Chapter 1 Novel photoinitiating systems for 3D printing

Di Zhu Jing Zhang Xiaotong Peng Jacques Lalevée Pu Xiao

The photosensitive formula for one-photon and two-photon 3D printing consists of a photoinitiating system (PIS) and photocurable monomers/oligomers [1, 2]. PIS refers to the group of components that are irradiated directly by light and produce active initiating species (free radicals and/or cations) [1, 2]. In more detail, the photoinitiator (PI) is excited by photons, then either homocleaved into free radicals or reacted with additives via oxidation and energy transfer to generate radicals and/or cations for polymerization initiation [1, 3]. Therefore, the properties of PIs in terms of functional group conversion (FC), rate of polymerization (RP,max), color, transparency and photobleaching affect the toughness of 3D-printed objects and 3D printer parameters such as printing layer thickness and printing duration for each layer [2].

Commercial PIs, such as 2,4,6-trimethylbenzoyldiphenyl phosphine oxide (TPO), phenylbis(2,4,6-trimethyl-benzoyl)phosphine oxide (BAPO), (1-hydroxycyclohexyl)(phenyl)methanone (Irgacure 184) and 2-benzyl-2-(dimethylamino)-1-(4-morpholinophenyl)butan-1-one (Irgacure 369), are efficient and commonly used in PISs for 3D printing [4]. However, 3D printing technology has been applied to various fields, such as tissue engineering, oral drug production and electrically conductive constructs [5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23]. Commercial PISs have notable drawbacks, such as UV initiation, toxicity and low efficiency, which limit their application in further advanced uses of 3D printing technology. To improve the efficiency of PISs and adapt them for use with low-intensity commercial 3D printer projectors in order to meet the standards required in specialized fields (e.g., medicine and scaffolds), additional requirements such as being nontoxic, visible light initiable and highly efficient must be considered. Numerous efficiently designed type I and type II PIs for 3D printing have been reported, including metal complexes, flavone derivatives and naphthalimide derivatives [24, 25, 26]. In this chapter, an overview of the newly developed PISs for both centimeter-scale and nanoscale 3D printing is discussed. Guidance on how to evaluate whether a PI or a PIS is suitable for 3D printing is provided as well.

1.1 Novel photoinitiating systems in 3D printing

1.1.1 Naturally originated and derived photoinitiators

Due to the abundance of brightly colored substances (i.e., dyes), nature is a large provider of potential PIs that are sensitive to visible light LED exposure. For instance, curcumin, a bright yellow turmeric extract, has been observed to be a blue-light-sensitive PI [27]; riboflavin, also called vitamin B2, is another well-investigated natural PI under blue light [28, 29]. Several naturally occurring and derived compounds have been reported to be efficient PIs [30, 31, 32, 33, 34, 35]. Flavone, chalcone, coumarin derivatives and vitamins are presented in this section to introduce their properties and abilities for 3D printing.

1.1.1.1 Flavone-derivative-based photoinitiating systems

Flavone derivatives, reported as antioxidants [36], have been discovered to be a new series of blue-light-sensitive PIs in recent years [24, 37]. A general mechanism for the photoinitiation process was proposed [24, 37]. Specifically, flavones are excited to a singlet or triplet state by photons (r1) and then react with NPG (N-phenylglycine) or Iod (diphenyliodonium hexafluorophosphate), forming intermediate free radicals (r2)-(r4). An alternate route involves two additives (i.e., NPG and Iod), which interact and produce a charge transfer complex (CTC) (r5). This CTC can then decompose into phenyl radicals (Ar) (r6). The Ar then initiates the subsequent polymerization:

(r1)Flavone1,3Flavone(hv) (r2)1,3Flavone+NPGNPG(-H)+Flavone-H (r3)NPG(-H)CO2+NPG(-H,-CO2) (r4)1,3Flavone+Ar2I+Flavone++Ar+Ar-I (r5)NPG+IodNPG-IodCTC (r6)NPG-IodCTCAr(hv)

Photoinitiators

3-Hydroxyflavone (3HF), 6-hydroxyflavone (6HF) and 7-hydroxyflavone (7HF) (Scheme 1.1) are monohydroxy-substituted flavone derivatives. Chrysin and myricetin (Scheme 1.1) are multi-hydroxyl-substituted flavone derivatives. Among all, myricetin is the most redshifted compound in terms of light absorption among reported flavone derivatives (Table 1.1: ?max?=?~375 nm). All presented flavone derivatives lack a maximum absorption peak in the visible light range, but their light absorption profiles overlap with the emission profile of LED@405 nm, and the corresponding extinction coefficients are listed in Table 1.1. Monohydroxy- and dihydroxy-substituted flavones exhibit extinction coefficients at LED@405 nm in the range of ~70 M-1 cm-1 to ~450 M-1 cm-1, while the hexahydroxy-substituted flavone, myricetin, has an extinction coefficient of 4,800 M-1 cm-1 at 405 nm.

Scheme 1.1: Chemical structures of flavone derivatives.

Table 1.1:Light absorption properties of flavone derivatives in methanol: maximum absorption wavelength (?max), molar extinction coefficients at ?max and at the maximum LED emission wavelengths (eLED) of irradiation sources.

PI ?max (nm) emax (M-1 cm-1) e405 nm (M-1 cm-1) Reference Flavone -? -? ~70 [37] 3HF ~350 ~15,000 ~250 [24] 6HF -? -? ~70 [37] 7HF ~305 ~1,900 ~427 [37] Chrysin ~310 ~10,000 ~336 [37] Myricetin 375 16,000 4,800 [37] ?

No obvious maximum absorption peak.

Non-hydroxy-substituted flavone (i.e., flavone itself) can trigger free radical polymerization (FRP) of methacrylates (bisphenol-A-glycidyl methacrylate (Bis-GMA) and triethylene glycol dimethacrylate (TEGDMA), 70 wt%/30 wt%) but cannot promote polymerization propagation (double bond FC of methacrylates at 100?s irradiation is only 4%). 3HF exhibits a minor extinction coefficient at 405 nm in methanol (e405?nm?=?~250 M-1?cm-1). In the presence of amine and NPG and 3HF-based PIS can efficiently initiate photopolymerization of the Bis-GMA/TEGDMA blend (thickness?=?1.4 mm) under LED@405 nm irradiation. The double bond conversion of...