US 6137502

I'll search for the patent's litigation and docket records, then summarize.

Summary — U.S. Patent No. 6,137,502

Bibliographic data (from the patent's front-page/Google Patents record)

Abstract (as issued)

An ink jet print head has first nozzles of a first (larger) diameter ejecting droplets of a first (larger) mass, and second nozzles of a second (smaller) diameter ejecting droplets of a second (smaller) mass. First and second heater-switch pairs are connected in parallel on the print head substrate; the first pairs include first heaters adjacent the first nozzles and the second pairs include second heaters adjacent the second nozzles, each heater being electrically resistive and occupying a respective heater area. Each switching device is in series with its heater and develops a voltage drop as current flows. The abstract notes two benefits: the first heater area is larger than the second heater area (matching heater area to nozzle diameter for more efficient thermal-energy transfer), and the voltage drop across each first switching device is substantially equivalent to that across each second switching device (reducing nozzle-to-nozzle variation in delivered energy).

Plain-language overview of the independent claims

Claim 1 — resistance-tuned series switches. A thermal ink jet print head with two nozzle populations (large-drop nozzles and small-drop nozzles). On the substrate, each large nozzle has a resistive first heater, and each small nozzle has a resistive second heater, with current flowing in the same direction through both. A first switching device (e.g., MOSFET) is in series with each first heater and a second switching device in series with each second heater. The single point of novelty recited is that the second switching devices' electrical resistance is greater than the first switching devices' electrical resistance. In the specification's worked example, the large heater is a 21 µm × 21 µm square (441 µm², 28 Ω from 28 Ω/sq TaAl), and the small heater is 11.75 µm × 23.5 µm (276 µm², 56 Ω); because the pairs are driven from a common V_dd rail, the series switch resistances are ratioed (4.3 Ω vs. 8.6 Ω) so that both heater types see the same voltage drop and operate at the same power/energy density, giving uniform bubble/drop sizes.

Claim 10 — area-tuned switches. Same two-nozzle, two-heater, two-switch architecture, but the distinguishing limitation is geometric and resistance-free: each first switching device occupies a larger switch area on the substrate than each second switching device (the spec achieves this by widening the source-drain line width of the large-drop device). This is essentially the structural/layout counterpart of claim 1.

Claim 19 — parallel pairs with matched switch voltage drops. Recites first heater-switch pairs (first heater + series first switching device) and second heater-switch pairs (second heater + series second switching device). The three conjunctive requirements are: (i) the first and second heater-switch pairs are connected electrically in parallel; (ii) the first heater area is larger than the second heater area; and (iii) the first switching device voltage drop is substantially equivalent to the second switching device voltage drop. This is the broadest "energy-balance" claim — it captures the design goal even where the specific switch-resistance or switch-area numbers of claims 1 and 10 are not met.

Claim 20 — dual voltage rails. The first heater-switch pairs are connected to a first voltage source and the second heater-switch pairs to a second voltage source that supplies a smaller voltage. In this embodiment, drop size is set by the supply voltages rather than by different heater resistances: the second heater area is smaller than the first, and the second heater's electrical resistance is substantially equivalent to the first heater's resistance (spec: both heaters square, 21 µm and 16.6 µm on a side, 441 µm² and 276 µm²). The specification's FIG. 9 wiring scheme shows how V_dd1 and V_dd2 rails can be routed to vertically alternating large and small heaters using only two metal layers.

Points of note / uncertainties

1. On-sale/priority context: Filing is 1999-08-27, so the 20-year term ran to 2019-08-27 — the patent is expired. Litigation dockets in 2026 for this number would therefore be unusual (any 2026 CAFC activity would most likely concern a different, later-expiring patent in the same Lexmark→Funai→Slingshot portfolio).
2. Cited prior art (15 references), including US 4,746,935 (Multitone ink jet printer), US 4,719,477, US 5,030,971, US 5,072,150, US 5,122,812, US 5,159,353, US 5,208,605, US 5,412,410, US 5,357,081, US 5,635,968, US 5,745,131, US 6,030,065, EP 0 613 781 A1 and EP 0 805 029 A2. The EP search report for the sibling EP 1 214 199 additionally cited EP 0 867 285 A2 (Lexmark), EP 0 785 072 A2 (Canon), US 5,521,622, EP 0 909 649 A2 and EP 0 674 994 A2.
3. Litigation — what I could and could not confirm. Google Patents' litigation metadata for this patent flags two W.D. Tex. filings (6:19-cv-00363 and 1:20-cv-00185), which correspond to the Slingshot Printing LLC v. HP Inc. campaign. Those cases asserted a large portfolio of Lexmark-origin printhead patents (e.g., the '299, '587, '738, '206, '596, '878 patents). I could not independently verify from the sources retrieved that US 6,137,502 itself was asserted in those suits or that it appears in any claim chart.
4. CAFC 2026 dockets — no match found. My searches of the Federal Circuit's 2026 argument calendar and docket feeds surfaced 2026 Slingshot Printing LLC v. Canon U.S.A., Inc. appeals (Nos. 25-1032 and 25-1033, argued July 6, 2026; also a 24-2132 docket), but the patents at issue in those appeals are 7,152,951, 7,195,341, 7,559,629, 7,938,523, 7,484,823, 7,594,708 and 7,290,864 — not 6,137,502. A targeted search for "6137502" together with "Federal Circuit" returned no results. I therefore cannot confirm any Court of Appeals docket tied to this specific patent number, and I am flagging that explicitly rather than inferring one. If you need a definitive answer, the authoritative check is PACER/CM-ECF for the Federal Circuit and USPTO Patent Center / Assignment records for the '502 patent.

Sources consulted

• Google Patents, US6137502A — https://patents.google.com/patent/US6137502A/en (full text, claims, assignments, litigation metadata)
• FreePatentsOnline, US 6,137,502 — https://www.freepatentsonline.com/6137502.html
• EPO Global Patent Index / PISE family data for EP 1 214 199 A1 and EP 1 520 712 A3 — http://data.epo.org/gpi/EP1214199A1-DUAL-DROPLET-SIZE-PRINTHEAD.html
• Federal Circuit scheduled-cases calendar (July 2026) — https://www.cafc.uscourts.gov/home/oral-argument/scheduled-cases/
• CourtListener docket feed, Slingshot Printing LLC v. Canon U.S.A., Inc. (24-2132) — https://www.courtlistener.com/docket/73304646/45/slingshot-printing-llc-v-canon-usa-inc/
• Unified Patents litigation pages, W.D. Tex. 6:19-cv-00363 and 1:20-cv-00185 — https://portal.unifiedpatents.com/litigation/Texas%20Western%20District%20Court/case/6%3A19-cv-00363
• Slingshot v. Canon E.D.N.Y. filings / PTAB mandatory notices (portfolio context) — https://ptacts.uspto.gov/ptacts/public-informations/petitions/[1550157](/patent/1550157)/download-documents

I'll search for litigation involving US patent 6137502.

Let me get more specifics on these two cases and their outcomes.

Let me pin down the outcomes and check for any IPR or additional cases.

Bottom line

Based on searches of litigation databases (Unified Patents litigation portal, Docket Alarm, Justia Dockets, CourtListener, PACER-derived dockets, PTAB records) and trade/legal press, I found one patent-infringement dispute involving US 6,137,502 ("Dual droplet size printhead"), and it is the same W.D. Tex. action listed twice on the patent's Google Patents page under two docket numbers (Waco filing number and post-transfer Austin number). I found no other lawsuits, and no IPR/PGR specifically challenging 6,137,502.

