational structures on a disc surface. Thus, there exists a
need for methods that permit such nonoperational structures to be disposed
on the surface of the disc in disaggregated forms and patterns.
The present invention presents solutions to this problem by exploiting
physical properties that inhere in the nonoperational structures to effect the
patter of their disposition on the disc surface.
In a first exemplary embodiment, the nonoperational structures are
magnetic and can be patterned on a disc surface by applying magnetic fields.
The nonoperational structures can be patterned into one or more discrete
spots on the disc surface, and can be patterned within each spot in a chosen
linear direction.
Paramagnetic beads in the sub-micron to sub-millimeter size range
have proven to be extremely useful in many different biologic and chemical
applications. With one member of a cognate pair of high affinity specific
binding groups attached to the surface of the bead - such as an antibody
highly specific for the binding of a ligand, streptavidin for the highly
speciific
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binding of a biotin, or a nucleic acid for the highly specific binding of its
nucleic acid complement - the second member of the cognate pair can readily
be purified from a fluid mixture by introduction of the beads followed by
application of a magnetic field. The magnetic field is believed to not exert
significant mechanical stress on the bound analyte, which can range in size
from eukaryotic cells, to long nucleic acids, to small molecule ligands, and
is
often rapid, highly scalable, low cost and nontoxic.
Paramagnetic beads are readily available commercially, either
derivatized with the most common specific binding groups, derivatized with
reactive chemical moieties for custom conjugation, or completely
underivatized.
Compositions of the beads vary, with some having a coated paramagnetic core,
others having Fe30 and or Fe203 and Fe304 evenly incorporated throughout
the beads, and still others having a polystyrene core with an iron oxide/poly-
styrene coating, with bead diameters varying from 50 nm (available, for
example, from Miltenyi Biotec, of Auburn, CA) to 7 m (available, for example,
from Spherotech, Inc., of Libertyville, IL). Noncolloidal superparamagnetic
beads with diameter between about 1 m and 10 m in diameter, have been
used in the present invention, although permanently magnetic or
paramagnetic particles can also be useful in the practice of the present
invention.
According to the present invention, magnetic beads can be patterned on
a disc surface using a device with embedded magnets, an embodiment of
which is shown in FIGS. 4a-4d.
FIG. 4a is an elevational side view of patterning device 40, which
includes disc support member 41 (and sometimes called a "platen"), and which
is dimensioned to support an optical disc on one (typically the upper)
surface,
hereinafter the "support surface."
Disc support member 41can be discoid, as shown best in FIGS. 4b and
4c, thus providing a circular support surface, with the support surface
diameter approximately equal to, more usually somewhat larger than, that of
the optical disc to be patterned. For standard optical discs, such as CD-ROM
or DVD discs, disc support member 41 would thus typically have a diameter of
at least 12 cm, although often somewhat larger, ranging from about 12 cm to
about 20 cm, more typically from about 12 cm to about 15 cm, most typically
from about 12 cm to about 14 cm. Since the Red Book standard further
contemplates optical discs of 8 cm diameter, the disc support surface can
alternatively be about 8 cm in diameter, or somewhat larger. In the latter
case,
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however, disc support member 41 will typically have a size sufficient to
support
either the 8 cm or the larger 12 cm disc.
Although shown as discoid, disc support member 41 can be any shape,
and need only be sufficiently large as to support the disc to be patterned and
to permit the application of magnetic fields to the area of the disc desired
to be
patterned. Typically, the disc support surface will be substantially planar,
to
permit close apposition of the disc to the disc support surface. Further to
facilitate that apposition, the disc support surface, although substantially
planar, can have an indentation therein (e.g., annular) to accommodate a
disc's
stacking ring.
Patterning device 40 can contain more than one disc support member
41.
Disc support member 41 will typically be made of non-magnetic solid
material, such as a nonmagnetic metal, glass, ceramic, or plastic.
