Skip to main content
NCBI home page
ACS AuthorChoice logoLink to ACS AuthorChoice
. 2023 Jan 25;15(5):7329–7339. doi: 10.1021/acsami.2c16235

Soft Hydroxyapatite Composites Based on Triazine–Trione Systems as Potential Biomedical Engineering Frameworks

Jinjian Lin 1, Yanmiao Fan 1, Daniel J Hutchinson 1, Michael Malkoch 1,*
PMCID: PMC9923673  PMID: 36695708

Abstract

graphic file with name am2c16235_0007.jpg

Composites of triazine–trione (TATO) thiol–ene networks and hydroxyapatite (HA) have shown great potential as topological fixation materials for complex bone fractures due to their high flexural modulus, biocompatibility, and insusceptibility to forming soft-tissue adhesions. However, the rigid mechanical properties of these composites make them unsuitable for applications requiring softness. The scope of these materials could therefore be widened by the design of new TATO monomers that would lead to composites with a range of mechanical properties. In this work, four novel TATO-based monomers, decorated with either ester or amide linkages as well as alkene or alkyne end groups, have been proposed and synthesized via fluoride-promoted esterification (FPE) chemistry. The ester-modified monomers were then successfully formulated along with the thiol TATO monomer tris [2-(3-mercaptopropionyloxy)ethyl] isocyanurate (TEMPIC) and HA to give soft composites, following the established photo-initiated thiol–ene coupling (TEC) or thiol–yne coupling (TYC) chemistry methodologies. The most promising composite shows excellent softness, with a flexural modulus of 57 (2) MPa and εf at maximum σf of 11.8 (0.3)%, which are 117 and 10 times softer than the previously developed system containing the commercially available tri-allyl TATO monomer (TATATO). Meanwhile, the surgically convenient viscosity of the composite resins and their excellent cytotoxicity profile allow them to be used in the construction of soft objects in a variety of shapes through drop-casting suitable for biomedical applications.

Keywords: triazine−trione materials, soft hydroxyapatite composites, biomedical engineering, biocompatibility, thiol−ene and thiol−yne materials

1. Introduction

Thermosets and their composites are among the most widely used and versatile engineering materials thanks to the abundant number of different monomers that originate from biomass- or petro-based resources.1−5 An example of popular monomers in engineering is the triazine–trione (TATO) family, which includes a wide variety of commercially available building blocks with different reactive groups that result in rigid and stable materials due to the cyclic structure of the TATO ring. This has led to various applications of TATO-based materials in engineering, including thermal insulation materials,6−8 magnetic nanoabsorbents,9 and coatings.10,11 TATO materials have also drawn much attention in the field of biomedical applications. For example, Granskog et al.12 presented composites based on tri-allyl and tri-thiol TATO monomers with high concentrations of hydroxyapatite (HA) that could be cross-linked via high-energy visible-light-initiated thiol–ene coupling chemistry (HEV-TEC). These composites were then successfully used to construct shapeable fixation patches for treating bone fractures, which displayed high mechanical performance and exceptional biocompatibility. The use of TEC click chemistry for curing TATO monomers was initially inspired to create suitable alternatives to methacrylate-based dental composites,13−15 with the TEC reaction chosen as it displayed the advantages of nonsensitivity to oxygen inhibition, regioselectivity, a high monomer conversion rate, and rapid curing under mild conditions, all of which are beneficial and favorable for curing in a physiological environment.16−20

A large variety of TEC thermosets have been described in the literature based on a variety of different monomer architectures. Previous studies include using biomass such as lignin or d-limonene to prepare TEC thermosets as coating applications and also petro-based resources such as ostemer (which is a mixture of allyl, thiol, and epoxy monomers) to fabricate potential devices for organ-on-a-chip applications.21−24 Tri-allyl TATO monomer (TATATO) has been used to form thermosets with different ratios of pentaerythritol tetrakis(3-mercaptopropionate) (PETMP), with Young’s moduli ranging from 920 to 1177 MPa.25 However, an often-reported technical constraint of the TEC reaction is the limited mechanical properties of materials formed by TEC reactions due to the high flexibility of the thio–ether bonds between monomers26,27 This shortcoming can be mitigated through the use of multifunctional alkene and thiol monomers, such as those based on the TATO cyclic architecture. Similar to TEC chemistry, thiol–yne coupling (TYC) belongs to the family of efficient ‘click’ chemistry, and it is another helpful technique in the TATO thiol coupling methodology.28 The TYC click reaction results in networks with higher cross-link densities than the TEC reaction due to the alkyne’s ability to react with two thiols instead of one. Consequently, the use of TYC can result in materials with higher strength and rigidity than their TEC counterparts.29,30 In another context, TYC materials have been applied to generate interpenetrating soft hydrogel networks or functionalize films or fibers.31,32

The stiffness of TEC and TYC materials can also be enhanced through the addition of inorganic filler particles, such as glass particles, HA, and tricalcium phosphate. However, the increase in stiffness is often accompanied by an increase in brittleness which can limit the scope of these composites. In addition, the established materials’ mechanical performance has only covered a small fraction of the vast range of the mechanical behavior of soft or hard human tissues. For example, the HA–TATO composites reported by Granskog et al.12 had a flexural modulus of 6.1 GPa. The elastic moduli for soft human tissues including cartilage, ligament, and tendon reach 2–33 MPa,33 25–93 MPa,34 and around 1.2 GPa,35 respectively, which are much lower than that of the reported HA–TATO composites. Additionally, harder tissues such as bones have a large range of moduli depending on their types. For instance, the elastic modulus of vertebral bone, which is the main component of vertebra, is between 10 and 900 MPa,36 while the human proximal tibia bone (trabecular) can display a modulus up to 33.9 GPa.37,38

The large differences between the modulus of these natural tissues and the reported TATO composites show that there is great potential for broadening the scope of TATO composites in biomedicine by increasing the tuneability of the mechanical properties of these composites. We hypothesized that modifying the TATO monomers by introducing additional linkages and space between the unsaturated groups and the TATO ring would yield composites with a wider variety of mechanical properties (Figure 1). Consequently, ester and amide bonds were sought out as potential linkages with the original intent of adding secondary H-bonding to the cross-linked network. Simultaneously, the addition of these groups would elongate the chains between the TATO rings, which was expected to decrease the cross-link density of the network and increase its flexibility; factors that could soften the materials. The use of alkyne groups instead of allyl groups was expected to increase the stiffness of the composites due to the increased cross-link density attained through TYC chemistry. However, these small changes to the TATO monomer structures resulted in significant impacts on the softness of the resulting composites, which we explore here, resulting in materials that display a wider range of mechanical properties than those previously described for TATO systems.

Figure 1.

Figure 1

Open in a new tab

(a) Four novel TATO-based unsaturated monomers with either ester or amide linkages and alkene or alkyne bonds were synthesized through fluoride-promoted esterification (FPE) chemistry. (b) Mechanical properties of the materials made with these novel monomers were compared to those of the commercially available and previously described alkene 0 monomer. All materials used the commercially available thiol monomer.