1. Slingshot Printing LLC v. HP Inc.

Sources: Google Patents/Unified Patents litigation entries for US 6,137,502 (https://portal.unifiedpatents.com/litigation/Texas%20Western%20District%20Court/case/6%3A19-cv-00363 and .../case/1%3A20-cv-00185); HP's Amended Preliminary Invalidity Contentions in 1:20‑cv‑00185‑ADA (May 22, 2020); Claim Construction Order, Nos. 1‑20‑cv‑00184‑ADA and 1‑20‑cv‑00185‑ADA (Sept. 15, 2020); Order of Dismissal With Prejudice, 1:20‑cv‑00189‑ADA (Mar. 24, 2021); The Recycler, "HP sued for patent infringement" (complaints of June 11, 2019 listing 6,137,502).

Caveats on this case. (a) Secondary sources are internally inconsistent about the Waco→Austin correspondence; one HP IPR petition described the action as "Case No. 6:19-cv-00185-ADA (formerly Case No. 1:19-cv-00363-ADA in the Western District of Texas, Waco Division)" (IPR2020-01086 Petition), i.e., the digits are transposed relative to the Google Patents/Unified Patents listing. I have not auto-corrected these; both numberings appear in the record. (b) I directly confirmed a with-prejudice dismissal order for the companion case 1:20-cv-00189-ADA, and the global settlement/termination of the parallel PTAB cases, but I did not independently pull the docketed dismissal order for 1:20-cv-00185-ADA itself — the dismissal there is inferred from the common settlement and contemporaneous dismissal of the companion actions, and should be verified in PACER. (c) The '502 patent's anticipated expiration was Aug. 27, 2019 (20 years from the Aug. 27, 1999 filing date), so the recoverable damages window in this suit was very short.

2. Related PTAB proceedings (none on 6,137,502)

HP filed a series of IPRs against sibling Slingshot/Lexmark patents in 2020, all terminated by the same settlement:

• IPR2020-01084 (US 6,394,593)
• IPR2020-01085 (US 6,243,115)
• IPR2020-01086 (US 7,014,299)
• IPR2020-01090 (US 7,244,015)
• IPR2020-01659 (US 7,410,246)

All were terminated without a final written decision (IPR2020-01084/-01085/-01086/-01090 by the March 29, 2021 Termination Due to Settlement After Institution of Trial; IPR2020-01659 terminated March 29, 2021, "Terminated‑Settled").

No IPR, PGR, reexamination, or other PTAB proceeding directed at US 6,137,502 was found in these searches. I cannot rule out a proceeding I did not surface, but nothing appeared in the dockets, IPR petitions, or the patent's own prosecution/litigation linkage.

3. Other litigation

• The only other "litigation" flag on the patent's Google Patents page is a Darts‑IP "First worldwide family litigation filed" link (family ID 23518820), which is a pointer to the family's litigation record, not a separate case.
• The later Slingshot suits against Canon U.S.A., Inc. / Canon Inc. (e.g., E.D.N.Y. No. 2:22-cv-00123 and 2:22-cv-01852) asserted other Slingshot patents (e.g., 7,152,951; 7,195,341; 7,559,629; 7,938,523; 7,484,823; 7,594,708) — not 6,137,502.
• No ITC investigation involving 6,137,502 was found.

Conclusion: The only known litigation involving US 6,137,502 is Slingshot Printing LLC v. HP Inc., W.D. Tex. (6:19-cv-00363-ADA / 1:20-cv-00185-ADA), filed June 11, 2019, resolved by settlement and dismissal with prejudice (companion dismissal order entered March 24, 2021) without a merits ruling on the '502 patent.

I'll verify the structured data with targeted searches before finalizing.

Let me run targeted searches on the specific patent number and the district court litigation.

One more pass to confirm the absence of any AIA trial on this specific patent.

Proceedings overview

Zero AIA trial proceedings are on file against US 6,137,502 — the USPTO ODP structured block returns no IPRs, PGRs, or CBMs, and targeted searches of PTAB docket aggregators (Docket Alarm, PTACTS/PTAB E2E, Law360 coverage of the Slingshot–Canon campaign) surfaced no petition ever docketed against the '502 patent. The defensive posture is therefore not "hardened by surviving IPRs" and not "claims canceled" — it is an untested patent that has now expired, which is a different and in some ways stronger defense: there is no PTAB estoppel on either side, every invalidity ground is still available in district court under § 282, and the patent's term lapsed on 2019-08-27, capping exposure to past conduct.

No AIA trial proceedings on file

• Type: N/A (no IPR / PGR / CBM ever instituted or filed)
• Filed: N/A
• Status: N/A — the canonical structured block ("USPTO Open Data Portal returns no AIA trial proceedings for this patent") is corroborated by public search; nothing found that the ODP has not indexed.
• Judge panel: N/A
• Petition grounds: N/A
• Institution decision: N/A
• Final Written Decision: N/A — no FWD exists, and no panel has ever construed a claim of this patent.
• Settlement / termination: N/A
• Appeal: N/A — no PTAB decision to appeal. (The Federal Circuit activity in Slingshot Printing LLC v. Canon U.S.A., Inc., e.g. Nos. 24-1956 and 2025-1032, involves sibling Slingshot patents — U.S. 7,195,341; 7,290,864; 7,938,523; 7,594,708; 6,394,593 — not US 6,137,502. Do not let a demand letter conflate them.)
• Defensive value: No PTAB record means no institution roadmap and no claim-construction ruling to borrow — but also no § 315(e)(2) estoppel binding anyone. A defendant is free to run any § 102/§ 103 theory in district court, and can still file its own IPR if a live past-damages theory is asserted (the Board has accepted IPRs on expired patents where past damages are at stake).

Adjacent, non-PTAB context (district court, not an AIA trial): The '502 patent was asserted — Slingshot Printing LLC v. HP Inc., W.D. Tex. (the two litigation entries in the structured block, 6:19-cv-00363 and 1:20-cv-00185). HP's amended preliminary invalidity contentions in 1:20-cv-00185 targeted claim 19 of the '502 patent and identified Tachihara et al. (US 6,447,088), Ishinaga et al. (US 5,731,828), Allen (US 4,746,935) and Bhaskar et al. (US 5,635,968) as invalidating references. (exhibit PDF). Note two of those references (Allen; Bhaskar) are already of record on the '502 face. I could not confirm the disposition of that case with confidence — treat any settlement-versus-judgment characterization as unverified.

Strategic summary

Claim status: nothing canceled, nothing sustained, nothing adjudicated. All twenty claims (independents 1, 10, 19, 20 and their dependents) remain as issued; no PTAB panel has ever invalidated or upheld a single claim of this patent. The commercially important claims are claim 19 (parallel heater-switch pairs with matched switching-device voltage drops) and claim 20 (dual voltage sources Vdd1/Vdd2 with substantially equivalent heater resistances) — claim 19 is the one Slingshot actually put in play against HP per HP's invalidity contentions. Those claims are untested, not "hardened."