FIG. 4b shows a disc support member 41, which contains a plurality of
embedded magnets 44. Typically, the surface of the embedded magnets 44 will
be co-planar with the disc support surface of disc support member 41,
although magnets can be recessed from the disc support surface (and thus
further from the disc supported thereon), or can alternatively extend above
the
support surface (and thus closer to the disc support thereon), although in the
latter case the combined surface area of the magnets must be sufficient as to
permit the disc to be supported thereon. It will be appreciated that magnets
44 need not be embedded at all, but could be merely mounted to a surface
(upper or lower) of support member 41.
Although electromagnets can be used, magnets 44 are permanent
magnets made of a rare earth alloy such as anisotropic sintered materials
composed of neodymium-iron-boron or samarium-cobalt, with a surface field
strength sufficient to attract a majority of the magnetic particles desired to
be
patterned. Surface field strengths of about 100 Gauss to 1 kiloGauss are
usually adequate to attract magnetic particles in the size range of about 1 m
to 10 m. Accordingly, patterning device 40 can employ magnets 44 in disc
support member 41 having a surface field strength of about 50 Gauss to about
50 kiloGauss, and even more typically about 100 Gauss to about 2.5
kiloGauss, with a range of 100 Gauss to about 1 kiloGauss being most
typically.
High energy permanent magnets made from neodymium-iron-boron or
samarium-cobalt and characterized by BHm. (maximum energy product) in the
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range of 25 to 45 MGOe (megaGauss Oersted) can also be used. Such magnets
can be obtained from International Magnaproducts Inc., of Valparaiso, IN, and
many other commercial sources.
Magnets 44 can be glued or fixed by standard mechanical means to disc
support member 41 or can, alternatively, be embedded therein, as, for
example, by polymerization or solidification of a liquid around the magnets.
Alternatively, disc support member 41 can be fashioned so that magnets 44
can be removed and replaced. This latter embodiment allows the magnetic
field strengths, as further described below, to be readily adjusted.
Magnets 44, shown in FIGS. 4a-4d, have a rectangular cross-section
and are oriented with their magnetic lines of force oriented in the plane of
the
disc support surface. Alternate cross-sectional shapes, orientations, and
magnetic pole orientation with respect to the platen are also envisioned,
depending upon the orientation and patterns desired to be imposed upon the
magnetic particles on the disc surface.
Disc support member 41 can additionally have a central hole 43, shown
in FIG. 4c, to permit mounting of the disc support member upon a spindle 42,
shown in FIG. 4a. Alternatively, spindle 42 can be integral to disc support
member 41. Spindle 42 can be rotatable by a motor 45, typically an electric
motor, as shown in FIG. 4a.
Spindle 42, when present, will typically be smaller in diameter than the
central mounting hole of the optical disc to be patterned, permitting optical
disc 10 to be placed on disc support member 41 with spindle 42 protruding
therethrough, as shown in FIG. 4d. The space between optical disc 10 and the
support surface of disc support member 41 is exaggerated for purpose of
illustration. To facilitate registrable positioning of disc 10 on the disc
support
surface of disc support member 41, spindle 42 will typically be dimensioned so
as to be only slightly smaller than the central mounting hole of disc 10,
permitting the spindle to snuggly engage disc 10.
Chuck 43, with a recess or throughvoid dimensioned to accommodate
spindle 42 therewithin or therethrough respectively, and with at least one
outer dimension exceeding the diameter of the mounting hole of disc 10, can
then be used reversibly to secure disc 10 to disc support member 41. Chuck
43 can engage spindle 42 in any conventional way, such as in a snappable or
screwable fashion.
When disc 10 is placed (typically reversibly fixed) on disc support
member 41, magnets 44 generate multiple magnetic fields on each of the
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planar surfaces of disc 10, and in each magnetic bead-containing liquid
droplet
46 applied thereupon. The magnetic field across the cross-section of a droplet
is characterized by a field gradient.
In one embodiment of the apparatus and methods of the present
invention, shown in FIG. 4c, magnets 44 and droplets 46 are so positioned
that,.as viewed from above, each droplet 46 is positioned on the disc so as to
lie at the edge of a magnet; this creates a magnetic field gradient in the
droplet
that is stronger closer to the magnet than elsewhere in the droplet.