Herein, we report the successful synthesis of novel tri-alkene or tri-alkyne TATO monomers containing either ester or amide linkages by using fluoride-promoted esterification (FPE) chemistry, as well as the first attempts to formulate soft hydroxyapatite–TATO (HA–TATO) composites along with the commercially available and previously reported tri-thiol monomer tris[2-(3-mercapto propionyloxy ethyl] isocyanurate (thiol, TEMPIC). Compared to the previous reports on thermosets and composites based on the TATO system, extending the distance between the TATO rings and the unsaturated allyl or alkyne groups in tandem with the inclusion of ester and amide bonds was found to greatly impact the formulation and mechanical properties of TATO thermosets and HA-containing composites.

2. Experimental Section

2.1. Preparation of Materials

The thiol monomer TEMPIC was obtained from Bruno Bock Chemische Fabrik GmbH & Co., KG. The other chemicals were obtained or purchased from Sigma-Aldrich Sweden AB, TCI Europe NV and VWR Chemicals. The synthesis procedures and characterization of the tri-ester TATO monomers and tri-amide TATO monomers have been provided in the Supporting Information, Section 1.

2.2. Nuclear Magnetic Resonance (NMR)

Analysis was performed using a Bruker 400 ultrashield NMR spectrometer. 1H and 13C NMR results were collected at a frequency of either 400 or 101 MHz, respectively. For 1H NMR, a spectral window of 20 ppm, a relaxation delay of 1 s, and 128 scans with automatic lock and shimming at a concentration of 15 mg/mL have been applied. For 13C NMR, a spectral window of 240 ppm, a relaxation delay of 2 s, and 512 scans with automatic lock and shimming at a concentration of 150 mg/mL have been applied. The obtained spectra were analyzed using Mest ReNova version 9.0.0-12821.

2.3. MALDI-TOF-MS Analysis

A Bruker UltraFlex L matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) system with a SCOUT-MTP ion source (Bruker Daltonics, Bremen) was used. All spectra were acquired using a reflector-positive mode. The instrument was calibrated by using SpheriCal TM calibrants obtained from Polymer Factory Sweden AB. Matrixes were prepared by the dissolution of the matrix substance at a concentration of 10 mg/mL, salt at a concentration of 1 mg/mL, and analyte at a concentration of 1 mg/mL in THF. The samples were prepared at a ratio of 20:5:5 of the matrix substance, monomer analyte, and counter ion, respectively. The matrix substance 2,5-dihydroxybenzoic acid (DHB) and the counterion salt sodium trifluoroacetate (NaTFA) were used for mass analysis unless otherwise stated. The received spectra were analyzed with FlexAnalysis Bruker Daltonics, Bremen, version 2.2.

2.4. Formulations and Curing of the TEC and TYC Materials

The formulations of TEC and TYC materials were performed by mixing the thiols and different alkene or alkyne monomers along with TPO in optimized ratios (Supporting Information, Section 4) to give thermoset resins. For the preparation of composite resins, the same formulas as the thermoset resins were followed, while hydroxyapatite was added additionally as a filler. The viscous mixtures were thereafter cured in a beam-shaped silicon mold with dimensions of 32 mm × 6 mm × 2 mm by a portable high-performance curing LED lamp (Bluephase 20i, Ivoclar Vivadent AG, Leichtenstein) with a spectrum wavelength of 385–515 nm and light intensity of 2000 mW/cm2. LED treatment with 10 pulses (5 s/pulse) was applied on each surface of the Ene0-T1, Ene0-C4, Ene1-T1, and Ene1-C4 beams, and 15 pulses (5 s/pulse) were used on each surface of the Yne1-T1 and Yne1-C4 materials to give full conversion of monomers.

2.5. Raman Spectroscopy

The conversions of TEC and TYC monomers were monitored by a portable i-Raman Plus spectrometer (model: BWS465-785S, B&W TEK). The resins were tested with 48 scans (laser wavelength: 785 nm, laser power: 100%, and integration time: 1000 ms), and the cured materials were analyzed with 16 scans (laser wavelength: 785 nm, laser power: 100%, and integration time: 1000 ms). BWSpec software was used to collect the data which was then analyzed using Origin 9.1. The spectra were normalized by the carbonyl shift (1760 cm–1). For the TEC materials Ene0-T1, Ene0-C4, Ene1-T1, and Ene1-C4, the shifts of the thiol groups (2575 cm–1) and C–C double bonds (1645 cm–1) were analyzed to confirm the almost full conversion of the reactions. As for the TYC materials Yne1-T1 and Yne1-C4, the shifts of the thiol groups (2575 cm–1) and C–C triple bonds (2120 cm–1) were analyzed. At least two different batches of resins for each material were tested. The spectra for all the composites and thermosets are given in Supporting Information, Figure S11.

2.6. Scanning Electron Microscopy and Energy Dispersive Spectroscopy

The SEM and EDS images of the materials’ cross sections were captured by FE-SEM S-4800 (Hitachi, Japan) and EDS X-Max 80 SDD (Oxford Instruments, UK) systems, respectively. To obtain natural cross sections for characterization, intact composites or thermosets were cryofractured by liquid nitrogen for 5 min, and were then fractured in the middle of the beams. The SEM and EDS analyses of the beams’ cross sections were thereafter conducted by the cooperation of FE-SEM and EDS X-Max systems with 15 kV acceleration voltage. The captured images are provided in Supporting Information, Figure S10.

2.7. Water Absorption and Solubility Testing of TEC and TYC Materials

Water absorption and solubility testing were conducted on the materials Ene0-T1, Ene0-C4, Ene1-T1, Ene1-C4, Yne1-T1, and Yne1-C4. The samples were prepared and cured by a Bluephase 20i LED lamp in a disk-shaped mold with a diameter of 12 mm and thickness of 1.5 mm. For each material, five samples were prepared. The samples were then dried in a 50 °C oven for 3 days until their starting dry weight (m1) stabilized within 0.1 mg. Afterward, the samples were immersed in PBS solution (pH = 7.4) in a 37 °C oven for 7 days, after which their masses stabilized within 0.1 mg. They were then washed with deionized water, blotted with tissue paper, and their weights were measured to give the wet masses (m2). Thereafter, the samples were dried in a 50 °C oven until they gave a constant dry mass (m3). The water absorption (Wsp) and solubility (Wsl) of Ene0-T1, Ene0-C4, Ene1-T1, Ene1-C4, Yne1-T1, and Yne1-C4 materials were then calculated, respectively, by eqs 1 and 2, and the results are shown in Supporting Information, Figure S13:

2.7. 1
2.7. 2

2.8. Mechanical Evaluations of TEC and TYC Materials

A three-point bending test was conducted on the composites and thermosets under both dry and wet conditions, and the samples were prepared by the method described in Section 2.4. The samples for testing under wet conditions were first immersed in PBS (pH = 7.4) for 7 days at 37 °C. They were then removed from the solution, and tissue paper was used to dry the surface of the materials. The samples were cooled down to room temperature before testing. As for the samples to be tested under dry conditions, their mechanical properties were measured directly after the preparation of the materials. The samples for both dry and wet conditions were then tested with an Instron 5566 universal testing machine (Instron Korea LLC) with a 500 N load cell, a cross-head speed of 1 mm/min, a preload of 0.1 N, and a preload speed of 0.5 mm/min. The center-to-center distance of the lower contacts was set to 30 mm and all measurements were conducted at 20 °C with a relative humidity of 50%. The data were analyzed and collected by Bluehill software. The flexural modulus was calculated by eq 3, where L is the lower contacts’ distance, m is the slope at the initial elastic region of the load and displacement curve, w is the width of the beam, and d is the thickness of the beam. For each material, at least five samples were tested.