The controlling fact is expiration, not PTAB. Per the structured record, the patent expired 2019-08-27 (20 years from the 1999-08-27 filing; all three maintenance fees paid — 2004, 2008, 2012 — and legal status "Expired – Lifetime"). Slingshot Printing LLC took assignment on 2019-03-29, roughly five months before expiry, and the HP suits followed in June 2019 / January 2020. For a defendant today, damages are limited to pre-expiry conduct and further capped by the six-year lookback of 35 U.S.C. § 286, plus any § 287 marking defense on unmarked product. Practically: the only exposure is a backward-looking damages case.

Estoppel landscape: clean both ways. Because no IPR was ever filed, § 315(e)(2) estoppel does not bind anyone — no petitioner is estopped, and no patent owner has had claims canceled to rely on. Every prior-art ground (patents, printed publications, and — unlike in an IPR — public-use/on-sale § 102(a)(1) art, plus § 112 and § 101 challenges) remains available in district court. If you do file an IPR, you can also raise grounds in court that were "not reasonably could have been raised" only if the IPR reaches FWD, so sequencing matters.

Pattern signals. Slingshot Printing LLC is an NPE asserting a Lexmark-origin portfolio it acquired from Funai Electric (assignment 2019-03-29). The 2019–2020 assertion wave against HP did not generate an IPR on the '502 patent, even though HP had invalidity contentions prepared — consistent with the patent expiring mid-litigation and with the economics of IPR on a nearly-dead patent. The later, much larger Canon campaign (IPR2022-01414, IPR2022-01416, IPR2023-00309, IPR2023-00312, IPR2023-00313) targeted other Slingshot patents. There is no evidence in the structured record that Unified Patents or any other defensive aggregator filed against this patent; Unified appears only as the host of the litigation-data links in the patent record, which is not participation.

Recommended next steps

• No PTAB activity exists — say so plainly, and use it. There is no Final Written Decision to quote and no institution decision to cite. Do not promise a client a cancelation that never happened.
• Run the expiration math first. Confirm the 2019-08-27 expiry and the maintenance-fee history against USPTO Patent Center and Google Patents. If the asserted products were sold only after 2019-08-27, the infringement theory fails at the threshold; if pre-expiry sales exist, scope damages to the § 286 window and check § 287 marking.
• Attack claim 19 head-on in court. HP's contentions (link above) supply a ready-made starting kit — Tachihara, Ishinaga, Allen, Bhaskar — against the "substantially equivalent switching device voltage drop" limitation, and Allen/Bhaskar are already on the face of the patent, so you will need a strong secondary-reference story rather than a cumulative-art theory.
• If a live past-damages demand arrives, an IPR is still viable on an expired patent where past damages are asserted. Any such petition would be filed against a patent with no prior PTAB record and no § 315 estoppel racing against you — but weigh IPR against the § 286 cap; on a 2019-expiry patent the recoverable stake may be too small to justify the filing fee.
• Monitor. Recheck PTAB E2E / PTACTS and CourtListener periodically: a newly-filed petition against an expired patent is unusual but not impossible if Slingshot pursues past-damages claims against a new defendant.

Confidence note: I am highly confident there is no AIA trial proceeding on US 6,137,502 (structured data plus independent search agree). I am not confident about the disposition of Slingshot v. HP, W.D. Tex. 1:20-cv-00185, and I have not verified the current ownership record beyond the structured assignment data — confirm both against PACER before relying on them in a filing.

I'll verify the assignment records and correspondent details against live sources before writing this up.

Let me dig into the correspondent of record and the parties' current status.

Assignment & Ownership Analysis — U.S. Patent No. 6,137,502

Verification note: The primary source of truth for this section is the assignment data accompanying the patent record (Google Patents legal-events feed, which mirrors the USPTO Assignment Center record). The USPTO Assignment Center front-end at https://assignmentcenter.uspto.gov/ renders the same reels/frames; I was able to confirm the reel/frame identifiers and execution/recording dates but not the per-record correspondent of record field, which is stripped from the mirrored data. Where I could not retrieve a correspondent, I say so rather than guess.

Inventors

Pattern assessment — no adverse signal. All four inventors assigned their entire right, title and interest to Lexmark by an instrument executed 1999-08-26 and recorded 1999-08-27 — i.e. the day before and the day of the 1999-08-27 application filing. That is the ordinary Lexmark practice of taking the employment-obligation assignment contemporaneously with filing, which is not the same tell as inventors transferring rights independently of the company. There is no record evidence of any inventor assigning separately, holding an undivided interest, or departing within 12 months of filing. I did not find documentary evidence of a near-term inventor exodus, and I am explicitly declining to infer one from the shared "Lexmark" address on the record.

Caveat: employment dates and departure dates are not exposed in the assignment record, PEDS-style bibliographic data, or the patent text. If a departure analysis matters, the corroborating source is the inventor-employment or issue-history record for each inventor's later filings, which I did not retrieve.

Original assignee

Lexmark International, Inc. (a Delaware corporation; principal place of business One Lexmark Centre Drive / 740 West New Circle Road, Lexington, Kentucky) — named as assignee on the issued patent, and the assignee of record on the assignment instrument itself, with the inventing entity's address of record during prosecution being Lexmark's own Intellectual Property Law Department, 740 West New Circle Road, Bldg. 082-1, Lexington, KY 40550-0999 (Customer No. 21972).

• Primary line of business: design, manufacture and sale of laser and inkjet printers, MFPs, cartridges and related software/services. Lexmark was spun out of IBM's printing division in 1991.
• Did it ship a product embodying the claims? Yes — the '502 patent is a Lexmark inkjet printhead patent (dual-nozzle, dual-drop-size architecture for multi-level gray-scale inkjet printing). It comes out of the period in which Lexmark designed and manufactured its own thermal inkjet printheads and cartridges in-house. I have not mapped the claim set to a specific Lexmark SKU with documentary support, so treat "embodying" as satisfied at the business-line level, not the model-number level.
• Current status: Operating (in a different configuration / different owner). Lexmark divested its entire inkjet hardware business in 2013 (see timeline), and in 2016 Lexmark itself was acquired by a Chinese-led consortium (Apex Technology, PAG Asia Capital, Legend Capital) and taken private. The Lexmark name and business continue. Note: the Google Patents legal-events record shows no change-of-name entry for Lexmark International, Inc., so the corporate name on the 1999 instrument is still the correct legal name for that link in the chain. (I flagged the 2016 transaction from background knowledge; my verification search on it was cut off before I could pull a primary SEC filing this session.)

Assignment timeline

Three assignments are recorded against US 6,137,502. No security agreement, license, merger, release, correction, or change-of-name instrument appears in the chain.

• 1999-08-26 (executed) / recorded 1999-08-27 — Reel 010210 / 0543

  • Conveyance: Assignment (of assignors' interest) — "ASSIGNMENT OF ASSIGNORS' INTEREST"; assignors: Anderson, Frank Edward; Bolash, John Philip; Cornell, Robert Wilson; and others
  • Assignor: Frank Edward Anderson; John Philip Bolash; Robert Wilson Cornell; George Keith Parish (all individual inventors)
  • Assignee: LEXMARK INTERNATIONAL, INC., Kentucky
  • Correspondent: Not retrievable from the mirrored record. The prosecution address of record for Lexmark-origin filings of this era is Lexmark's in-house IP Law Department, Lexington KY (Customer No. 21972) — but I cannot state that this was the recording correspondent without the Assignment Center abstract, and I will not assert it.
  • Context: Ordinary pre-filing employment assignment from inventors to the operating employer — not a transfer of a going-concern asset.

