A Biomimetic 3-Layered Head Phantom
for Non-Invasive Brain Stimulation
Abstract
"A Biomimetic 3-Layered Head Phantom with Tunable Electrical Properties for Neurostimulation Testing"
The objective of this work was to develop an electrically conductive three-layered head phantom mimicking the geometry and conductivity of a living head.
The phantom layers represent the major head tissues - outer soft tissue, skull bone, and brain.
To achieve tissue-level conductivity, we formulated conductive polymer composites using corn starch/graphene oxide (GO) mixed matrix membranes. The phantom presented in this work consists of an outer layer emulating the impedance of skin/flesh, a middle layer representing cranial bone, and an inner layer simulating the bulk impedance of the brain.
The phantom provides a monkey head analogue with layered electrical properties mimicking the major tissues of a living head. This allows testing of neurostimulation techniques like those being explored for Parkinson’s disease treatment prior to clinical use.
In summary, we fabricated an anatomically shaped and electrically mimicking 3-layered head phantom using inexpensive biomaterials and a straightforward technique can serve as a test platform for the development of emerging neurostimulation therapies.
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Project Defenition
Tissue mimicking phantoms have been indispensable tools in biomedical research for evaluating therapeutic techniques and medical devices in vitro. While animal models provide biological fidelity, their use faces ethical concerns and human translational challenges. Anatomically shaped physical models that replicate human tissue morphology and properties present a safe, ethical alternative for device testing and protocol optimization before clinical trials .
This is especially pertinent for emerging electromagnetic brain stimulation therapies that hold promise for treating neurological disorders like Parkinson’s disease . However, the development and safe implementation of such techniques requires extensive prior testing in realistic tissue environments. Existing head phantoms are limited in replicating the layered impedances of the scalp, skull, and brain tissues.
In this work, we present the fabrication of a novel 3-layered head phantom with tailored electrical properties emulating the major tissue layers - skin/flesh, bone, and brain. Conductive hydrogel composites of amylomaltase-treated corn starch and graphene oxide (GO) serve as the tissue-mimicking materials. By optimizing GO concentrations, the impedances of each phantom layer are tuned to match physiologically relevant values.
The biomimetic phantom provides a human head analogue to realistically simulate the effects of neurostimulation therapies for protocol optimization and safety evaluation prior to clinical trials. The anatomically accurate morphology and tissue-specific impedances enable robust characterization of the induced electric field effects. In summary, this cost-effective phantom fabricated using simple techniques can serve as an ethical and practical test bed to accelerate the development of emerging electromagnetic brain therapies.
Textile-Based Head Phantom
Tseghai et al. (2021) developed a textile-based head phantom for EEG electrode testing. They used 3D printing and molding of fabric layers to mimic head anatomy and generate realistic EEG signals. This phantom was durable, customizable and simulated scalp electrical properties.
3D Printed Anthropomorphic Head Phantom
J et al. (2017) presented a 3D printed anthropomorphic head phantom for ultrasound imaging. High-contrast 3D printing resins were used to mimic bone, vessel and soft tissue structures. This allowed evaluation of ultrasound image quality and testing of beamforming algorithms. The modular design also enabled variability across phantoms.
Silicone Rubber Phantoms
Synthetic materials like silicone rubbers have also been used to fabricate tissue-mimicking phantoms Forte et al. (2016). Optimized mixtures of silicones and additives were able to match acoustical and mechanical properties of soft tissues. Phantoms based on these rubbers proved useful for multimodal imaging techniques.
Prior Synthesized Tissue Mimicking materials
| Matrix | Filler | ||||
| Polyacrylamide | Water-lipid compositions | Rice Starch | PMMA | Electrically Conductive Gelatine | |
|---|---|---|---|---|---|
| Gellan gum | Corn Starch | PVC | Graphene | PANI | Agarose |
| PVA | PHY | Jelly | Carbon nanotubes (CNTs) | PPy | Silicone-RTV2 |
| Silicone-RTV-1 | PDMS | Hyaluronic acid | Carbon Black (CB) | Carbon Active | 3D printing materials |
| Polyester | PEG | PVP | Gold nanoparticles | Silver nanoparticles | Textiles |
| Polyurethane | Alginate | Resins | Ionic Compounds | ||
Methods and Experiences
Material Selection
Material selection is key for phantoms intending to simulate the head’s electrical properties. The matrix determines baseline mechanical and electrical characteristics, while added fillers enhance conductivity. Design priorities like anatomical fidelity, manufacturing feasibility and cost constraints also influence material choices. Affordable, available materials facilitate simpler phantom constructions for preliminary analyses before investing in more complex designs needing specialized conductive composites.
Comparison
Silicones
- Are inherently insulating materials
- Commercial silicones often contain additives like curing inhibitors, plasticizers, etc.
- Possible matrix-filler debonding during thermal cycling due to CTE difference
Ionic Fillers
- The freely mobile ions can build up surface charge densities at the electrode-phantom interface, forming polarization layers that hinder charge transfer.
PANI
- Expensive to purchase
- Electrical conductivity is highly dependent on environmental factors (Oxidation state, H+, etc.)
CB
- Expensive to synthesize
- Tendency to agglomerate into large particles or processing to disperse
- Negatively impacts mechanical properties of polymer matrix
- Electrical conductivity is highly variable and anisotropic depending on morphology
Measuring the Conductivity
Two common methods are used:
- Two-point probe measurement
- Four-point probe measurement
Four-Point Probe Resistivity Measurement Fixture
We reverse-engineered the ETS volume resistivity measurement fixture model and made our own to be able to estimate the samples' electrical conductivities.
I had the opportunity to design and construct this device for the team.
Shows original model vs our model:
| Original Model | Our Model |
|---|---|
| Image of original fixture | Image of our fixture |
|
|
Four-Point Probe Resistivity Measurement Fixture
Includes calculations:
ρv = Vwd/iL
ρv = Volume resistivity in Ohm-cm
σ = 1/ρ V = Potential difference across potential electrodes
i = Current through specimen w = width of specimen (cm)
d = Thickness of specimen (cm)
L = distance between potential electrodes (cm)
Remaining Tasks and Future Directions
- Characterizing the Electrical Behavior
- Illustrating the Relationship between Filler Content and Electrical Conductivity
- Synthesis of the final 3-layered head phantom
Experiments Gallery
Matrix: PDMS | Filler: Cu(SO4)2
poroused PDMS drowned in Cu(SO4)2 solution
Matrix: Starch | Filler: Carbon Black
Gelatinized starch stirred with CB
4-Point Resistivity measurement fixture
We made our own fixture to measure samples' conductivities
Testing our 4-point probe measurement fixture
The results were evaluated with 2-point probe measurement. The results were promissing.
The molded sigle layer brain phantom
The mold was 3D printed and the sample was starch/NaCl-based.
GO (Graphene Oxide) Flakes Awards
Synthesizing GO using modified Hummer's method
A set of starch-based samples with varied CB and PANI percentages
The black samples are CB, while the others have PANI as the filler.
2-Point probe resistivity measurement
Using a syringe for precise volumetric values to measure a sample's resistivity.
Gelatinized starch stirred with Cu(SO4)2 solution
Had a pretty high electrical conductivity, but the setback was drying out too soon.
Our samples 1
Our samples 2
Our samples 3