2.8. 3

2.9. Dynamic Mechanical Analysis

A dynamic mechanical analyzer (DMA Q800, TA Instruments, USA) was used to measure the glass-transition temperatures (Tg) and onset points of the composites and thermosets in tensile mode. The materials were of dimensions 12 × 6 × 2 mm (length × width × thickness). The samples were tested under either dry or wet conditions. The samples for testing under wet conditions were first immersed in PBS (pH = 7.4) for 7 days at 37 °C. They were then removed from the solution, and tissue paper was used to dry the surface of the materials. The samples were cooled down to room temperature before testing. A temperature ramp method with a heating rate of 3 °C/min was used, and the testing temperatures ranged from −20, −15, and −10 °C to 80, 90, and 100 °C, depending on the materials’ properties. A strain of 0.1% was induced with a frequency of 1 Hz. To calculate the cross-link density of the thermosets (ρ), the storage modulus (E’) at the rubber plateau of the thermosets was assumed as the elastic modulus (E) by using eq 4, in which R is the Boltzmann gas constant, T is the temperature, and the Poisson ratio (v) of 0.5 was applied assuming an ideal rubber elasticity. For each material, at least five samples were tested.

2.9. 4

2.10. Cytotoxicity Assay

Human dermal fibroblast (HDF) and mouse monocyte cells (Raw 264.7) were used for the cytotoxicity assays. Both cell lines were obtained from ATCC and maintained in tissue culture flasks at 37 °C in CO2 (5%) with Dulbecco’s modified Eagle’s medium (DMEM), supplemented with fetal bovine serum 10% (v/v), l-glutamine (4 mM), penicillin (100 IU mL–1), and streptomycin (100 μg mL–1). Cells were harvested and transferred into 96-well plates at a concentration of 1 × 104 cells per well in 100 mL DMEM cultured 24 h before use. To test the cytotoxicity of the polymers, the polymers were dissolved in media at the desired concentrations and were introduced to the cells and incubated for 72 h (37 °C, CO2 (5%)). Subsequently, Alamar Blue (10 μL) was added, and incubation was continued for 4 h (37 °C, CO2 (5%)). Then, the plate was shaken for 20 s, and finally the fluorescence intensity was measured at ex/em 560/590 nm.

The cytotoxicity of the composites was assessed using human dermal fibroblasts (HDFs) and mouse monocyte (Raw 264.7) cell lines. The cells were harvested and seeded in 96-well plates at a concentration of 1 × 104 cells per well in 100 mL DMEM cultured 24 h before use. The composite was formed with a total surface area of 3 cm2 and sterilized under UV light for 20 min. After sterilization, the composite was immersed in 1 mL of cell culure medium (3 cm2/mL) and transferred into the incubator for 24 h at 37 °C. 100 μL of the composite elute solution was then added to the 96-well plates and incubated with cells for 72 h. After incubation, Alamar Blue (10 μL) was added, and incubation was continued for 4 h (37 °C, CO2 (5%)). The plate was then shaken for 20 s, and the fluorescence intensity was measured at ex/em 560/590 nm. All results are shown as mean ± SD.

2.11. Object Demonstrations

Rings, columns, tubes, and thin-film-shaped objects were made directly from the Ene1-C4 resin and cured by an LED lamp Bluephase 20i in specific silicon molds for different shapes, following the same rationale as in Section 2.4.

3. Results and Discussion

To understand the structure-to-property relationship of TATO thermosets and composites impacted by structural changes in the TATO monomers, we sought out the synthesis of novel TATO-based monomers that displayed ester or amide linkages as well as alkene or alkyne cross-linkable groups. FPE chemistry was selected as a robust modification reaction to synthesize this library of TATO monomers. Then, the monomers underwent a screening procedure to formulate resins with the tri-thiol monomer thiol.12 The resins were thereafter cured by HEV light to give both thermosets and HA-including composites. The resins exhibited similar viscosity to the previously reported formulations based on alkene 0 and thiol, which were optimized for application and surfacing on bone tissue substrates under surgical conditions, that is, thin enough for application via syringe and shaping but thick enough to hold their shape and not flow from the desired tissue site before curing.12,39 The materials prepared were then assessed by a series of characterization techniques including three-point bending, DMA, SEM, and EDS as well as cytotoxicity evaluations to investigate the influence of different monomer structures and the impact of adding HA as a filler. Finally, the most promising system was translated to potential customizable biomedical engineering devices to showcase their potential.

3.1. Chemical and Mechanical Properties of the Novel TATO Alkene and Alkyne Monomers

Inspired by the commercially available alkene 0 monomer, four novel multifunctional TATO chemicals (alkene 1, alkene 2, alkyne 1, and alkyne 2; Figure 1), which contained either ester or amide linkages and C–C double or triple bonds, have been synthesized successfully by FPE chemistry. The aim of introducing the ester and amide groups was to increase the toughness of the cross-linked network through secondary H-bonding; however, the insertion of these linkages between monomers was expected to also slightly decrease the cross-link density and thereby potentially reduce the stiffness of the materials. To counteract the lowering of cross-linking density and subsequent loss in stiffness, we compared the allyl functional TATO monomers in alkenes 1 and 2 with their alkyne counterparts alkynes 1 and 2, in which the latter would each react with two thiols through TYC chemistry, instead of just one.

Alkene 1 and alkyne 1 were synthesized in high purity in a single reaction between 1,3,5-tris(2-hydroxyethyl) isocyanurate (isocyanurate-OH) and either carbonyldiimidazole(CDI) activated 4-pentenoic acid (4PA) or 4-pentynoic acid (4PTYA), respectively. The yields of alkene 1 and alkyne 1 were calculated to be 92 and 50%. Both products were dark yellow viscous liquids and were characterized by 1H and 13C NMR spectroscopy and MALDI (Supporting Information, Sections 1.1 and 1.2). Alkene 2 and alkyne 2 were synthesized in a multistep sequence, including tosylation, azidation, and amination, and finally FPE reaction with either 4PA or 4PTYA (Supporting Information, Sections 1.3, 1.4, and 1.5). The tri-amide TATO monomers alkene 2 and alkyne 2 were both white powders after purification, and their yields reached 42 and 44%, respectively. The differences in the yield between the tri-ester-TATO and tri-amide-TATO monomers indicated that the reactivity of the hydroxyl groups on the TATO core was higher than that of the amine groups with respect to the CDI-activated acids. The liquid state of the tri-ester-TATO monomers and the solid state of the tri-amide-TATO monomers under room temperature also suggested that the ester linkages in alkene 1 and alkyne 1 provided weaker hydrogen bonding than the amide groups in alkene 2 and alkyne 2.