• 2013-04-01 (executed) / recorded 2013-05-14 — Reel 030416 / 0001

  • Conveyance: Assignment
  • Assignors: LEXMARK INTERNATIONAL, INC. (and, per the SEPARATE-SELLER structure, its wholly-owned Swiss subsidiary LEXMARK INTERNATIONAL TECHNOLOGY, S.A. — "LITSA")
  • Assignee: FUNAI ELECTRIC CO., LTD, Japan
  • Correspondent: Not retrievable from the mirrored record. (Do not confuse this with Fitch Even or MoloLamken — those are Slingshot's later PTAB counsel, not this recording's correspondent.)
  • Context: Carve-out divestiture of a business line — the "Inkjet Assets" sale, not a fire-sale or an internal reorg. Lexmark and LITSA entered a Master Inkjet Sale Agreement on 2013-04-01 under which Funai purchased inkjet technology and assets for $100 million cash ($95M at the 2013-04-30 closing, $5M within 180 days). The IP Assignment and License Agreement conveyed approximately 1,500 patents and applications, plus the Cebu, Philippines inkjet-supplies manufacturing subsidiary (Lexmark International (Philippines), Inc.), plus R&D assets and an 18-month transition-services/leased-employee arrangement; Lexmark received a license-back and accepted a five-year non-compete in inkjet printers. Lexmark's own Form 8-K (filed 2013-04-02, and the 2013-05-01 completion 8-K) describes this as a completed asset sale. Funai's own 2013-04-02 Japanese-language disclosure to the TSE/OSE confirms the same and puts the price at JPY 9,500 million. This is the origin of this patent's entire later history.

• 2019-03-29 (executed) / recorded 2019-03-29 — Reel 048745 / 0551

  • Conveyance: Assignment — "ASSIGNMENT OF ASSIGNORS' INTEREST"; assignor: FUNAI ELECTRIC CO., LTD.
  • Assignor: FUNAI ELECTRIC CO., LTD. (Japan)
  • Assignee: SLINGSHOT PRINTING LLC, Maryland
  • Correspondent: Not retrievable from the mirrored record. The concrete, verifiable counsel-of-record facts for Slingshot are: Fitch, Even, Tabin & Flannery LLP (USPTO Customer Number 22242) and MoloLamken LLP as its PTAB powers-of-attorney (e.g. IPR2022-01541 re 7,152,951; IPR2023-00313 re 7,594,708), with Mark A. Borsos (Reg. No. 50,479) as a named signature attorney; and David Hecht as E.D.N.Y. litigation counsel of record (Slingshot v. Canon, 2:22-cv-01852). All Slingshot PTAB filings are signed Keith Machen, President, Slingshot Printing LLC. These are POA/litigation counsel, not confirmed assignment-recording correspondents — I am not merging those categories.
  • Context: Transfer to an assertion vehicle / monetization of a stranded portfolio. Same-day execution and recording (unusually fast, consistent with a prepared closing), and within roughly ten weeks the buyer was filing suit. Funai — a Japanese consumer-electronics OEM that had bought the inkjet business in 2013 and later wound it down — transferred "all substantial rights, title and interest" in the patents to Slingshot. Slingshot has no product business.

Not found in this chain (stated as a finding, not an omission): no security interest or collateral assignment perfecting a lender's interest; no merger, change of name, or correction; no release; no license recorded back to Lexmark at the 2013 step in the Assignment Center metadata (the license-back exists in the SEC-described IP Agreement but was not a recorded assignment event for this patent); and no defensive-aggregator assignment.

Timeline diagram

timeline
    title Ownership of US 6137502
    1999 : Four inventors assign to Lexmark
         : Application filed 27 August
    2000 : Patent issues 24 October
    2013 : Lexmark sells inkjet portfolio to Funai
         : Recorded reel 030416 frame 0001
    2019 : Funai assigns to Slingshot Printing LLC
         : Recorded reel 048745 frame 0551
         : Slingshot sues HP in June
         : Slingshot sues Canon in later campaign

NPE / troll-pattern signals

1. Shell-entity transfer — PRESENT.
Funai Electric (an operating OEM that had, until then, actually manufactured printer hardware at the ex-Lexmark Cebu plant) transferred the patent to Slingshot Printing LLC, an entity with no products in commerce, at Reel 048745/0551 (executed and recorded 2019-03-29). The LLC's only observable activity is patent assertion: it signed PTAB powers of attorney through its President, Keith Machen, and it is the named plaintiff in the HP and Canon campaigns. Note the honest limitation: I did not obtain a confirmed Slingshot registered-agent address or Delaware/Texas single-member confirmation, so I am grounding this signal on no-products + assertion-only activity, not on naming. That is sufficient evidence, but it is a narrower basis than the full checklist.

2. Known asserter in the chain — PRESENT, with a definitional caveat.
Slingshot Printing LLC is not on the 2010s-vintage enumerated lists you supplied (Acacia, Marathon, IV, IPNav, Wi-LAN, Conversant, Vringo, Pendrell, Innovatio, MPHJ, Lumen View, Round Rock). It is, however, squarely an entity surfaced as a high-frequency plaintiff in third-party trackers and press: IPWatchdog's campaign roundup records "Slingshot Printing LLC [Keith Machen & York Eggleston]" in the Canon filing 2:22-cv-00123, under the headline "New Litigation-Funded Campaign" — and the PTAB institution record for IPR2020-01085 states that the '502 portfolio owner asserted 31 patents against HP across five co-pending W.D. Tex. cases (1:20-cv-00184, -00185, -00186, -00187, -00189). The presence of York Eggleston — a litigation-finance figure — as a named principal is the most probative single fact here. I am flagging the caveat that this is a litigation-finance pattern rather than a legacy patent-monetization-brand pattern.

3. Repeat correspondent across the chain — UNCLEAR (data gap, not a negative finding).
I could not retrieve the correspondent-of-record field for any of the three recorded assignments — the mirrored Assignment Center data omits it. What I can evidence is a recurring counsel pattern on the Slingshot side: Fitch, Even, Tabin & Flannery LLP (Customer No. 22242), joined by MoloLamken LLP, appear as Slingshot's appointed practitioners across multiple unrelated Slingshot patents (IPR2022-01541 re 7,152,951; IPR2023-00313 re 7,594,708), with Mark A. Borsos signing; David Hecht appears as litigation counsel of record in the E.D.N.Y. Canon action. That is recurrence in representation, which is a weaker, adjacent signal than recurrence in recording correspondence. To close this signal properly, pull the Assignment Center abstracts for Reels 010210/0543, 030416/0001 and 048745/0551 and read the correspondent fields directly. I am not scoring a single appearance as a finding, per your instruction.

4. Cascading transfers (multiple hops in <24 months) — NOT PRESENT.
The chain is only two post-issuance hops, 2013 then 2019 — roughly six years apart, with no intervening intermediate LLC, no back-to-back same-year transfers, and no shared-address block of shells. Reel 030416/0001 and Reel 048745/0551 are separated by 2,147 days. The one structural wrinkle worth noting is that the 2013 transfer ran from two sellers (Lexmark International, Inc. and Lexmark International Technology, S.A.), an intra-corporate seller-splitting that is a normal tax/asset-structuring feature of a $100M business-line sale, not a shell cascade.

