🔬 applied physics

Spatiotemporal Thermal Modulation and Patterning using a Programmable 1024 Element Microheater Array

This paper presents a programmable 32x32 microheater array utilizing row-column addressing and high-voltage multiplexed electronics to achieve independent control of 1,024 sub-300-micrometer platinum heating elements, enabling precise spatiotemporal thermal modulation for applications such as heat imaging and liquid metal patterning.

Original authors: Rahul Goyal, Jang-Hwan Han, Sadaf Pashapour, Peer Fischer

Published 2026-02-26

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Rahul Goyal, Jang-Hwan Han, Sadaf Pashapour, Peer Fischer

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you have a giant, high-tech heating pad, but instead of just getting warm all over, it can heat up tiny, specific spots on its surface to create pictures, letters, or even melt metal into shapes. That is essentially what this paper is about.

Here is the story of how the researchers built this "smart heating blanket" and what they did with it, explained in simple terms.

1. The Problem: The "Too Many Wires" Dilemma

Think of a standard electric blanket. It has a few heating wires running through it. If you wanted to make a picture with heat, you'd need a separate wire for every single pixel (dot) in that picture.

If you tried to make a picture with 1,000 dots, you would need 1,000 separate wires coming out of the blanket. That would be a tangled, impossible mess of cables. This is why, until now, scientists could only make small heating grids. They couldn't scale up because the wiring became too complicated.

2. The Solution: The "Light Switch" Trick

The researchers solved this by using a clever trick called Row-Column Addressing.

Imagine a giant grid of light bulbs (like a massive LED screen). Instead of wiring every single bulb individually, you have:

32 horizontal wires (Rows).
32 vertical wires (Columns).

To light up one specific bulb, you don't need a unique wire for it. You just turn on the specific Row switch and the specific Column switch that cross at that bulb. The bulb at that intersection lights up, while all the others stay off because they aren't getting power from both sides.

The researchers applied this to 1,024 tiny heaters (a 32x32 grid) on a chip the size of a postage stamp. By using a fast electronic "traffic controller" (a multiplexer), they could flip these switches so quickly that it looked like they were heating many spots at once, even though they were actually turning them on one by one, very fast.

3. The Hardware: Tiny Platinum Spirals

The heating elements themselves are tiny squares made of Platinum (the same metal used in jewelry and catalytic converters).

Why Platinum? It's tough, doesn't expand much when hot, and resists electricity well, which makes it great for generating heat.
The Shape: Each heater is a double-spiral (like a tiny snail shell). This shape packs a lot of wire into a small space, ensuring the heat is spread evenly across that tiny square.
The Size: Each heater is smaller than a grain of sand (about 300 micrometers).

4. What Can It Do?

The researchers showed off two cool things they could do with this "smart heating pad":

A. Drawing with Heat (Thermal Images)

They programmed the chip to turn on specific heaters to create a "Smiley Face" in infrared light.

How it works: They heated the pixels that make up the smiley face to about 45°C (113°F) while keeping the rest cool.
The Result: If you looked at it with a special heat camera, you would see a glowing smiley face floating in the dark. Because the heaters are so close together but well-insulated, the heat didn't blur into the neighbors, keeping the picture sharp.

B. "Thermal Printing" with Liquid Metal

This is the most magical part. They used Gallium, a metal that is solid at room temperature but melts into a liquid at just 29.7°C (80°F)—roughly the temperature of a hot summer day.

The Setup: They put a thin layer of solid Gallium over the chip and kept the whole thing cool (around 5°C) so the metal stayed solid.
The Trick: They turned on specific heaters to melt the Gallium only in the shape of letters (M, P, I, M, R).
The Result: The melted liquid Gallium flowed away or was wiped off, leaving behind solid Gallium in the shape of the letters. It's like using a hot iron to burn a pattern into wood, but in reverse: they used heat to melt away the metal to reveal a shape.

Why Does This Matter?

This isn't just a cool science experiment. It opens the door to new technologies:

Better 3D Printing: Imagine printing circuits or sensors directly onto flexible materials using heat instead of ink.
Robotics: Tiny robots that can move or change shape using heat.
Medical Devices: Devices that can gently warm up specific parts of the body or activate drugs with heat.
Security: Creating "thermal messages" that are invisible to the naked eye but show up on a heat camera.

The Bottom Line

The researchers built the largest, most precise heating grid ever made by solving the "wiring mess" problem with a smart electronic switch system. They proved that you can control heat with the same precision as a computer controls pixels on a screen, allowing them to "draw" with heat and even sculpt liquid metal. It's a giant leap forward for making tiny, smart devices that can manipulate the world through temperature.