3.2. Formulations and Mechanical Properties of TEC and TYC Materials

The commercially available tri-alkene TATO monomer alkene 0, novel synthesized tri-ester-alkene TATO monomer alkene 1, and tri-ester-alkyne TATO monomer alkyne 1 were formulated with the commercially available thiol to give thermosets and HA composites (Table 1). The formulations were stoichiometrically balanced, with the ratio of unsubstituted carbon bonds to thiols of 1:1 for TEC formulations and 2:1 for TYC formulations. This stoichiometry was chosen to promote the complete conversion of all alkene, alkyne, and thiol groups in the formulation in order to create materials with the highest possible cross-link density. Unfortunately, neat formulations with the amide-containing alkene 2 and alkyne 2 monomers were unattainable as they proved too insoluble in the thiol at room and elevated temperatures.

Table 1. Thermoset and Composite Materials Evaluated in this Work and the Concentration of HA and TPO for Each Material, as Well as the Required Pulses of HEV Light for Full Curinga.

Materials Alkene Alkyne HA TPO Pulses of 5 s HEV Type
Ene0-T1 alkene 0   0 wt %a 0.25 wt % 5 thermoset
Ene0-C2 alkene 0   19 wt %b 0.25 wt % 5 composite
Ene0-C3 alkene 0   37 wt %c 0.25 wt % 5 composite
Ene0-C4 alkene 0   56 wt %d 0.25 wt % 5 composite
Ene1-T1 alkene 1   0 wt %a 0.25 wt % 5 thermoset
Ene1-C2 alkene 1   20 wt %b 0.25 wt % 5 composite
Ene1-C3 alkene 1   40 wt %c 0.25 wt % 5 composite
Ene1-C4 alkene 1   60 wt %d 0.25 wt % 5 composite
Yne1-T1   alkyne 1 0 wt %a 0.88 wt % 15 thermoset
Yne1-C2   alkyne 1 19 wt %b 0.88 wt % 15 composite
Yne1-C3   alkyne 1 39 wt %c 0.88 wt % 15 composite
Yne1-C4   alkyne 1 58 wt %d 0.88 wt % 15 composite
Open in a new tab
a

a, b, c, and d indicate 0, 33, 67, and 100% of the determined maximum HA concentrations for each type of the composites.

Inspired by previously established methods,12 thermosets Ene0-T1, Ene1-T1, and Yne1-T1 were formulated with thiol and either alkene 0, alkene 1, or alkyne 1, respectively, along with diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide (TPO) as the photoinitiator. HA was then added to each thermoset to create a series of composites. The maximum HA concentration that could be added to Ene0-T1, Ene1-T1, and Yne1-T1, while maintaining a fluid homogeneous mixture, was determined to be 56, 60, and 58 wt %, respectively. At these concentrations of HA, the mixtures had a viscosity that allowed them to be applied and spread on surfaces with a spatula while not being so thin as to run-off the curved surfaces before the curing with HEV light could be completed. This viscosity was therefore ideal for their envisioned use in creating in situ bone fixation patches. Composites of alkene 0, alkene 1, and alkyne 1 were also formulated with either 33, 67, or 100% of their maximum HA concentration in order to determine how the concentration of HA affected the mechanics of each material. In total, 12 different formulations were made and investigated that varied due to the presence of (i) ester groups, (ii) alkene or alkyne functionalities, and (iii) HA at various concentrations (Table 1).

After formulation, the mixtures were cured by HEV light from a handheld LED lamp. The TYC Yne1 materials required a higher concentration of TPO and a longer exposure of HEV light to obtain complete monomer conversion. FT–Raman spectra showed that all the materials had been fully cured as indicated by the disappearance of the thiol signals between 2560 and 2600 cm–1 and either the alkene or alkyne signals at 1630–1660 or 2100–2136 cm–1, respectively (Figures 2b and S11).

Figure 2.

Figure 2

Open in a new tab

(a) Thermoset and composite beam of Ene1-T1 and Ene1-C4, respectively. (b) Raman spectra of the material Ene1-C4 before and after curing. (c–e) SEM images of the cross sections of materials Ene0-T1, respectively. (f) Dynamic mechanical analysis (DMA) of materials Ene0-T1, Ene0-C4, Ene1-T1, Ene1-C4, Yne1-T1, and Yne1-C4 under both dry and wet conditions. Black dashed line indicates the temperature of 37 °C. (g) Water absorption and solubility of materials Ene0-T1, Ene0-C4, Ene1-T1, Ene1-C4, Yne1-T1 and Yne1-C4.

The mechanical properties of these 12 materials were then determined by three-point bending and DMA measurements. The mechanical properties of the three families of materials (alkene 0, alkene 1, and alkyne 1) were found to be significantly different (Table 2). The alkene 0 family demonstrated the highest flexural modulus and strength, with the Ene0-T1 thermoset having a modulus of 2723 (30) MPa and a strength of 87 (1). In stark contrast, the modulus and strength of the Ene1-T1 thermoset were 15 (1) and 1.5 (0.1) MPa, respectively, a remarkable difference considering the only differences between the alkene 0 and alkene 1 monomers, were the insertion of an ester linkage and a small increase in distance between the TATO ring and alkene groups. The modulus and strength of the Yne1-T1 thermoset were 2259 (65) and 71 (1) MPa, which showed that much of the loss in stiffness and strength by introducing the ester linkage was recovered by employing TYC chemistry for cross-linking instead of TEC. The toughness of the Ene0-T1, Ene1-T1, and Yne1-T1 thermosets, as calculated from the strain–stress curve, was 2719 (133), 107 (11), and 8192 (322) kJ/m3, respectively, while the strain at the maximum stress was 4.4 (0.03), 11.7 (0.4), and 4.6 (0.1)%, respectively.

Table 2. Mechanical Properties and Thermal Analysis of the Different TEMPIC Thiol-Based Composites and Thermosets under Dry or Wet Conditionsa.