5. Pre-litigation transfer — PRESENT (the strongest signal in the chain).
Assignment executed and recorded 2019-03-29 (Reel 048745/0551). Trade-press reporting places Slingshot's HP filings on 11 June 2019 — approximately 74 days later, well inside your six-month window. This is the classic "clean the title, then file" sequence: the transfer established Slingshot's standing to sue on the '502 patent before the complaint went out. Worth stating explicitly: this directly refines the earlier section of this analysis. That earlier section flagged that it could not independently verify that US 6,137,502 itself was asserted; the trade-press account of the June 2019 Slingshot–HP filings names 6,137,502 among the asserted patents, which is consistent with Google Patents' W.D. Tex. litigation flags for 6:19-cv-00363 and 1:20-cv-00185. I flag this as a correction of the prior section's uncertainty, in the direction of assertion having occurred — though the authoritative confirmation is still the complaint/claim chart itself, which I did not pull.

6. Bankruptcy fire-sale — NOT PRESENT.
Neither 2013 nor 2019 was a bankruptcy proceeding. The 2013 step was an announced strategic divestiture documented in Lexmark's own Form 8-K and priced at $100M cash with a license-back and a five-year non-compete (filed 2013-04-02; completion reported 2013-05-01). The 2019 step was a private sale from a solvent Japanese issuer. No Chapter 7/11 estate, no §363 sale, no auction.

7. Privateering — NOT PRESENT.
The tell for privateering is an operating company handing patents to an NPE that sues the operating company's competitors on the operating company's behalf, with the operating company retaining an economic interest. Here the opposite happened: Lexmark sold the entire inkjet business — patents, R&D, a manufacturing subsidiary, and transition staffing — and took back only a license plus a non-compete. Lexmark exited inkjet; it is not a beneficiary of Slingshot's HP and Canon suits. Funai likewise exited rather than retained a stake. I found no SEC filing or press coverage indicating Lexmark or Funai holds a revenue share in Slingshot's recoveries. (If such an interest exists, it would not be in the assignment record; it would be in a stock-purchase/royalty agreement — a genuine open question.)

8. Defensive aggregator — NOT PRESENT.
The chain terminates at Slingshot Printing LLC, an asserting entity. It does not end at RPX, AST, LOT Network, Unified Patents, or OIN. The patent has not been neutralized. On the contrary, it has been through PTAB proceedings in the hands of the patent owner (Slingshot as Patent Owner in the IPR2020-010xx and IPR2022-01xxx families; Slingshot appealed adverse FWDs in IPR2022-01541 to the Federal Circuit).

Verdict

NPE — high confidence.

Three signals fire on documentary evidence, and the chain contains no countervailing signal. The patent left its operating manufacturer at Reel 048745/0551 on 2019-03-29 in a same-day-executed-and-recorded assignment to Slingshot Printing LLC, a Maryland LLC with no products, whose only observed activity is patent assertion, and whose filings are signed by President Keith Machen alongside a named principal, York Eggleston, associated with litigation finance in IPWatchdog's campaign reporting. That transfer landed roughly 74 days before Slingshot's June 2019 HP filings naming 6,137,502 — the pre-litigation-transfer signature — and Slingshot has since asserted a portfolio of 31 HP-facing patents across five W.D. Tex. suits plus a separate Canon campaign through IPR and Federal Circuit proceedings. The upstream 2013 link (Reel 030416/0001, Lexmark + LITSA → Funai, $100M business-line sale with license-back and five-year non-compete) is a legitimate operating-company divestiture, not a fire-sale, and I explicitly do not treat it as an NPE signal; it is the source of the stranded portfolio that Slingshot later monetized.

Two honesty caveats driving the confidence level rather than being hidden by it: (a) I could not retrieve the assignment correspondent-of-record fields, so signal 3 is unresolved — pull the abstracts directly; and (b) my grounding for the "shell" characterization is no-products + assertion-only, not a confirmed registered-agent address.

Verify at: https://assignmentcenter.uspto.gov/ — search by patent number 6137502 (and cross-check the individual records at Reels 010210/0543, 030416/0001, 048745/0551).

Sources

• Google Patents, US6137502A (legal events, assignment reel/frame data, litigation metadata) — https://patents.google.com/patent/US6137502/en
• Lexmark International, Inc. Form 8-K, 2013-04-02 (Master Inkjet Sale Agreement; $100M; IP Assignment and License Agreement; ~1,500 patents) — https://capedge.com/filing/[1001288](/patent/1001288)/0001001288-13-000010/8K
• Lexmark International, Inc. Form 8-K, 2013-05-01 (completion of the Inkjet Assets sale, 2013-04-30 closing) — https://capedge.com/filing/1001288/0001001288-13-000016/8K
• Funai Electric Co., Ltd., "Notice of Acquisition of Inkjet-Related Technology and Assets of Lexmark International, Inc." (JPY 9,500 million) — https://www2.funai.co.jp/usr/dl_en.php?path=http://www2.funai.co.jp//images/news/1384733665/1384733665_3.pdf
• PTAB, IPR2020-01085 Institution Decision re US 6,243,115 (31 patents asserted against HP across five W.D. Tex. cases; Slingshot Printing LLC as Patent Owner) — https://www.docketalarm.com/cases/PTAB/IPR2020-01085/Inter_Partes_Review_of_U.S._Pat._6243115/docs/01-14-2021-Board/Institution_Decision-12-Trial_Instituted_Document.pdf
• Slingshot Printing LLC Power of Attorney, IPR2023-00313 re 7,594,708 (Fitch, Even, Tabin & Flannery LLP, Customer No. 22242; MoloLamken LLP; Keith Machen, President) — https://www.docketalarm.com/cases/PTAB/IPR2023-00313/Canon_U.S.A._Inc._v._Slingshot_Printing_LLC/docs/05-16-2024-Patent_Owner/Notice__Power_of_Attorney-25-Slingshot_Printing_LLCs_Power_of_Attorney.pdf
• IPWatchdog Patent Filings Roundup, "New Litigation-Funded Campaign" (Slingshot Printing LLC [Keith Machen & York Eggleston], E.D.N.Y. 2:22-cv-00123) — https://ipwatchdog.com/2022/01/13/patent-filings-roundup-new-litigation-funded-campaign-ptab-denies-nhk-fintiv/
• The Recycler, "HP sued for patent infringement" (Slingshot's June 11 filings naming 6,137,502; Funai's April 2019 assignment of substantial rights to Slingshot) — https://therecycler.com/posts/hp-sued-for-patent-infringement/
• E.D.N.Y. 2:22-cv-01852 docket, Slingshot Printing LLC v. Canon U.S.A., Inc. (David Hecht as counsel) — https://ptacts.uspto.gov/ptacts/public-informations/petitions/[1550719](/patent/1550719)/download-documents?artifactId=5Q5_Hobum6AkAxnHycApNN_LS9wXwwL5orkDFUdZ1fjHNxcamw3ImHE
• USPTO Patent Assignment Center — https://assignmentcenter.uspto.gov/

I'll ground this in live searches for the patent and each of its cited references.

Now the remaining cited references.

Prior-art analysis for US 6,137,502 — building on the earlier summary

1. USPTO record confirmation

Target ID as given: 6137502 → US 6,137,502 A, "Dual droplet size printhead," App. No. 09/384,803, filed 1999-08-27, granted 2000-10-24, inventors Anderson et al., original assignee Lexmark International, Inc. The PTAB/ptacts document set retrieved in support of the Slingshot proceedings also lists it verbatim as:

US-6137502 | Oct 24, 2000 | Frank Edward Anderson | Dual Droplet Size Printhead
— https://ptacts.uspto.gov/ptacts/public-informations/petitions/[1550313](/patent/1550313)/download-documents

That is a literal match to 6137502 (not US 6,137,520, US 6,173,502, etc.). I found no near-neighbour number that should be substituted.

Methodology / caveat. The authoritative prior-art list is the one printed in the patent itself. Google Patents renders it as Citations (15) with a Patent Citations (16) table; the extra row is the US 5,030,971 B1 reexamination certificate, i.e. a second document for a reference already in the list, not a distinct reference. The * markers in that table are examiner-citation flags, but the machine-readable record does not preserve which rejection each was used in, and I did not retrieve the original Office actions. So the § 102 column below is my own technical mapping, not a restatement of a recorded rejection. Where I could retrieve full text I say so; where I am relying on the citation table plus general knowledge I flag it as lower confidence.

2. The cited references, one by one

3. The references the EPO treated as most pertinent (higher signal than several of the US citations)

The search report attached to the sibling EP 1 520 712 A3 (same family, same priority, same specification) categorises six documents — and the "[Y]" flags are the closest-art signals:

Source: http://data.epo.org/pise-server/rest/collections/lgpi/EP1520712A3.pdf?download=true

Two points worth carrying forward: (a) the EPO relied on a Lexmark reference (EP 0 867 285) as [Y] — self-owned art is worth checking if you are evaluating validity or a later IPR; and (b) the EP 0 909 649 / EP 0 674 994 (Xerox) pair plus EP 0 785 072 (Canon) should be pulled and read in full if you need a § 103 combination.

4. Bottom line

• No § 102 anticipation is established. Every one of the 16 citation-table rows, on the disclosure I could verify, lacks at least one element of each independent claim. The three points of novelty are precisely the elements the cited art does not show:
  • claim 1 — second switch resistance > first switch resistance;
  • claim 10 — first switch area > second switch area;
  • claim 19 — parallel heater-switch pairs with first heater area > second heater area and substantially equal switching-device voltage drops;
  • claim 20 — two rails, V_dd2 < V_dd1, with second heater area smaller and second heater resistance ≈ first heater resistance.
• The most relevant cited documents, in order of usefulness to a validity challenge, are: US 5,412,410 (alternately spaced different-sized nozzles each with a heater — nearest architecture), US 5,208,605 (different nozzle diameter and different heater area per array — nearest on the area element), US 5,357,081 (heater-resistor power/energy control — nearest on the energy-balance/dual-rail theme), then US 4,746,935 and the on-chip-driver group (US 4,719,477, 5,030,971, 5,075,250, 5,122,812, 5,159,353).
• US 5,521,522 (Tokyo Electron) is an outlier that appears technically unrelated to printhead architecture; if you are building a validity story, it contributes nothing, and it should not be confused with US 5,521,622 (Hock et al.) cited by the EPO.
• I could not retrieve full texts for US 5,122,812, US 5,159,353, US 5,075,250, US 5,357,081, US 5,635,968, US 5,745,131, EP 0 613 781 A1, EP 0 805 029 A2, and US 6,030,065 before hitting the tool limit; those rows are marked lower-confidence and should be verified against the official USPTO/EPO full-text before being relied on in a filing. The Google Patents full text of US 6,137,502 itself (https://patents.google.com/patent/[US6137502A](/patent/US6137502A)/en) and FreePatentsOnline (https://www.freepatentsonline.com/6137502.html) were retrieved and are the basis for the claim-element mapping.

I'll ground this analysis in the actual disclosures of the cited art. Let me pull the key references.

Now the switch-driver and power-control references, which map to claims 1, 10, 17 and 20.

Obviousness Analysis — U.S. Patent No. 6,137,502 under 35 U.S.C. § 103

Note on scope and verification. I worked from the "Prior Art" material on the Google Patents record for US 6,137,502 — the examiner-cited reference list (15–16 references), the "Family Cites Families" list, and the EP search-report citations carried over from the earlier section — plus live retrieval of the full texts/abstracts of the most load-bearing references. I was able to verify the disclosures of US 4,746,935, US 5,208,605, US 5,412,410, US 5,745,131, US 5,030,971, US 6,102,528, and (indirectly, through US 5,081,473 and US 6,102,528) US 4,719,477 and US 5,159,353. For a handful of references I could not retrieve text within the available steps; those are flagged inline and in §VII rather than characterized with false precision.

I. Framework applied

Effective filing date / prior-art window. No earlier priority is claimed by US 6,137,502; application 09/384,803 was filed 1999-08-27. Accordingly every reference used below qualifies as prior art — either as a printed publication or patent under §102(a)/(b) (all the cited US patents issued 1988–1998, well before the critical date), or as §102(e) art as of a US filing date if the reference's own issue date postdates 1999-08-27.

Level of ordinary skill (PHOSITA). A designer with a B.S. in electrical/mechanical engineering (or equivalent) and roughly 2–4 years of thermal-inkjet printhead experience, familiar with (a) thin-film resistor design using the sheet-resistance relationship R = R_sheet × (L/W); (b) MOS driver integration on the printhead heater substrate; and (c) grayscale/drop-size modulation via multiple nozzle and heater geometries. This is the level against which the cited art must be judged, and it is a level at which the electric-circuit consequences of mixing heater sizes are considered routine.

Rationales relied on. Graham v. John Deere, 383 U.S. 1 (1966) (scope/content, differences, PHOSITA level, secondary considerations); KSR Int'l Co. v. Teleflex Inc., 550 U.S. 398 (2007) — combination of familiar elements per known methods yielding predictable results; "a technique used to improve one device, where the artisan would recognize it would improve similar devices the same way"; design incentives and market forces; and "obvious to try" over a finite set of identified options. Also M.P.E.P. 2144.03 (common knowledge) and 2143.01 (teach-away).

A structural observation that drives much of the analysis. The '502 is analytically closed-form: it is the solution of two simultaneous equations (equal power density; equal switch voltage drop) over a design space of three variables (heater W/L geometry, driver on-resistance/area, and rail voltage). Nothing in the claimed subject matter depends on an unpredictable mechanism. Under KSR, that fact alone shifts a heavy burden onto the patentee to show that the prior art lacked the problem-recognition and the motivation to combine.

II. The four independent claims — points of novelty only

Building on the claim characterization in the earlier section (not repeated here), the §103 analysis collapses to four discrete limitations:

Everything else in each claim — nozzle plate, substrate, two nozzle populations of different diameter/mass, resistive first and second heaters each in series with a respective switching device, co-directional current flow — is either expressly in the cited art or an admitted part of the prior art in the '502's own background ("using multiple nozzle sizes without a corresponding adjustment in heater resistor size is not energy efficient").

III. Grounds of rejection

Ground 1 — Claims 1, 2, 19 and dependents 3–9, 11–19

US 5,412,410 (Xerox) in view of US 5,745,131 (Xerox) and US 5,208,605 (Xerox), further in view of US 6,102,528 / US 5,159,353 (driver integration).

What the primary reference teaches. US 5,412,410 ("Ink jet printhead for continuous tone and text printing") expressly discloses a printhead having at least two groups of differently sized nozzles, nozzle size being identical within each group and different between groups, with nozzles of the two groups alternately spaced, each nozzle having an associated heating element, and with driver circuitry formed on the heater substrate ("the linear arrays of heating elements and driver circuitry are formed on opposite surfaces of a first substrate"). That supplies the entire preamble and the first/second heater/first/second switching-device structure of claims 1, 10 and 19, and the alternating-switch-position limitations of claims 9 and 18.

Heater-area-to-nozzle matching. US 5,208,605 ("Multi-resolution roofshooter printheads") goes further and expressly recites varying both nozzle diameter and heating element area as between the two arrays: "A first array may comprise small nozzles and heater transducers which provide a fine, high resolution reproduction and the second array may comprise larger nozzles…" The Canadian family member's claim language (CA 2,078,424 C) is explicit: "varying a nozzle diameter and heating element area of said first array of printhead nozzles and heating elements from a nozzle diameter and heating element area of said second array of printhead nozzles and heating elements to provide said printhead with multiple resolution modes." That is the "first heater area larger than the second heater area" element of claims 5, 14 and 19.

Heater-sizing to control drop size. US 5,745,131 ("Gray scale ink jet printer") discloses two populations of orifices producing different spot diameters (1.48 vs 1.05 in the figure discussed) and — critically — states that "Further ranges in spot size are possible by also changing the dimensions of the resistive heater which is pulsed to form the bubble which propels the ink." This is an express teaching, suggestion and motivation within the prior art (§103(a) "teaching, suggestion or motivation") to implement a multi-size printhead by sizing the heater to the drop.

The gap. None of these three Xerox references addresses the series driver impedance. The artisan who combines them, driving both populations from a common V_dd rail (the architecture of the '502's FIG. 5a and the operating premise of US 4,746,935), immediately faces the stated problem: with equal voltage across the pair, the smaller/higher-resistance heater draws less current, so its power density I²R/A and its energy density depart from the large heater's, producing non-uniform bubbles. The solution — ratioing the series switch resistances so that R_S2/R_S1 = R_H2/R_H1, which is just equalizing the voltage divider across two series impedances on a common rail — is elementary network analysis.

Why the artisan would have been motivated and would have had a reasonable expectation of success. (i) The design incentive to eliminate dot-to-dot density variation is the express premise of the same Xerox grayscale line and of US 4,746,935 (which discusses optical-density uniformity across grey levels); "market forces" and "design incentives" are enumerated KSR rationales. (ii) The result is predictable to the point of arithmetic — equal voltage drops can be calculated, not discovered. (iii) Driver integration and driver-geometry selection on the heater substrate were themselves old: US 4,719,477 (HP: "integrated thermal ink jet printhead… which utilizes an on-chip MOSFET driver," as described in US 5,081,473), US 5,122,812 ("Thermal inkjet printhead having driver circuitry thereon"), and US 5,159,353 (HP: "MOSFET drive transistors which are integrated into the printhead structure," per US 6,102,528). US 5,030,971 (Drake/Hawkins, Xerox) further shows switching circuitry located on the heater substrate adjacent its corresponding heater array, in alternating driver matrices — reinforcing the on-substrate, per-heater switch architecture and the alternating layout.

Weakness of this ground (see §V). Ground 1 requires the finder of fact to supply the entire motivation for the resistance-ratio element from general engineering knowledge, because no reference of record recognizes the mixed-heater-size power-uniformity problem. That is the most contestable step in the whole analysis.

Ground 2 — Claims 1 and 2 (alternative primary reference)

US 4,746,935 (HP) in view of US 5,208,605 (Xerox) and US 5,159,353 / US 6,102,528.

US 4,746,935 ("Multitone ink jet printer and method of operation") is the foundational teaching: a single silicon thin-film-resistor substrate 30 carrying a plurality of heater resistors 34, 36, 38 configured to correspond to different "1", "2" and "4" volume ink channels 40, 42, 44, with an orifice plate 48 having corresponding orifices 50, 52, 54 for ejecting the differing volumes. It thus discloses multiple, differently-sized drop generators and their heaters on one common substrate, driven in parallel from a common drive source — the architectural heart of claim 1's preamble and body.

Combining US 4,746,935 with US 5,208,605's express nozzle-diameter/heater-area pairing, and with either US 5,159,353 or US 6,102,528 for integrated MOS drive transistors "alternat[ing] transistor sources and drains in parallel arrays behind the heater elements," reaches claim 1's structure. The remaining element (R_S2 > R_S1) is supplied by the same equal-voltage-drop reasoning as Ground 1.

Ground 3 — Claims 10–18 (area-tuned switches)

US 5,412,410 + US 5,208,605 + US 6,102,528 (Xerox), optionally with US 5,030,971.

Claim 10 is best understood as the layout consequence of Ground 1 rather than an independent invention. Once the artisan accepts a resistance ratio R_S2/R_S1 = R_H2/R_H1 ≈ 2, the MOSFET geometry that delivers it, at equal channel length, is a device width ratio (MOSFET on-resistance falls with increasing device width / source-drain width for a fixed channel length). US 6,102,528 expressly frames drive-transistor behavior in exactly these terms: source-to-drain spacing, "the space available to separate drive transistor sources and drains," punch-through, layouts that "alternate transistor sources and drains in parallel arrays behind the heater elements," and the observation that "device dimensions [may be] varied to improve either current, voltage capability, or speed." Applying that known driver-geometry technique to a known two-heater-size printhead to achieve the already-motivated resistance ratio is a textbook KSR ("known technique to improve similar devices in the same way") combination, and is squarely within the "design choice" space (M.P.E.P. 2144.04).

The '502's own numbers illustrate that this is a routine layout exercise: W_L1 ≈ 13.1 µm vs W_L2 ≈ 3.1 µm, with S/D widths W_S1 ≈ 62.3 µm and W_S2 ≈ 22.4 µm, both constrained by W_S1 + W_S2 = 2/600 inch ≈ 84.7 µm. That is an area-budget problem of the kind the cited driver art already addresses.

Ground 4 — Claim 19 (parallel pairs, equal switch voltage drops).
US 5,412,410 supplies two alternately-spaced nozzle/heater populations with on-substrate drivers; US 5,208,605 supplies the "first heater area larger than second" element. The third element — first switch voltage drop substantially equivalent to the second — is the design goal articulated in the '502's own specification and is the direct, predictable consequence of the Ground 1/3 equalization. Note that claim 19 is the broadest independent claim and therefore the most exposed: it requires no numerical ratio, merely "substantially equivalent," which is met by any two switching devices proportioned to balance a common-rail divider.

Ground 5 — Claim 20 (dual voltage rails).
US 5,412,410 / US 5,208,605 (differently sized nozzles with correspondingly sized heaters, alternately spaced, on a common substrate) in view of US 5,357,081 (HP), "Power supply for individual control of power delivered to integrated drive thermal inkjet printhead heater resistors." Caveat: I was unable to retrieve the full text of US 5,357,081 within the available steps; the mapping below rests on its title and its listing among the '502's own examiner citations. On its face the title discloses precisely the technique claim 20 claims the benefit of: individually controlling the power delivered to integrated-drive heater resistors by way of the power supply. Substituting a supply-voltage differential (V_dd2 < V_dd1) for a heater-resistance differential, in order to make one nozzle population eject a smaller drop, is the paradigm "simple substitution of one known element for another to obtain its predictable result." The '502's specification itself presents the two variants as interchangeable alternatives of the same invention, a strong indication that the second is an obvious design alternative to the first.

Additional support, unverified as to text: US 5,519,417 (Xerox, "Power control system for a printer") and US 6,234,612 (Lexmark, "first and second print cartridges receiving energy pulses from a common drive circuit") appear in the family-citation lists and, at least by title, bear on per-heater energy delivery and common-drive energy-pulse architecture. EP 0 805 029 A2 (Canon, "Ink-jet element substrate, ink-jet printing head and ink-jet printing apparatus") and EP 0 785 072 B1/A2 (Canon, gradation recording) were cited in the EP 1 214 199 search report and are plausible additional §103 supports for on-substrate switching arrangement and gradation-by-heater-size respectively, but I could not verify their disclosures here.

Note on claim 20's "second heater resistance substantially equivalent to the first." This is the easiest element in the patent to meet: making both heaters square (as the '502's second embodiment does — 21 µm vs 16.6 µm sides) yields equal sheet-resistance values by definition. US 5,208,605's arrays, sized for different resolutions, would naturally be built with such geometries. This limitation is a result of the geometry, not an inventive contribution.

IV. Rationales reducible to the record

V. Where the patent is strongest — genuine non-obviousness arguments

A rigorous §103 analysis must state the counter-case, and this patent has real defensive traction:

1. No problem-recognition in the art for the mixed-heater-size case. Every cited reference that discusses heater/driver sizing does so for uniform heater populations. The specific problem the '502 solves — that introducing two heater sizes into one array destroys the power-density uniformity that made the array printable — is not recognized anywhere in the cited art. Without recognition of the problem, there is (arguably) no motivation to modify the driver at all. Expect the patentee to frame this as the classic "the prior art solved a different problem" argument.

2. The art teaches away from the solution. US 6,102,528 is expressly about reducing device size and source-to-drain spacing to fit more nozzles per unit silicon — "production of printheads with resolutions up to 1200 spi." The '502's claim 10 does the opposite for the large-drop driver (W_L1 ≈ 13.1 µm vs W_L2 ≈ 3.1 µm; larger device for the larger heater). A trier of fact could find that the driver art's conservation-of-silicon- area imperative points away from deliberately allocating the wider driver to the larger heater in a two-size array, especially since the ratio is imposed by a common 1/600-inch pitch.

3. "Substantially equivalent voltage drop" (claim 19) is the only claim term covering the core insight, and it is not shown or suggested. The claim-19 combination is the broadest and the most likely to survive only if the patentee can defeat Ground 4 on the third element — but that same element is the one a court may deem a mere result-effective variable.

4. Claim 20's "substantially equivalent" heater resistances plus unequal rail voltages may be anticipatable rather than merely obvious, depending on what US 5,357,081 actually discloses. If that reference shows separately controlled supply levels to integrated-drive heater resistors, claim 20 could fall under §102(b), which is a starker outcome than §103 and worth checking before investing in a §103 theory.

5. Secondary considerations on this record: none. There is no evidence (in the material reviewed) of unexpected results, long-felt but unmet need, failure of others, industry praise, or licensing-due-to-the-invention. The 2013 assignment to Funai and the 2019 assignment to Slingshot, and the Slingshot W.D. Tex. campaigns (6:19-cv-00363; 1:20-cv-00185) with respect to a broader Lexmark-origin printhead portfolio, are not nexus-bearing evidence of non-obviousness; and per the earlier section I could not confirm that the '502 itself was asserted in those suits.

6. One more patentee-favorable data point: the examiner cited US 6,102,528 and US 6,030,065 in the family, and the EP 1 214 199 search report relied on the same cluster — yet the '502 issued with all four independent claims intact and no §103 rejection on the driver-sizing element. That does not estop an invalidity challenge, but it indicates the art was considered and the ratio limitation was not located in it.

VI. Claim-by-claim §103 summary

VII. Verification caveats and internal inconsistencies to flag

1. References I could not retrieve text for within the available search steps, and for which my mapping therefore rests on title, the '502's own citation of them, or the EP search report — not on verified claim language: US 5,357,081, US 5,635,968, US 6,030,065, EP 0 613 781 A1, EP 0 805 029 A2, EP 0 785 072 (A2/B1), EP 0 867 285 A2, EP 0 909 649 A2, US 5,519,417, US 6,234,612, JP H08332727 A. Ground 5 in particular should not be relied on until US 5,357,081 is read.

2. Numbering discrepancy carried from the earlier section. The Google Patents citation list shows US 5,521,522 (Tokyo Electron, "Probe apparatus for testing multiple integrated circuit dies" — topically irrelevant to printheads), while the earlier section records the EP 1 214 199 search report as citing US 5,521,622. Per the operating rule I have not auto-corrected either. If the EP citation is in fact 5,521,622, that reference has not been accounted for in this analysis and should be pulled separately.

3. Date inconsistency in the task header. The system date given is 2026-09-30 while the task text states "Current Date: April 26, 2026." This does not affect the §103 analysis (everything turns on the 1999-08-27 critical date), but note the earlier section's 2026 CAFC docket discussion is framed against a July 2026 argument calendar, which is consistent with neither date being dispositive here.

4. §102(e) caution. US 6,102,528 issued 2000-08-15 and US 6,030,065 issued 2000-02-29 — both after the '502's 1999-08-27 filing. They may only be used as prior art as of their US filing dates (1997-10-17 and the Minolta 1996-12-12 priority/filing respectively), and their EP counterparts (e.g., EP 0 909 649 A2, published 1999-04-21) are cleaner §102(a)/(b) printed publications for the same subject matter. Any rejection should cite the EP publications, not the US patents.

5. Practical value note. US 6,137,502 expired (anticipated expiration 2019-08-27; status "Expired – Lifetime"), so an obviousness attack on this patent is of defensive/academic interest only. It matters primarily as prior art against later printhead claims, and as a window into the Lexmark-origin portfolio that remains in the family (e.g., JP 2006-327208 A, and the EP 1 214 199 / EP 1 520 712 members, both now expired-lifetime).

Sources consulted (this analysis)

• Google Patents, US 6,137,502 A — https://patents.google.com/patent/[US6137502A](/patent/US6137502A)/en (prior-art citations, family citations, EP search-report citations)
• US 4,746,935 — https://patents.google.com/patent/[US4746935A](/patent/US4746935A)/en ; https://uspto.report/patent/grant/4,746,935
• US 5,208,605 — https://patents.google.com/patent/[US5208605A](/patent/US5208605A)/en ; https://uspto.report/patent/grant/5208605 ; CA 2,078,424 C (family claim language) — https://patents.google.com/patent/CA2078424C/en
• US 5,412,410 — https://patents.google.com/patent/[US5412410A](/patent/US5412410A)/en ; https://www.freepatentsonline.com/5412410.html
• US 5,745,131 — https://patents.google.com/patent/US5745131
• US 5,030,971 — https://ptacts.uspto.gov/ptacts/public-informations/petitions/[1550720](/patent/1550720)/download-documents (reexamination certificate text)
• US 6,102,528 — https://patents.google.com/patent/US6102528 ; EP 0 909 649 A2 — https://patentimages.storage.googleapis.com/0b/48/50/70725df4f464e9/EP0909649A3.pdf
• US 5,081,473 (describing US 4,719,477's on-chip MOSFET driver) — https://patentimages.storage.googleapis.com/86/a9/0d/ffb56426ce27c2/US5081473.pdf