1. Problem Statement

Programmable microheater arrays are critical for applications ranging from gas sensing and microfluidics to 3D printing and biosensors. However, scaling these devices to large arrays (high density and high element count) presents two major challenges:

Scalability of Wiring: Direct wire addressing for every individual element becomes impractical as the number of elements increases, leading to complex and unmanageable interconnects.
Thermal and Electrical Constraints: As feature sizes decrease, resistance drops, making it difficult to generate sufficient heat. Furthermore, maintaining uniform electrical characteristics across thousands of elements and achieving steep thermal gradients without excessive heat diffusion to neighboring pixels is difficult.
Current Limitations: Existing demonstrations are largely limited to single elements or small arrays, lacking the architecture to control large-scale, individually addressable thermal patterns.

2. Methodology

The authors developed a solution combining advanced microfabrication with high-speed multiplexed electronics to create a scalable addressing architecture.

Device Fabrication:

Substrate: A 3-inch Si/SiO₂ (300 nm thermal oxide) wafer.
Heater Material: Platinum (Pt) was chosen for its low thermal expansion coefficient (~~8.9 × 10⁻⁶ K⁻¹) and high resistivity (~~10.1 × 10⁻⁸ Ω m), which ensures stability and efficient Joule heating.
Geometry: The array consists of 1024 elements arranged in a 32 × 32 grid. Each element is a double-spiral square with a side length of 297.5 μm and a minimum feature size of ~2 μm.
Isolation: To prevent short circuits at the crossing points of the row and column bus lines, a 1 μm thick HfO₂ dielectric layer was deposited. The top Au bus lines are patterned over this insulator.
Packaging: The device is coated with a ~1 μm parylene layer for protection.

Control Architecture (Row-Column Addressing):

Multiplexing: Instead of individual wires, the array uses a row-column addressing scheme with 64 independent control channels (32 rows + 32 columns).
Electronics: A custom high-voltage multiplexer board controls the array. It utilizes a 5-bit digital input (1 chip-select + 4 wordline bits) to select specific rows and columns.
Operation:
- Time-Division Multiplexing (TDM): Elements are activated sequentially using a high-frequency clock (up to 25 kHz).
- Thermal Control: Temperature is regulated not by varying voltage amplitude alone, but by adjusting the duty cycle (%DC) of square-wave voltage pulses (e.g., 25% to 100% ON time) while keeping the peak voltage ( $V_{DD}$ ) constant (up to 75 V).
- Refresh Rate: The system can activate all 1024 elements and maintain them at a target temperature (e.g., 45°C) within a 3-second refresh cycle.

3. Key Contributions

Largest Microheater Array: The authors present what is, to their knowledge, the largest microheater array to date, featuring 1024 individually addressable elements on a single chip.
Scalable Architecture: They successfully demonstrated that row-column addressing combined with high-speed multiplexing can overcome the wiring bottleneck of large arrays, allowing for the independent operation of any pixel.
Spatiotemporal Patterning: The system enables the generation of complex, dynamic thermal images with high spatial resolution.
Subtractive Liquid Metal Patterning: A novel application was demonstrated where the array is used to pattern liquid metal (Gallium) via a subtractive process (melting and removing specific areas) to create solid metal structures.

4. Key Results

Thermal Performance:
- Individual elements can reach temperatures up to 150°C without delamination.
- At a target temperature of 41°C, the thermal diffusion to neighboring elements is minimal. The Full Width at Half Maximum (FWHM) of the temperature curve corresponds to approximately 4 heating elements, allowing for distinct thermal patterns.
- A temperature difference of at least 15°C was achieved between activated and non-activated elements.
Pattern Generation:
- The team successfully generated complex thermal images, including a "smiley face," with temperatures ranging from 20°C to 45°C.
- Infrared imaging confirmed the ability to display these patterns over time (demonstrated at 0s, 25s, 50s, 65s, and 75s).
Liquid Metal Patterning:
- Using a thermoelectric cooling plate to maintain a base temperature of 5°C (below Gallium's melting point of 29.76°C), the array selectively heated specific regions to 35°C.
- This melted the Gallium in targeted areas, which was then removed, leaving behind solid Gallium structures shaped like the letters "M, P, I, M, R".
- Numerical simulations (FEM in MATLAB) aligned well with experimental thermal distributions.

5. Significance and Future Impact

This work represents a significant leap in on-chip thermal device capabilities. By solving the scalability issue of microheater arrays, it opens new avenues for:

Advanced Manufacturing: Enabling high-resolution, heat-controlled additive or subtractive manufacturing, particularly for soft electronics and liquid metal circuits.
Micro-actuation Systems: Facilitating novel micro-actuation systems driven by precise thermal gradients.
Sensing and Robotics: Enhancing the resolution and functionality of haptic displays, electronic skin, and gas sensors.
Biosensing: Improving DNA amplification and other thermal cycling applications through precise, localized temperature control.

The demonstrated ability to control 1024 pixels independently with a refresh rate of 3 seconds suggests that this architecture is a viable platform for next-generation thermal management and patterning systems.