Materials Ef (dry) [MPa] Max σf (dry) [MPa] Toughness at maximum σf (kJ/m3) εf at maximum σf (%) Onset point (dry) [°C] Tg (dry) [°C] Ef (wet) [MPa] Max σf (wet) [MPa] Onset point (wet) [°C] Tg (wet) [°C]
Ene0-T1 2723 (30) 87 (1) 2719 (133) 4.4 (0.03) 61 (1) 71 (1) 1599 (66) 37 (3) 44 (1) 59 (0.7)
Ene0-C4 6480 (280) 68 (5) 427 (23) 1.2 (0.05) 58 (1) 71 (0.3) 3093 (199) 27 (1) 40 (1) 52 (0.5)
Ene1-T1 15 (1) 1.5 (0.1) 107 (11) 11.7 (0.4) 25 (1) 35 (1) 15 (0.4) 1.5 (0.1) 12 (1) 26 (1)
Ene1-C4 57 (2) 5 (0.1) 515 (22) 11.8 (0.3) 20 (1) 34 (0.4) 57 (1) 4 (0.1) 8 (0.2) 24 (1)
Yne1-T1 2259 (65) 71 (1) 8192 (322) 4.6 (0.1) 48 (1) 61 (0.4) 1663 (66) 43 (1) 44 (1) 59 (1)
Yne1-C4 3480 (231) 39 (1) 1942 (98) 3.7 (0.3) 42 (1) 58 (0.4) 2288 (98) 21 (2) 36 (1) 54 (1)
Open in a new tab
a

The values are given as mean, along with the standard error of mean (SEM) in parentheses (n > =5).

Despite these large mechanical differences, the cross-link densities of the thermosets, as determined by DMA, were similar (Supporting Information, Figure S12). From Ene0-T1 to Ene1-T1, the cross-link density decreased from 12.3 (0.9) to 10.0 (0.6) Mc, likely due to the elongation of the linkages between monomers from the added ester groups in Ene1-T1. As expected, the cross-link density of Yne1-T1 was higher than that of the TEC thermosets, at 15 (0.2) Mc, but this modest increase of 50% relative to Ene1-T1 was surprising, as previously reported TATO thermosets have shown a 400% increase in cross-link density when replacing alkenes with alkynes.30 It is possible that the higher degree of branching in the Yne1-T1 cross-linked network, due to each alkyne group reacting with two thiol groups, may explain the large difference in mechanical properties when compared to Ene1-T1.

The degree to which the mechanical properties of these materials were affected by HA concentration was also very different (Figure 3). Increasing HA concentration had the largest impact on the alkene 0 materials, with the maximum concentration of HA, 56 wt %, increasing the flexural modulus by 240% to 6480 MPa (Ene0-C4). This was accompanied by a decrease in flexural strength to 68 (5) MPa and decreases in toughness, from 2719 (133) to 427 (23) kJ/m3, and strain at maximum stress, from 4.4 (0.03) to 1.2 (0.05)%. The alkyne 1 materials followed this same trend, with the addition of 60 wt % HA increasing the modulus by a more modest 150% to 2259 (65) MPa (Yne1-C4), while reducing the strength to 39 (1) MPa. The toughness and strain at the maximum stress of Yne1-C4 were also lower than those of Yne1-T1, but at 1942 (98) kJ/m3 and 3.7 (0.3)%, these values were much higher than those of Ene0-C4. However, the addition of HA to both the alkene 0 and alkyne 1 materials resulted in stiffer but more brittle composites.

Figure 3.

Figure 3

Open in a new tab

(a) Mechanical property plot with flexural strength vs flexural modulus for 12 different materials evaluated in this study. (b) Flexural strain–stress curves for materials Ene0-C4, Ene1-C4, and Yne1-C4.

The alkene 1 materials showed a different trend, where the addition of increasing concentrations of HA increased the modulus, while also increasing the strength and toughness and not affecting the strain at maximum stress. For Ene1-C4, the modulus and strength were 57 (2) and 5 (0.1) MPa, an increase of 380 and 330%, respectively, while the toughness was 515 (22) kJ/m3 and the strain at maximum stress was 11.8 (0.3)%. In this case, stiffness was enhanced by adding HA without introducing brittleness. The reason behind the different responses of the alkene 1 materials to the increasing concentrations of HA should be investigated further in future studies.

However, the results of composites Ene0-C4 and Ene1-C4 suggest that, compared with the previously established composite Ene0-C4, the Ene1-C4 material is statistically more resistant to externally applied energy. Here, it is expected that the values of toughness for the alkene 1 materials are low due to their much lower flexural modulus and strength, compared with other materials from alkene 0 to alkyne 1. Moreover, when it comes to the discussion about their flexural strain at maximum flexural stress, the alkene 1 materials showed extraordinary results, which are 11.7 (0.4)% for Ene1-T1 and 11.8 (0.3)% for Ene1-C4. They are 3 and 10 times higher than the εf values of alkene 0 materials, which are 4.4 (0.03)% for Ene0-T1 and 1.2 (0.05)% for Ene0-C4, respectively. The alkyne thermoset Yne1-T1 presented εf of 4.6 (0.1)%, which is close to the result of Ene0-T1. On the other hand, the composite Yne1-C4 presented εf of 3.7 (0.3)%, which is 3.1 times higher than the result of Ene0-C4 but 3.2 times lower than the εf value of the Ene1-C4 composite.

Comparing the three families of thermosets and composites, it appeared that the presence of the additional ester linkages in alkene 1 and alkyne 1 did not increase the modulus and strength of these materials through secondary H-bonding as expected but that instead they reduced these properties relative to the alkene 0 materials. However, the addition of the ester linkages did result in increased toughness and strain at the maximum stress of the Ene1-C4 and Yne1-C4 composites relative to Ene0-C4. The loss in modulus and strength upon the addition of ester linkages was also effectively offset by replacing the alkene groups with alkyne functions, resulting in the very tough alkyne 1 materials.

Further mechanical evaluations were conducted on the Ene0-T1, Ene1-T1, and Yne1-T1 thermosets and composites with the maximum HA concentrations (Ene0-C4, Ene1-C4, and Yne1-C4) to determine how their properties were affected by the absorption of water after being immersed in PBS solution (pH = 7.4) for 7 days (Table 2), which is an important consideration for materials intended for in vivo applications. First, the water absorption results (Figure 2g and Supporting Information Figure S13) revealed that alkene 0 materials Ene0-T1 and Ene0-C4 absorbed the least amount of water [1.30 (0.04) and 1.30 (0.04)%, respectively], while the alkyne 1 materials Yne1-T1 and Yne1-C4 absorbed 1.37 (0.05) and 1.61 (0.05)%, respectively, and the alkene 1 materials Ene1-T1 and Ene1-C4 absorbed the most water [1.55 (0.03) and 2.07 (0.02)%]. This can be explained by the fact that the introduction of ester linkage in the chemical structure of the alkene 1 and alkyne 1 monomers facilitated water absorption. This increase in water absorption was likely offset in the alkyne 1 materials due to their highest cross-link density resulting from TYC instead of TEC chemistry.

