Photocuring Basics

In the additive manufacturing space, photocuring is a common process, especially when working with hydrogels. Photocuring is the process of hardening a substance by exposing it to a specific wavelength of light. Below, we discuss the basics of photocuring in bioprinting.

How Photocuring Works in a Hydrogel

A hydrogel is a network of polymers that retain water. In most cases, nothing in that structure is photo-reactive. The most common way to make a hydrogel photocurable is to modify its polymers and introduce a photoinitiator. In this case, the photoinitiator is a reagent that releases free radicals from its structure when exposed to a certain wavelength. Typically, you modify the hydrogel by adding acryl or tyrosine functional groups to the polymer chains. These functional groups react to the free radicals released by the photoinitiator. This causes the polymers in the hydrogel to form more interlinking bonds, in a process referred to as crosslinking. A higher degree of crosslinking gives the hydrogel firmer physical properties.

The extent of crosslinking (and therefore stiffness) is controlled primarily by the concentration of photoinitiator in the hydrogel, the intensity of the light source, and the duration of the curing. In general, the entire hydrogel will be composed of photoinitiator-compatible polymers. This ensures that the photoinitiator is the limiting reagent in the crosslinking process. In general, more concentrated photoinitiators have the capacity to release more free radicals, causing more crosslinking and therefore a firmer gel. Similarly, using a higher intensity light source over longer periods provides more energy to release free radicals, leading to a firmer gel.

For more details about the Crosslinking Section of the Allevi Software, check out our Complete Guide to Allevi Bioprint Online.

Why Use Photocuring?

Bioprinting commonly uses hydrogels which encapsulate cells well but often have poor mechanical properties for building constructs. Photocuring is an excellent technique for adding some needed firmness. It offers lots of flexibility as a crosslinking method since you can change the amount of light exposure you use and the chemical composition of your hydrogel to fine-tune the stiffness. It also works much faster than other crosslinking methods. This can be especially helpful when working with cells that are outside of their native environment during a print. Photocuring also provides an alternative to thermal or chemical crosslinking for experiments that cannot tolerate them.

Comparing Photoinitiators

There are two basic types of photoinitiators: type 1 and type 2. Type 1 initiators activate with exposure to their requisite wavelength, then they release free radicals into the surrounding material. Type 2 initiators involve two reagents the first of which is photosensitive. When activated by light exposure it becomes reactive with the second reagent which releases free radicals.

Beyond their mode of activation, photoinitiators differ mainly on their solubility before activation and their specific range of activation wavelengths. When working with hydrogels, it is often helpful to have a water-soluble photoinitiator. This helps ensure even distribution into the water-rich interior of the hydrogel. In the broader field of material science, many photoinitiators are activated by ultra-violet light. This can be problematic for bioprinting as the UV rays can harm cell viability. Fortunately, there are many photoinitiators that are activated by blue visible light wavelengths. These, however, may produce less crosslinking relative to their UV activated counterparts. It’s also important to consider if your particular hydrogel will be reflective with any range of wavelengths.

Photoinitiator Cheat Sheet

Photoinitiator	Activation Wavelength	Technical Name	Additional Needed Reagents
LAP	365 nm – 405 nm	Lithium phenyl-2,4,6-trimethylbenzoylphosphinate	N/A
Ruthenium	400 nm – 450 nm	Ruthenium Photoinitiator	Sodium Persulfate
Irgacure	385 nm – 420 nm	Diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide	N/A

Photocurable Hydrogels

Currently, there are some great hydrogel options that are already modified to work with photoinitiators! Below we give some quick details on some popular options.

Hydrogel	Main Advantage	Technical Name
PhotoCol	Good for mimicking collagen dense tissue types like skin, connective tissue, bone, endothelia, etc.	Methacrylated Collagen
PhotoHA	Hyaluronic Acid (HA) has a unique and useful structure, this version adds photocuring; good for mimicking HA dense tissues like skin	Methacrylated Hyaluronic Acid
GelMA	Ubiquitous and well understood, GelMA is great for general use	Gelatin Methacrylate

Do you have more questions about photocuring? Email the Allevi Customer Success Team at [email protected].