When the droplet contains a suspension of magnetic beads, typically
paramagnetic or superparamagnetic beads, the field gradient causes movement
of the magnetic beads as the stronger magnetic field in the vicinity of the
magnets pulls the beads. As the beads are pulled, they move and form bead
chains. In this embodiment, these chains are oriented in a tangential
direction(i.e., a direction that is substantially perpendicular to the track).
FIG. 5 presents a light microscopic photograph of the surface of an optical
disc
upon which paramagnetic beads have been aligned into several chains using
such externally applied magnetic fields. While not wishing to be bound by
theory, it is presently believed that the chains result from the magnetic
dipoles
induced in the beads by the externally applied magnetic fields provided by
magnets 44.
Depending on the number of beads present in droplet 46, multiple
parallel chains can be formed. These chains have been observed to migrate
toward a magnet and this movement is arrested when the leading bead of the
chain reaches the limit of the fluid droplet.
A careful control of the magnetic field strength is important so that the
bead chain is not subjected to excessive fluid drag forces, which would tend
to
cause the chains to break apart or to move out of the droplet boundary and
become an aggregated mass; conversely, too low a field precludes the formation
of chains. A suitable magnetic field strength can be calculated on the basis
magnetic mass susceptibility of the beads, buoyancy, and fluid friction.
However, for any given disc, patterning device 40, bead composition and size,
the intensity of the magnetic field or fields can readily, and will often, be
determined empirically.
Furthermore, it will be appreciated that the field strength at the desired
disc surface can readily be adjusted for any given disc support member 41 by
altering the distance between its disc support surface and optical disc 10,
through interposition of variable numbers of blank (e.g., single-layer
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polycarbonate) discs therebetween. Such blank discs can usefully have one or
more structural features, such as interleaving tabs or lips, that facilitate
stacking.
Field strength can also usefully be varied by interchanging disc support
members 41 in disc patterning device 40, each of said disc support members
having magnets 44 of different field strength and/or orientation. In this way,
a
gross adjustment of field strength and/or orientation can be made by selecting
an appropriate disc support member 41, with finer adjustment made by use of
varying numbers of blank between disc 10 and the support surface.
Although disc support member 41 shown in FIGS. 41-4d includes a
plurality of magnets of identical strength, it will also be appreciated that a
disc
support member can include a plurality of magnets having different field
strengths, depending upon the pattern desired.
One such desired pattern is to dispose the magnetic beads substantially
in the tangential direction. As shown in the trace chart in FIG. 3b, the data
features created by each bead can be more readily distinguished and thus
characterized in the tangential (Y axis) direction than in the radial (X axis)
direction.
In addition to choice of droplet location and change in field intensity,
further physical patterning can be effected by applying centrifugal forces.
As noted above, disc support member 41 can be fixed to rotatable
spindle 42 of a motor 45; to same effect, spindle 42 can be integral to disc
support member 41, and attachable to motor 45.
Motor 45 can be any type of motor, and is preferably an electric motor,
such as an electric step motor capable of providing a stop-wise change of a
predetermined distance in the relative angular position of disc support member
41. Step rotation of predefined angles can be effected by means of an
electronic motor control (not shown) according to devices and techniques well
known in the art. Application of centrifugal force by means of disc rotation
will
permit bead chains to be disposed at angles different from those that would be
effected by the magnets with the disc support member and applied disc
stationary. Preferably, such centrifugal forces are applied while the beads
remain in fluid suspension; after drying or evaporation of droplet 46, the
magnetic beads will thereafter typically adhere to the disc surface by
noncovalent and potentially other interactions.
The use in disc patterning device 40 of rotatable disc support members
41 further serves to facilitate application of magnetic bead-containing
droplets
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46 in predetermined patterns useful for analyte-specific assay. Geometries
that are useful in such assays are set forth in. detail in
U.S. Patent No. 6,342,349.
Thus, rotation of disc support member 41 can be coordinated with
robotic fluid dispensers well known in the art. Angular step movement of disc
support member, _ 41 in conjunction with the linear displacement of the
dispenser permits addressable applicable of droplets 46 at any chosen location
on the surface of the disc. If desired, time delays between the angular
movement of the disc or the linear movement of the dispenser can be
interposed for inspection purpose or other process needs. FIG. 6 illustrates
physical patterning on disc 10 with multiple bead chains 60, each having an
angular component, each originating from a droplet applied robotically to the
disc surface.