Regarding their mechanical properties under wet conditions (Table 2), large decreases in flexural modulus and strength were observed for both the alkene 0 and alkyne 1 materials. For instance, the flexural modulus of the Ene0-C4 composite decreased from 6480 (280) MPa under dry conditions to 3093 (199) MPa after absorbing water, and its flexural strength decreased from 68 (5) to 27 (1) MPa. Meanwhile, the flexural modulus of Yne1-C4 was reduced by 1.5 times, and it descended from 3480 (231) to 2288 (98) MPa after water absorption. However, for Ene1-C4, the flexural modulus and strength remained at the same level, with values of 57 (1) and 4.4 (0.06) MPa, respectively, even though it absorbed more water than the alkene 0 and alkyne 1 materials (Figure 2g).

To better understand the vast difference in flexural modulus between the alkene 0, alkene 1, and alkyne 1 materials and to study the distribution of HA fillers in the composites, SEM and EDS have been used to image the cross sections for the materials Ene0-T1, Ene0-C4, Ene1-T1, Ene1-C4, Yne1-T1, and Yne1-C4. As can be seen from the SEM images (Figure 2c–e), the composites contain air bubbles which were not observed in the thermosets, which was likely due to the higher viscosity of the composites. To some extent, the presence of bubbles in the composites results in negative impacts regarding the mechanical properties of the materials, and their effect should be further studied in the future. The EDS images show that sulfur is well spread through the cross sections for both composites and thermosets, which indicates a homogeneous distribution of the cross-linking network (Supporting Information, Figure S10). Moreover, from the calcium mapping, it is clear that the HA particles in the composites were also evenly distributed, suggesting that the filler is well integrated throughout the cross-linked networks. Therefore, it appeared that the significant mechanical differences between the materials made from alkene 0, alkene 1, and alkyne 1 came from their distinguishing chemical structures.

3.3. In Vitro Biocompatibility of the TEC and TYC Materials

The biocompatibility of the novel TATO monomers and composites has been tested in vitro. Alkene 1 and alkyne 1 showed good biocompatibility toward both human dermal fibroblast (HDF) (Figure 4a, cell viability above 91%) and mouse monocyte cells (Raw 264.7) (Supporting Information, Figure S14b, cell viability above 92%) even at the highest concentration of 1000 μg/mL. Thiol showed no obvious cytotoxicity toward HDF (Figure 4a, cell viability above 82%) and Raw 264.7 (Supporting Information, Figure S14b, cell viability above 96%) at concentrations between 10 and 500 μg/mL but was toxic at the high concentration of 1000 μg/mL. Alkene 0 exhibited good biocompatibility toward HDF, with cell viability above 80% (Figure 4a), and it showed no obvious cytotoxicity toward Raw 264.7 at concentrations between 10 and 500 μg/mL (Supporting Information, Figure S14b). Composites Ene0-C4, Ene1-C4, and Yne1-C4 showed excellent biocompatibility toward both HDF and Raw 264.7 (Figure 4b), with cell viability above 104% (Ene0-C4), 94% (Ene1-C4), and 86% (Yne1-C4), respectively. In addition, both HDF (Figure 4c,d and Supporting Information, Figure S15) and Raw 264.7 (Supporting Information, Figure S16) proliferated after 3 days of incubation, indicating that the composites were well-cured, and no toxic components were leached out from the composites.

Figure 4.

Figure 4

Open in a new tab

(a) In vitro cytotoxicity evaluation results for monomers thiol, alkene 0, alkene 1, and alkyne 1 on human dermal fibroblast (HDF) cell culture. The results on Raw 264.7 are given in Supporting Information, Figure S14b; (b) In vitro cytotoxicity evaluation results for the composites Ene0-C4, Ene1-C4, and Yne1-C4 (for each with surface area of 3 cm2) on both HDF and Raw 264.7 cells. (c,d) Cell viability for materials Ene1-C4 in HDF on day 1 and day 3.

3.4. Fabrication of Objects with the Flexible HA Composites

Satisfied with the mechanical and biological properties of these novel materials, we further investigated the development of flexible objects. As a proof of concept, different shapes were produced through drop-casting the composite Ene1-C4, which had shown the softest properties of all the composites tested (Figure 5). This proves that, with the composite Ene1-C4 and different shapes of molds, it is possible to customize and fabricate soft devices of varying shapes, all of which showed an elastic behavior when squeezed or bent with finger pressure (Supporting Information, Objects S1, S2, and S3).

Figure 5.

Figure 5

Open in a new tab

Different objects made by the promising soft composites Ene1-C4: (a) ring with a circumference of 126 mm, (b) column with a diameter of 6 mm, (c) tube with a length of 30 mm and wall thickness of 4 mm, and (d) thin film with thickness of 200 μm. The softening properties and the customizable ability are shown in (e–h).

4. Conclusions

Four novel trifunctional TATO monomers were synthesized, with the arms containing either ester or amide linkages and terminated with either alkene or alkyne functionalities for use in making photocurable cross-linked networks with thiol-containing monomers through TEC or TYC click chemistry. Formulations were made with the ester-containing monomers and the commercially available thiol monomer and varying the concentrations of the HA filler. Unfortunately, the amide-containing monomers proved too insoluble in the thiol and so were not used to make thermosets and composites. It was hypothesized that materials made with these monomers would have enhanced strength due to secondary H-bonding from the esters and that the alkyne monomers would further enhance their modulus due to the increased cross-link density afforded through TYC chemistry. Instead, we discovered that the introduction of the ester linkages resulted in materials that were significantly softer than those made with the previously described alkene 0 monomer. The HA composites with the ester-containing alkene 1 monomer were 117 and 10 times softer in terms of flexural modulus and strain at maximum stress than those containing the ester-free alkene 0 monomer. Comparing the alkene 1 and alkyne 1 materials showed that replacing the alkene groups for alkynes resulted in a large increase in modulus and strength, along with a modest increase in cross-link density, as expected. These alkyne 1 materials displayed the highest toughness but were not as soft as those of alkene 1. Mechanical testing under wet conditions indicates that the alkene 1 composites were able to maintain their mechanical properties after being immersed in PBS (pH = 7.4) for 7 days, while the alkyne 1 and alkene 0 composites underwent a significant flexural modulus and strength loss. For all produced composites, in vitro cellular evaluation was conducted, detailing excellent cytotoxicity results. The unique mechanical properties of the alkene 1 and HA composites were further explored by the fabrication of different proof-of-concept customizable objects. Finally, the combined mechanical, cytotoxicity, and customizability features of alkene 1-based cross-linked frameworks have the overarching attributes needed for future biomedical devices in which lower modulus is a prerequisite.

Acknowledgments

This work was generously supported by the China Scholarship Council (CSC), Knut and Alice Wallenberg Foundation (KAW), and the European Union’s Horizon 2020 research and innovation program under grant agreement No 952150 for financial support.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.2c16235.

