Mechanism and Fabrication

This guide will walk you through the process of designing, fabricating, and using TH-Wood, an actuator that responds to both temperature and humidity changes. Inspired by the bi-layer deformation of pinecones, TH-Wood combines wood veneer with 3D-printed Polyhydroxyalkanoate (PHA) stripes, creating an actuator with flexible and responsive actuation.

Understand How It Works

The actuator works by exploiting the different expansion behaviors of its two layers:

• Wood veneer responds to humidity (hygroscopic expansion), bending toward the PHA layer.
• PHA responds to temperature changes (thermal expansion), bending away from the wood veneer.

These combined behaviors create precise, controlled deformation based on environmental conditions.

Materials Preparation

• Elm Veneer (0.2 mm thick) with uniform fiber orientation, ideal for thermal-driven actuation. You can also choose other thin, anisotropic wood types with noticeable expansion properties.
• Pine Veneer (0.5 mm thick), also with uniform fiber orientation, is suitable for humidity-driven actuation. This type of wood offers higher mechanical strength and rigidity.
• Polyhydroxyalkanoate (PHA) filament for 3D printing.
• Soy Wax for optional coating to reduce humidity-driven deformation. For better control over deformation, use wood veneer with fibers running parallel to the sheet edges.

Fabricating a Thermal-Driven Actuator

Step 1: Material Selection

• For thermal-driven actuation, use 0.2 mm elm veneer.
• Wax Coating: Apply a hydrophobic soy wax coating to the wood veneer to minimize deformation caused by humidity.
• Use PHA filament for the 3D-printed stripes.

Step 2: Hot Pressing

Hot press the wood veneer to ensure it remains flat and stable, avoiding warping:

• Temperature: 150°C
• Time: 20 minutes

This step prepares the veneer for 3D printing, ensuring a smooth and compact surface.

Step 3: 3D Printing

Print the PHA stripes directly onto the prepared wood veneer to form the bi-layer actuator:

Printer Settings:

• Speed: 200 mm/s
• Nozzle Temperature: 220°C
• Layer Height: 0.1 mm
• Toolpath Width: 0.4 mm

Tip: If double-sided printing is required, ensure precise alignment of the printed stripes on both sides to maintain consistent deformation.

Step 4: Laser Cutting

Laser cut the printed actuator into the desired shape.

• Cutting Speed: 50 mm/s
• Cutting Power: 40–60%

Once cut, the actuator will bend into the desired shape as the environmental temperature changes.

Step 5: Wax Coating

For thermal-driven actuation only, apply a soy wax coating to both sides of the wood veneer:

• Wax Temperature: 60°C
• Application: Apply a thin layer to both sides, allowing the wax to cover and penetrate the wood fibers.

Note: The wax coating is unaffected by typical ambient temperatures during use, ensuring it won’t melt or interfere with the actuator’s performance.

After completing the above steps, you will have a temperature-driven actuator ready for use.

Fabricating a Hygro-Driven Actuator

The fabrication process for the humidity-driven actuator is similar to that for the thermal-driven actuator, with two key differences:

1. Use 0.5 mm thick pine veneer instead of elm veneer.
2. Skip the wax coating step, as it is not needed for humidity-driven actuation. Following these steps, you will have a humidity-driven actuator.

Following these steps, you will have a humidity-driven actuator.

Coordinating Modes of Actuation

TH-Wood actuators can be controlled in various modes depending on environmental changes. There are two basic modes and three coordinating modes to choose from:

Basic Modes:

1. Thermal-Driven Mode: The actuator deforms when the temperature reaches a specified level.
2. Hygro-Driven Mode: The actuator deforms when humidity reaches a certain threshold. Coordinating Modes:
   1. Synergistic Mode (AND Logic): The actuator deforms only when both temperature and humidity meet their specific conditions. If only one factor meets the requirement, the actuator remains inactive.
   2. Substitutive Mode (OR Logic): The actuator deforms when either temperature or humidity reaches the specified condition.
   3. 3 Serial Mode: The actuator responds sequentially to changes in temperature and humidity. When one factor reaches a certain threshold, a specific function is triggered. When the other factor reaches another threshold, a subsequent function is activated. These functions operate in sequence, without interfering with each other.

Tip: Choose the appropriate control mode based on the requirements of your application for precise deformation behavior. Learning application examples helps you better understand coordinating modes.

Application Examples