Although described with particular emphasis on magnetic patterning of
magnetic beads, the present invention comprehends other means of physically
patterning nonoperational structures on the surface of an optical disc.
For example, aggregation of nonoperational structures can be prevented
by using beads or other structures that have a net surface charge sufficient
to
cause the structures, when in fluid suspension on a disc surface, to repel one
another. This would prevent aggregation. Furthermore, application of
localized electric fields can be used to effect further physical patterning.
Moreover, aggregation can be prevented using patterned adhesive, which can
be patterned on a surface using any conventional lithographic technique, as
well known in the semiconductor arts.
If the nonoperational structures are both charged and (para)magnetic,
both electric and magnetic fields can be used, permitting further
discrimination in the physical patterning effected. It will be appreciated
that
some nonoperational structures can be charged, some magnetic, some both,
and that the charged structures can all be of the same net charge, or can
alternatively include structures with opposite charge.
It will, therefore, be appreciated that the present invention permits a
desired spatial distribution (physical pattern) of readable nonoperational
structures to be effected on a surface of a trackable optical disc. In
particular,
the invention permits a desired spatial distribution of data-encoding
nonoperational structures, readable by an optical disc reader, to be effected
on
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a surface of an optical disc, and especially on the surface of a trackable
optical
disc.
Broadly spealnng, the present invention provides methods and
apparatus for superimposing a second mastering process, involving mastering
nonoperational structures, upon a first mastering process that involves data
digitally encoded within the disc. The second mastering process involves
application of exernal magnetic and/or electric fields to the disc.
It will also be appreciated that the physical properties of certain
nonoperational structures, such as magnetic beads, that are usefully employed
in separating analytes prior to disposition on an optical disc, can also be
used
to pattern such structures on the surface of the disc- for maximal detection.
This provides efficiencies that are useful when sample sizes, are obligately
small.
Although particularly described with respect to facilitating the
acquisition of discrete electrical signals by disaggregating plural
nonoperational structures on the disc surface, the invention is useful in
other
ways as well.
For example, some assays conducted on, optical discs wi1T tether
multiple nonoperational signaling structures, such as beads, to a single
analyte, such as a nucleic acid. In such assays, each of said nonoperational
signaling structures is capable of reporting a discrete physical property of
the
analyte, and their physical proximity reports the concurrent presenee on a
single analyte of the respective properties. As in the case described above,
aggregation of the nonoperational structures can interfere with analysis of
the
electrical signals acquired during reading of the dise. The present invention
presents a useful means and -apparatus to effect disaggregation of these
nonoperational structures.
As another example, the methods and apparatus of the present
invention prove useful in increasing the electrical signal generated by each
individual nonoperational structure. Signal intensity will be maximized when
the nonoperational structures are centered on a tracking feature, such as a
wobbled feature. Application of a physical force, as by application of a cover
to
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the disc surface, can usefully compel the nonoperational structures into such
grooves; magnetic and electric fields, as described herein, can also be so
used.
The present invention will prove useful not only in iunproving signal
acquisition, but in facilitating disc operation as well. Since the
nonoperational
structures to be patterned may not themselves contribute to the disc
operation, they can be patterned so as to minimizr interference with disc
trac.idng, focus, and synchronization.
For example, with respect to traclarig, it is desirable to pattern the
nonoperational elements so that they do not, through aggregation, cause a
signal of sufficient width as to interfere with tracking. With respect to
focus, it is
desirable to pattern the nonoperational structures so that they do not
aggregate along the optical axis (piling up).
Furthermore, although magnetic bead alignment has been described
above in the context of an optical disc surface, it will be appreciated that
such
I5 alignment could alternatively be on the surface of an optical disc cover.
All patents, patent publications, and other published references
mentioned herein are hereby incorporated by reference in their entirety as if
each had been individually and specifically incorporated by reference herein.
While preferred illustrative embodiments of the present invention are
described, it will be apparent to one skilled in the art that various changes
and
modifications can be made therein without departing from the invention, and it
is intended in the appended claims to cover all such changes and modifications
which fall within the true spirit and scope of the invention.