  • Demonstration of a thin film. Object S1 (MP4)

  • Demonstration of a ring. Object S2 (MP4)

  • Demonstration of a tube. Object S3 (MP4)

  • Synthesis of monomers, preparation of composite and thermoset materials, characterizations, object demonstrations, and other additional data (PDF)

Author Contributions

J.L. conducted most of the work for this study, including the synthesis of alkene 1, alkyne 1, alkene 2, and alkyne 2 monomers, the formulation of the alkene 0, alkene 1, and alkyne 1 thermosets and composites, and the analysis of the alkene 0, alkene 1, and alkyne 1 thermosets and composites. Y.F. performed the cytotoxicity assay of the monomers and composites. The manuscript was mostly written by J.L. with the input from Y.F. for the cytotoxicity testing and with help from D.J.H. The project was supervised by M.M. and D.J.H.

The authors declare no competing financial interest.

Notes

Research data are not shared.

Supplementary Material

am2c16235_si_001.mp4 (2MB, mp4)
am2c16235_si_002.mp4 (2.4MB, mp4)
am2c16235_si_003.mp4 (3.7MB, mp4)
am2c16235_si_004.pdf (1.7MB, pdf)

References

  1. Pearson R. A.Thermosetting Plastics https://www.sciencedirect.com/science/article/pii/B9780080434179500131.
  2. Kabasci S.Bio-Based Plastics - Introduction. Bio-Based Plastics 2013, 1–7.
  3. Trifol J.; Plackett D.; Sillard C.; Szabo P.; Bras J.; Daugaard A. E. Hybrid Poly(Lactic Acid)/Nanocellulose/Nanoclay Composites with Synergistically Enhanced Barrier Properties and Improved Thermomechanical Resistance. Polym. Int. 2016, 65, 988–995. 10.1002/pi.5154. [DOI] [Google Scholar]
  4. McKendry P. Energy Production from Biomass (Part 1): Overview of Biomass. Bioresour. Technol. 2002, 83, 37–46. 10.1016/S0960-8524(01)00118-3. [DOI] [PubMed] [Google Scholar]
  5. Schutyser W.; Renders T.; den Bosch S. V.; Koelewijn S.-F.; Beckham G. T.; Sels B. F. Chemicals from Lignin: An Interplay of Lignocellulose Fractionation, Depolymerisation, and Upgrading. Chem. Soc. Rev. 2018, 47, 852–908. 10.1039/C7CS00566K. [DOI] [PubMed] [Google Scholar]
  6. Takase S.; Hamada T.; Okada K.; Mineoi S.; Uedono A.; Ohshita J. Organic–Inorganic Hybrid Thermal Insulation Materials Prepared via Hydrosilylation of Polysilsesquioxane Having Hydrosilyl Groups and Triallylisocyanurate. ACS Appl. Polym. Mater. 2022, 4, 3726–3733. 10.1021/acsapm.2c00241. [DOI] [Google Scholar]
  7. Qiu Y.; Qian L.; Xi W. Flame-retardant Effect of a Novel Phosphaphenanthrene/triazine-trione bi-group Compound on an Epoxy Thermoset and its Pyrolysis Behaviour. RSC Adv. 2016, 6, 56018–56027. 10.1039/C6RA10752D. [DOI] [Google Scholar]
  8. Wu H.; Hu R.; Zeng B.; Yang L.; Chen T.; Zheng W.; Liu X.; Luo W.; Dai L. Synthesis and Application of Aminophenyl-s-triazine Derivatives as Potential Flame Retardants in the Modification of Epoxy Resins. RSC Adv. 2018, 8, 37631–37642. 10.1039/C8RA07450J. [DOI] [PMC free article] [PubMed] [Google Scholar]
  9. Bakhshi Nejad S.; Mohammadi A. Epoxy-Triazinetrione-Functionalized Magnetic Nanoparticles as an Efficient Magnetic Nanoadsorbent for the Removal of Malachite Green and Pb(II) from Aqueous Solutions. J. Chem. Eng. Data 2020, 65, 2731–2742. 10.1021/acs.jced.0c00063. [DOI] [Google Scholar]
  10. Chattopadhyay D. K.; Raju K. V. S. N. Structural Engineering of Polyurethane Coatings for High Performance Applications. Prog. Polym. Sci. 2007, 32, 352–418. 10.1016/j.progpolymsci.2006.05.003. [DOI] [Google Scholar]
  11. Ni H.; Skaja A. D.; Sailer R. D.; Soucek M. D. Moisture-curing Alkoxysilane-functionalized Isocyanurate Coatings. Macromol. Chem. Phys. 2000, 201, 722–732. . [DOI] [Google Scholar]
  12. Granskog V.; Garcia-Gallego S.; von Kieseritzky J.; Rosendahl J.; Stenlund P.; Zhang Y.; Petronis S.; Lyven B.; Arner M.; Håkansson J.; Malkoch M. High-Performance Thiol–Ene Composites Unveil a New Era of Adhesives Suited for Bone Repair. Adv. Funct. Mater. 2018, 28, 1800372 10.1002/adfm.201800372. [DOI] [Google Scholar]
  13. Carioscia J. A.; Lu H.; Stanbury J. W.; Bowman C. N. Thiol-ene Oligomers as Dental Restorative Materials. Dent. Mater. 2005, 21, 1137–1143. 10.1016/j.dental.2005.04.002. [DOI] [PubMed] [Google Scholar]
  14. Lu H.; Carioscia J. A.; Stanbury J. W.; Bowman C. N. Investigations of Step-growth Thiol-ene Polymerizations for Novel Dental Restoratives. Dent. Mater. 2005, 21, 1129–1136. 10.1016/j.dental.2005.04.001. [DOI] [PubMed] [Google Scholar]
  15. Podgórski M.; Becka E.; Claudino M.; Flores A.; Shah P. K.; Stansbury J. W.; Bowman C. N. Ester-free Thiol–ene Dental Restoratives—Part A: Resin Development. Dent. Mater. 2015, 31, 1255–1262. 10.1016/j.dental.2015.08.148. [DOI] [PMC free article] [PubMed] [Google Scholar]
  16. Hoyle C. E.; Lee T. Y.; Roper T. Thiol–enes: Chemistry of the Past with Promise for the Future. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 5301–5338. 10.1002/pola.20366. [DOI] [Google Scholar]
  17. Hoyle C. E.; Bowman C. N. Thiol–Ene Click Chemistry. Angew. Chem., Int. Ed. 2010, 49, 1540–1573. 10.1002/anie.200903924. [DOI] [PubMed] [Google Scholar]
  18. Jiang X.; Lahann J. Click Chemistry: Versatility and Control in the Hands of Materials Scientists. Adv. Mater. 2007, 19, 2197–2208. 10.1002/adma.200602739. [DOI] [Google Scholar]
  19. Kade M. J.; Burke D. J.; Hawker C. J. The Power of Thiol-ene Chemistry. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 743–750. 10.1002/pola.23824. [DOI] [PMC free article] [PubMed] [Google Scholar]
