Four-order stiffness variation of laser-fabricated photopolymer biodegradable scaffolds by laser parameter modulation Balázs Farkas a , Ilaria Romano a , Luca Ceseracciu a , Alberto Diaspro a , Fernando Brandi a,b , Szabolcs Beke a, ⁎ a Department of Nanophysics, Istituto Italiano di Tecnologia (IIT), Via Morego 30, 16163 Genova, Italy b Istituto Nazionale di Ottica, Via Moruzzi 1, 56124 Pisa, Italy abstract article info Article history: Received 23 October 2014 Received in revised form 30 March 2015 Accepted 17 May 2015 Available online 21 May 2015 Keywords: Stereolithography Excimer laser Biodegradability Poly(propylene fumarate) Scaffolds The effects of various fabrication parameters of our Mask Projection Excimer Laser StereoLithography (MPExSL) system were investigated. We demonstrate that laser parameters directly change the physical properties (stiffness, thermal degradation, and height/thickness) of the poly(propylene fumarate) (PFF) scaffold structures. The tested parameters were the number of pulses, fluence per pulse and laser repetition rate. We present a four- order tuning capability of MPExSL-fabricated structures' stiffness without altering the resin composition or using cumbersome post-treatment procedures. Thermogravimetric analysis and differential scanning calorimetry con- firmed this tuning capability. Prototype-segmented scaffold designs are presented and analyzed to further expand the concept and exploit this in situ stiffness tuning capability of the scaffolds for tissue engineering and regenerative medicine applications. © 2015 Published by Elsevier B.V. 1. Introduction Tissue engineering (TE) [1–4] is an expanding interdisciplinary field with the purpose of growing tissues directly on controlled microenvi- ronments called scaffolds. The design and fabrication of structures are among those parameters that strongly affect the successful outcome of tissue formation. Therefore, before these artificial structures can be con- sidered for medical applications, they must fulfill some general require- ments, such as proper porosity and mechanical properties, making them capable of supporting the engineered tissue [5]. One of the major goals of tissue engineering is to provide a rapid and reliable production of well-designed and functional scaffolds, capable of fulfilling the aforementioned requirements. The tuning of these charac- teristics is also highly desired — in some cases, even within the same structure, in order to achieve multi-phase constructs [6,7]. It is therefore crucial to be able to reliably adjust all physical param- eters of the scaffolds, including the stiffness (Young's modulus), the degradation (rate), and the geometry (size and porosity) with a high level of biocompatibility. All structures presented here were fabricated by our novel fabrication process called Mask Projection Excimer Stereolithography (MPExSL) [8] using poly(propylene fumarate) (PFF) resin. MPExSL is a simple and highly efficient method optimized for rapid prototyping of single-layer (2D) and multilayer (3D) TE scaffolds of various sizes and porosities. It is based on pulsed excimer laser photocuring of a biocompatible photosensitive resin. The biocompati- bility of MPExSL-fabricated structures has already been investigated by our group in previous in vitro [9] and in vivo [10] studies. Besides, elastine [11,12] and titanate nanotubes [13,14] were utilized as func- tional coatings on these PPF scaffolds. MPExSL applies several variable laser parameters — such as the number of pulses, the pulse fluence and laser repetition rate. The main resin composition parameter is the photoinitiator concentration [15–18]. All of these parameters are potentially capable of tuning the physical properties of the scaffolds [19] due to the changes in the initial photocross-linking density [16], and the reversion occurring under in- tense exposure [20]. The current study aims at unraveling the effects of these aforementioned parameters on the height, thermal degrada- tion, and stiffness of the fabricated structures. Scaffolds with properly adjusted stiffness are indispensable to mimic the tissue's extracellular matrix [21–24]. High stiffness is also important for greater structural integrity, for instance, in osteochondral tissue re- generation [25–27]. Usually though, the stiffness tuning capability of these scaffolds is limited, leading to constructs fabricated from various metals, polymers, and ceramics, depending on their functionality. The lack of a facile tuning process becomes apparent when tissue interfaces are repaired using composite scaffolds [25–29] and these structures are made of multiple materials employing lengthy and costly processes. In this study, we present an in situ, 4-order stiffness tuning (4 MPa to 4 GPa) of biodegradable and biocompatible scaffolds without altering the PPF resin composition. The modulation of the scaffold's degradation rate by hydrolysis in 37 °C DMEM is also reported. Materials Science and Engineering C 55 (2015) 14–21 ⁎ Corresponding author at: Department of Nanophysics, Italian Institute of Technology, Via Morego 30, 16163 Genova, Italy. E-mail address: szabolcs.beke@iit.it (S. Beke). http://dx.doi.org/10.1016/j.msec.2015.05.054 0928-4931/© 2015 Published by Elsevier B.V. Contents lists available at ScienceDirect Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec