Sensor Deposition Unit | Emre Soydemir

Bench setup showing the manifold, gasket, and BVT chips in the same workflow.

Intro

The Sensor Deposition Unit is an attachable electrical subsystem for Terry/ASATS, a lower-cost IsoPure-style magnetic bead agitation platform used in the lab. Terry/ASATS handles the bead-processing side of the workflow. The SDU adds the electrical layer: controlled deposition and electrochemical sensing across multiple biosensor chips.

The project is active. The board and parts have been ordered, and the next stage is soldering, bring-up, and comparison against baseline EmStat Pico measurements.

Purpose

The unit routes deposition and sensing signals across multiple BVT chips while preventing signal paths from touching when they should not. Deposition and sensing are part of the same workflow, but electrically they ask for very different behavior.

During deposition, the system drives a square-wave signal across the chips. During sensing, the EmStat Pico needs a clean electrochemical path for cyclic voltammetry and square-wave voltammetry. That difference is why signal clarity and isolation are the priority. A useful sensing result depends on knowing that the potentiostat is measuring the selected chip, not a path accidentally influenced by the deposition circuit, a neighboring electrode, or a wet mechanical leak.

System context

Terry/ASATS: lab magnetic bead agitation and processing platform
Sensor Deposition Unit: attachable electrical subsystem
BVT chips with three electrode contacts per chip
More than 24 electrode connections across the full system
EmStat Pico for electrochemical readout
555-timer deposition path for the deposition signal
Arduino Pro Mini at 8 MHz / 3.3 V for local control
Simple board-side controls and a single external handoff signal from Terry/ASATS

Operating modes

Deposition Mode

10 Vpp square wave at 50 Hz
Applied to all chips simultaneously
Generated through a 555-based deposition path

Sensing Mode

EmStat Pico potentiostat
Cyclic voltammetry and square-wave voltammetry support
One chip selected at a time
Switching isolation protects sensing electronics from deposition drive

Implementation

The architecture separates deposition from sensing. In deposition mode, the 555-based signal path is routed to the chips together. In sensing mode, the deposition path is disconnected and the EmStat Pico is routed to one chip at a time.

That multiplexing choice is the central design tradeoff. It reduces hardware count and keeps a validated potentiostat in the workflow, but it pushes difficulty into relay or switch selection, channel isolation, state timing, and validation. The board has to prove that the selected electrode path is the only meaningful path.

The system behaves like a small state machine. It has to know which mode is active, which path is allowed to connect, and when a signal is safe to pass through. No deposition and sensing path should fight the other.

The manifold work is just as important as the PCB. The fixture must apply enough pressure to seal a single silicone gasket, hold BVT chips and their contacts consistently, and sit beside the magnetic bead agitation process without making the workflow harder.

Design decisions

Decision	Selected direction	Why it fits the system	Tradeoff / next step
Control interface	Board-side controls	Keeps the subsystem focused on reliable mode selection and lab use	Less flexible than a full software interface
Sensing	EmStat Pico routed to one chip at a time	Preserves trusted potentiostat behavior while adding multiplexing	Sequential readout limits simultaneous sensing
Deposition	Shared 555-based deposition path	Applies the deposition condition across chips together	Isolation must be proven before full workflow use
Controller	Arduino Pro Mini, 8 MHz / 3.3 V	Small, familiar, and enough for state and switching control	Not the final answer if UI or logging grows
Integration	Single handoff signal from Terry/ASATS	Simple boundary between magnetic processing and electrochemical sensing	More automation can be layered on after bring-up

Current status

The PCB and parts have been ordered. The next steps are assembly, board bring-up, and validation.

Planned validation sequence:

Solder and inspect the board
Confirm board power rails
Verify switching logic and mode transitions
Compare EmStat Pico one-plex baseline against the SDU-routed path
Confirm signals do not interact across channels
Verify deposition routing across all chips
Validate one-chip sensing before full multi-chip operation
Integrate with Terry/ASATS after subsystem validation

Results so far

System architecture defined
Board and parts ordered
Deposition and sensing modes specified
EmStat Pico sensing path selected
Manifold and leakage test fixtures modeled
Integration target defined through Terry/ASATS signal handoff

Future work

Assemble and bring up the ordered board
Validate isolation behavior
Compare EmStat Pico measurements before and after SDU routing
Refine the manifold and gasket pressure structure
Automate sequential sensing across chips
Explore longer-term full workflow automation, including solution movement

Why multiplex the signal

The SDU exists because the lab does not just need one chip to work on a bench. It needs a way to add deposition and electrochemical sensing to Terry/ASATS across many BVT chips without turning the system into a pile of repeated instruments.

One option would be one readout chain per chip. That is clean on paper and ugly in reality: more cost, more calibration burden, more board area, more failure points, and more channels that may not behave identically. Another option is to keep one trusted electrochemical reader and route it carefully.

Multiplexing is the compromise. Deposition can be applied across the chip set, then sensing can route the EmStat Pico to one selected chip at a time. The advantage is scale without multiplying the most sensitive hardware. The cost is that switching and isolation become the project.

SDU manifold, gasket, and BVT chip bench setup — Manifold, gasket, and BVT chips in the same bench workflow.

Deposition and voltammetry cannot share a path

Deposition and voltammetry ask for different electrical behavior. Deposition drives a square-wave condition across chips. Sensing uses the EmStat Pico potentiostat to run cyclic voltammetry or square-wave voltammetry on one selected chip.

Those two modes cannot casually share copper. A deposition path left connected during sensing can pollute the measurement. A neighboring electrode path can make the selected chip look better or worse than it is. With more than 24 electrode connections in the system, one unintended path is enough to make a result ambiguous.

That is why the board is really a discipline problem: which path is allowed to exist, when it is allowed to exist, and how to prove every other path is irrelevant.

Back side of SDU PCB — Back side of the PCB, where routing and isolation are central to keeping sensing paths clean.

Board-side control is a boundary choice

A separate GUI layer was an older direction, but it would have moved attention away from the risky part too early. The current plan keeps control on the board and waits for one handoff signal from Terry/ASATS.

That boundary is deliberate. Terry/ASATS handles magnetic bead motion. The SDU handles deposition, sensing, switching, and isolation. If that boundary is clear, bring-up can prove the electrochemical subsystem before more software is layered on top.

This is the same reason the board behaves like a small state machine. It should be obvious which mode is active, which chip is selected, and which signal path is disconnected. The interface can become more polished after the electrical behavior is proven.

Front side of SDU PCB — Front side of the ordered SDU PCB. Bring-up comes next.

Mechanical sealing is part of measurement

The electrochemical design only works if the manifold seals, the gasket pressure is stable, and chip contacts repeat. This is not mechanical side work. It is measurement quality.

A leak can create a path the schematic does not show. Uneven gasket pressure can change the fluid geometry. Bad contact can look like a board problem. If the EmStat Pico reports a strange result, the system has to know whether it is reading the selected chip, a leak path, a contact issue, or a geometry problem disguised as electrochemistry.

Ethanol leak tests and connector-holder fixtures belong in the same design story as the multiplexing. They protect the meaning of the measurement.

SDU ethanol leak test — Ethanol leak testing exposes sealing failures before electrochemical measurements are trusted.

What validation has to prove

Bring-up cannot stop at "the board powers on." The important test is whether a one-chip baseline through the EmStat Pico matches the SDU-routed path closely enough to trust the switch network. After that, the board has to prove non-selected channels stay electrically irrelevant.

The validation order matters: inspect the soldering, confirm rails, test logic states, verify switching, compare one-plex baseline to routed sensing, check channel isolation, then try deposition routing across the chip set. If that order is skipped, a later voltammetry failure becomes too ambiguous to debug.

The useful result will be boring in the best way: one selected chip measured through a routed path, neighboring chips disconnected, deposition isolated when sensing, and sensing hardware protected when deposition is active.

SDU connector holder render — Connector-holder concept for making repeated BVT chip contact possible across the multi-chip fixture.