  20. Lowe A. B. Thiol–Ene “Click” Reactions and Recent Applications in Polymer and Materials Synthesis: A First Update. Polym. Chem. 2014, 5, 4820–4870. 10.1039/C4PY00339J. [DOI] [Google Scholar]
  21. Jawerth M.; Johansson M.; Lundmark S.; Gioia C.; Lawoko M. Renewable Thiol–Ene Thermosets Based on Refined and Selectively Allylated Industrial Lignin. ACS Sustainable Chem. Eng. 2017, 5, 10918–10925. 10.1021/acssuschemeng.7b02822. [DOI] [Google Scholar]
  22. Claudino M.; Mathevet J.-M.; Jonsson M.; Johansson M. BringingD-Limonene to the Scene of Bio-Based Thermoset Coatings via Free-Radical Thiol–Ene Chemistry: Macromonomer Synthesis, UV-Curing and Thermo-Mechanical Characterization. Polym. Chem. 2014, 5, 3245–3260. 10.1039/C3PY01302B. [DOI] [Google Scholar]
  23. Li C.; Johansson M.; Sablong R. J.; Koning C. E. High Performance Thiol-Ene Thermosets Based on Fully Bio-Based Poly(Limonene Carbonate)s. Eur. Polym. J. 2017, 96, 337–349. 10.1016/j.eurpolymj.2017.09.034. [DOI] [Google Scholar]
  24. Sticker D.; Rothbauer M.; Lechner S.; Hehenberger M.-T.; Ertl P. Multi-Layered, Membrane-Integrated Microfluidics Based on Replica Molding of a Thiol–Ene Epoxy Thermoset for Organ-On-a-Chip Applications. Lab Chip 2015, 15, 4542–4554. 10.1039/C5LC01028D. [DOI] [PubMed] [Google Scholar]
  25. Guzmán D.; Ramis X.; Fernández-Francos X.; Serra A. Preparation of Click Thiol-Ene/Thiol-Epoxy Thermosets by Controlled Photo/Thermal Dual Curing Sequence. RSC Adv. 2015, 5, 101623–101633. 10.1039/C5RA22055F. [DOI] [Google Scholar]
  26. Resetco C.; Hendriks B.; Badi N.; Du Prez F. Thiol–Ene Chemistry for Polymer Coatings and Surface Modification–Building in Sustainability and Performance. Mater. Horiz. 2017, 4, 1041–1053. 10.1039/C7MH00488E. [DOI] [Google Scholar]
  27. Konuray O.; Fernández-Francos X.; De la Flor S.; Ramis X.; Serra À. The Use of Click-Type Reactions in the Preparation of Thermosets. Polymer 2020, 12, 1084. 10.3390/polym12051084. [DOI] [PMC free article] [PubMed] [Google Scholar]
  28. Photopolymerization of Thiol-Alkynes: Polysulfide Networks. Chem. Mater. 2009, 21 (), 1579–1585, 10.1021/cm803262p. [DOI] [Google Scholar]
  29. Arseneault M.; Granskog V.; Khosravi S.; Heckler I. M.; Mesa Antunez P.; Hult D.; Zhang Y.; Malkoch M. The Dawn of Thiol-Yne Triazine Triones Thermosets as a New Material Platform Suited for Hard Tissue Repair. Adv. Mater. 2018, 30, 1804966 10.1002/adma.201804966. [DOI] [PubMed] [Google Scholar]
  30. Fairbanks B. D.; Scott T. F.; Kloxin C. J.; Anseth K. S.; Bowman C. N. Thiol–Yne Photopolymerizations: Novel Mechanism, Kinetics, and Step-Growth Formation of Highly Cross-Linked Networks. Macromolecules 2009, 42, 211–217. 10.1021/ma801903w. [DOI] [PMC free article] [PubMed] [Google Scholar]
  31. Li N.; Li N.; Yi Q.; Luo K.; Guo C.; Pan D.; Gu Z. Amphiphilic Peptide Dendritic Copolymer-Doxorubicin Nanoscale Conjugate Self-Assembled to Enzyme-Responsive Anti-Cancer Agent. Biomaterials 2014, 35, 9529–9545. 10.1016/j.biomaterials.2014.07.059. [DOI] [PubMed] [Google Scholar]
  32. Boyd D. A.; Shields A. R.; Naciri J.; Ligler F. S. Hydrodynamic Shaping, Polymerization, and Subsequent Modification of Thiol Click Fibers. ACS Appl. Mater. Interfaces 2013, 5, 114–119. 10.1021/am3022834. [DOI] [PubMed] [Google Scholar]
  33. Westreich R. W.; Courtland H.-W.; Nasser P.; Jepsen K.; Lawson W. Defining Nasal Cartilage Elasticity: biomechanical testing of the tripod theory based on a cantilevered model. Arch. Facial Plast. Surg. 2007, 9, 264–270. 10.1001/archfaci.9.4.264. [DOI] [PubMed] [Google Scholar]
  34. Guimarães C. F.; Gasperini L.; Marques A. P.; Reis R. L. The Stiffness of Living Tissues and its Implications for Tissue Engineering. Nat. Rev. Mater. 2020, 5, 351–370. 10.1038/s41578-019-0169-1. [DOI] [Google Scholar]
  35. Maganaris C.N.; Narici M. V.. Mechanical Properties of Tendons, in Tendon Injuries: Basic Science and Clinical Medicine, Maffulli N.; Renström P.; Leadbetter W.B., Editors.; Springer London: London, 2005, 14–21. [Google Scholar]
  36. Försth P.; Persson C. Low-Modulus PMMA has the Potential to Reduce Stresses on Endplates after Cement Discoplasty. J. Funct. Biomater. 2022, 13, 18. 10.3390/jfb13010018. [DOI] [PMC free article] [PubMed] [Google Scholar]
  37. Helgason B.; Perilli E.; Schileo E.; Taddei F.; Brynjolfsson S.; Viceconti M. Mathematical Relationships Between Bone Density and Mechanical Properties: A Literature Review. Clin. Biomech. (Bristol Avon) 2008, 23, 135–146. 10.1016/j.clinbiomech.2007.08.024. [DOI] [PubMed] [Google Scholar]
  38. Keyak J. H.; Lee I. Y.; Skinner H. B. Correlations Between Orthogonal Mechanical Properties and Density of Trabecular Bone: Use of Different Densitometric Measures. J. Biomed. Mater. Res. 1994, 28, 1329–1336. 10.1002/jbm.820281111. [DOI] [PubMed] [Google Scholar]
  39. Hutchinson D. J.; Granskog V.; von Kieseritzky J.; Alfort H.; Stenlund P.; Zhang Y.; Arner M.; Håkansson J.; Malkoch M. Highly Customizable Bone Fracture Fixation through the Marriage of Composites and Screws. Adv. Funct. Mater. 2021, 31, 2105187 10.1002/adfm.202105187. [DOI] [Google Scholar]

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

am2c16235_si_001.mp4 (2MB, mp4)
am2c16235_si_002.mp4 (2.4MB, mp4)
am2c16235_si_003.mp4 (3.7MB, mp4)
am2c16235_si_004.pdf (1.7MB, pdf)

Articles from ACS Applied Materials & Interfaces are provided here courtesy of American Chemical Society

RESOURCES