Introduction

Alzheimer's disease (AD) is a neurodegenerative disease that causes deaths in millions of people across the globe. AD is marked by accelerated cognitive loss, memory loss and behaviour modification, and the individual diagnosed with AD, their families and carers are in deep trouble. Alzheimer's disease is not curable and its therapies can do only symptomatic good, even after decades of research. One challenge in AD research is that there are no model animals that accurately represent the disease's neurobiology and brain chemistry. The cellular and biochemical landscape of the human brain is complex enough for two-dimensional (2D) cell cultures and animal models, but they can't simulate this complexity quite well. And that's where cutting-edge technology like 3D printing comes in, which allows for new ways to more accurately model Alzheimer's.

Figure 1. A new 3D triculture system. (Karmirian K, et al.; 2023)

3D printing or additive manufacturing is changing everything from manufacturing to medicine. 3D printing is now available for biomedical research, and can be used to build accurate, detailed structures, like real tissues. In the case of Alzheimer's, micro-models that mimic the structure of the brain and AD's pathology (amyloid plaques, neurofibrillary tangles) can be 3D printed. This post looks at the potential of 3D printing Alzheimer's disease micro-models to show what progress, issues and opportunities there are for this novel technique.

The Expansion of AD Models: Why We Need Better Models?

Even current Alzheimer's models – such as transgenic animals and 2D cell cultures – have been informative about how the disease works. But they're weak on a number of fronts. Although useful, animal models don't entirely resemble the human brain, and ethical objections also limit their use. 2D cell cultures aren't capable of the kind of three-dimensional coupling that exists in living tissues, and they're unsuitable for studying complicated cellular processes and treatment response.

The brain is a complex organ with cells of all sorts: neurons, astrocytes, microglia and so on, all interacting in a 3D extracellular framework. Alzheimer's disease follows a series of cascades: amyloid-beta (A) aggregation, hyperphosphorylation of tau protein, neuroinflammation and synaptic loss. For such processes to be studied and for treatments to be made, scientists need brain models capable of capturing the 3D space and pathology of the brain.

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Use of 3D Printers in Biomedical Engineering

In recent years, 3D printing has advanced considerably, and it is now more precise and adaptable to fabricate biological models than ever before. It's a technology that assembles layers of materials layer by layer into structures with biocompatible polymers, hydrogels, and even cells. Because 3D printing is capable of printing large, individual pieces of architecture, it is a perfect method for building complex, biological tissue models.

With Alzheimer's disease, we can use 3D printing to create micro-models of the brain with essential pathology of AD. These models can be used to map out the environment of disease more fully so that scientists can study disease mechanisms and trial possible treatments.

Creating Alzheimer's Disease Micro-models

The process of making 3D-printed micro-models of Alzheimer's disease consists of several steps — design the model, pick the right materials, add the features of Alzheimer's disease. This is to produce a model that faithfully describes the cellular and extracellular environments of the AD brain.

Design and Fabrication

The building design process begins with digital design of the building to be built. This model can be derived from image information of human brain tissue, and the model will represent microarchitecture of the brain. Thanks to powerful software, the geometry of the model can be precisely controlled, from the distribution of cell types to the distribution of extracellular matrix materials.

Once the model is finalized, it is 3D printed. We have printing methods such as extrusion printing, inkjet printing and stereolithography. Each approach is good in its own way, and extrusion-based printing works best for constructing large multicellular structures.

Material Selection

You have to get the materials right to make realistic micro-models of Alzheimer's disease. It is common to use biocompatible polymers and hydrogels that enable cells to grow and differentiate. Hydrogels, for instance, can be infused with biochemical signals based on the extracellular matrix of the brain and encouraged to build neural networks.

A key part of the micro-models, apart from scaffold materials, are cells. iPSCs from Alzheimer's disease patients can be made into brain cells such as neurons, astrocytes and microglia. Such cells can then be plugged into the 3D-printed model, which gives a more realistic view of the disease setting.

Incorporating Disease Features

A realistic model of Alzheimer's would have to include hallmark pathology – amyloid plaques and tau tangles, for example. We can do that by adding genetic changes or biochemical treatments that cause them to form in the 3D-printed model. For example, iPSCs can be genetically altered to secrete too many amyloid-beta or tau proteins and create the plaques and tangles that we find in AD patients.

Other solutions are to embed amyloid-beta peptides or tau fibrils in the printed scaffold to provide a substrate on which plaque and tangles can form. We can then examine in these models how plaques and tangles develop, and how they influence neuronal activity and survival.

Applications and Benefits

3D-printed Alzheimer's disease micro-models have a few distinct advantages over models: they're a more lifelike, generic toolkit for research.

Drug Screening and Development

One of the best use cases for 3D-printed AD micro-models is drug screening. They represent the truer environment of the human brain, and so researchers can evaluate the efficacy and toxicity of potential therapies in a real-world environment. This can accelerate the process of drug development as candidates for future growth can be found and refined before they are put to use in animals and in trials.

Mechanistic Studies

It's important to be able to identify how Alzheimer's behaves before there are treatments. Micro-models printed in 3D could be used to study cell interaction, the effect of amyloid plaques and tau tangles on neurons, and the impact of neuroinflammation on disease. This can help to understand the molecular and cellular drivers of AD and help to shape individualised treatments.

Personalized Medicine

3D printing has possibilities for personalized medicine, too. Using iPSCs from individual patients, scientists could build patient models of Alzheimer's. They can then investigate the effects of diverse genetic and environmental environments on disease course and therapeutic responses to create personalised treatments.

Challenges and Future Directions

Although there are huge opportunities for 3D-printed Alzheimer's disease micro-models, there are still challenges. Models that simulate all the intricate details of the brain are not easy to produce, and some models may still be incomplete – the blood-brain barrier, for example, or all the different types of brain cells. Furthermore, 3D printing technology has yet to scale as sculpting large quantities of identical models for high throughput screening is slow and expensive.

But these are just some of the challenges, and there will be ongoing innovations in 3D printing and biomaterials that will remove a lot of them. It's only when printers become more advanced with more resolution and print speeds that we'll be able to make more detailed, more complex models. In addition, the modelling will be made more realistic by the use of high-tech biomaterials, including materials that mimic mechanical and biochemical characteristics of brain tissue.

Future work will also involve more accurately incorporating disease-specific information – like more precise models of amyloid plaques and tau tangles, and more accurate replicating the brain microenvironment. These models might also be made even more functional if 3D printing is used in combination with other technologies, including microfluidics, to study dynamic phenomena like blood flow and nutrient exchange.

Conclusion

Three-dimensional printing also gave Alzheimer's research new possibilities: more realistic and adaptable human brain models. In building micro-models of Alzheimer's disease in sufficient detail that they mimic some of its hallmarks, scientists can better understand how the disease works, and to design more effective drugs. Still there are hurdles, but continued progress with 3D printing and biomaterials promises to break through these and open up a pathway for new discoveries and treatments for Alzheimer's disease.

Reference

1. Karmirian K, et al.; Modeling Alzheimer's Disease Using Human Brain Organoids. Methods Mol Biol. 2023, 2561:135-158.